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Journal of Geographical Sciences >

2018 , Vol. 28 >Issue 5: 611 - 628

DOI: https://doi.org/10.1007/s11442-018-1494-9

Research Articles

Response of the water use efficiency of natural vegetation to drought in Northeast China

  • LIU Dan , 1, 2, 3 ,
  • YU Chenglong , 1, 2, 3 ,
  • ZHAO Fang 4
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  • 1. Northeast China Ecological and Meteorological Open Innovation Laboratory, China Meteorological Administration, Harbin 150030, China
  • 2. Meteorological Academician Workstation of Heilongjiang Province, Harbin 150030, China
  • 3. Heilongjiang Province Institute of Meteorological Sciences, Harbin 150030, China
  • 4. College of Agronomy, Ningxia University, Yinchuan 750021, China

Author: Liu Dan (1974-), PhD, specialized in ecological meteorology. E-mail:

*Corresponding author: Yu Chenglong (1973-), PhD, specialized in ecological meteorology. E-mail:

Received date: 2017-10-18

  Accepted date: 2018-01-06

  Online published: 2018-03-30

Supported by

Foundation of Northeast China Innovation and Opening Laboratory of Eco-Meteorology, CMA, No.stqx2017zd01

Special Projects of Climate Change of CMA, No.CCSF201512

Foundation of Institute of Atmospheric Environment in Shenyang, CMA, No.2016SYIAE11

National Natural Science Foundation of China, No.41165005

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Journal of Geographical Sciences, All Rights Reserved
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Abstract

Drought has become a problem that is universally faced by global terrestrial ecosystems. Northeast China is located in a region sensitive to global climate changes, and one of the main impacts of climate changes in Northeast China is manifested as drought in growing seasons. This study analyzes the spatio-temporal evolution law of the water use efficiency (WUE) of the main natural vegetation (i.e., cold-temperate coniferous forests, temperate pine-broad-leaved mixed forests, warm-temperate deciduous broad-leaved forests, and grasslands) in Northeast China based on public MODIS data products, including MCD12Q1, MOD15A2H, MOD16A2, and MOD17A3H, and meteorological data from 2002 to 2013. The influence of drought events on the WUE of different vegetation types and their response to drought events are also investigated. The study findings are as follows: (1) drought in Northeast China frequently occurs in the regions stretching from 114.55°E to 120.90°E, and the percentage of drought area among the forests is lower than that among the grasslands during these years; (2) the annual average WUE of the natural vegetation ranges from 0.82 to 1.08 C/kg-1H2O, and the WUE of forests (0.82 to 1.08 C/kg-1H2O) is universally higher than that of grasslands (0.84 to 0.99 C/kg-1H2O); (3) in 2008, the regions where the WUE in drought conditions is higher than that in normal water conditions account for 86.11% of the study area, and a significant linear positive correlation is found between the WUE in drought conditions and the WUE in normal water conditions, whereas the degree of drought does not influence the WUE of the natural vegetation in an obviously linear manner; and (4) the WUE for the cold-temperate coniferous forests and temperate pine-broad-leaved mixed forests with a high ET or low NPP is more likely to rise in drought conditions; the WUE for the grasslands with a low Evapotranspiration (ET), Net Primary Production (NPP), and Leaf Area Index (LAI) is more likely to rise in drought conditions; and the ET, NPP, and LAI have no significant influence on the WUE for the warm-temperate deciduous broad-leaved forests in drought conditions. This study contributes to improving the evaluation of the influence of drought on natural ecosystems.

Key words： natural vegetation; drought; water use efficiency (WUE)

Cite this article

LIU Dan , YU Chenglong , ZHAO Fang . Response of the water use efficiency of natural vegetation to drought in Northeast China[J]. Journal of Geographical Sciences, 2018 , 28(5) : 611 -628 . DOI: 10.1007/s11442-018-1494-9

1 Introduction

Drought has a far-reaching influence on global terrestrial ecosystems (Yu et al., 2016). Both the frequency and intensity of drought increase as global climate changes are aggravated (Houghton, 1995). Therefore, the ecological academia has paid great attention to the response and adaptability of terrestrial ecosystems to drought, and studied this issue from different angles of view. Regarding the response of terrestrial ecosystems to drought, studies have reached the following consensus: (1) drought causes a decline in stomatal conductance, maximum photochemical efficiency, and quantum yield, thereby weakening the photosynthesis of plants (Hernandez et al., 2016; Vanlerberghe et al., 2016) and reducing the GPP of terrestrial ecosystems (Joo et al., 2016; Wang et al., 2017); (2) drought causes a decline in the activity of metabolic enzymes of certain plants, thereby weakening the respiratory action of their aboveground and underground parts (Zhang et al., 2017; Hasibeder et al., 2015); and (3) the respiratory action of soil microbes calls for an optimal range of moisture content, hence, the influence of drought on soil respiration is somewhat different. Drought has a negative influence on soil respiration across the world (Raich et al., 2002).
The response of terrestrial ecosystems to drought is mainly studied by two types of methods, namely experimental and model simulations. Most of the experimental simulation methods ascertain the response of plants to water shortage through a simulation experiment on water controlling (Hoover et al., 2017; Ledger et al., 2011). Such methods can obtain accurate experimental data, which can be used to study the mechanism of the response of plants to drought. The current experimental simulation methods are mainly used to study grasslands and farmlands, but seldom used to study forest ecosystems, because of the limitations in the plant growth cycle, degree of sensitivity of plants to drought, and experimental equipment. In addition, such methods are only applicable within a small scope. Therefore, extending the applicable objects of research conclusions from individuals or groups to ecosystems is somewhat challenging. Based on a biogeochemical model, model simulation methods mainly simulate the influence of drought on the productivity of terrestrial ecosystems by adjusting the rainfall variable (PaiMazumder et al., 2016; Shiogama et al., 2013). Such methods can largely study the influence of drought on terrestrial ecosystems. However, most of these methods overlook the adaptability of terrestrial ecosystems to water shortage. Therefore, research conclusions have a certain deviation from actual ones. To address this problem, advanced remote sensing technologies can be used to inversely analyze various indices on terrestrial ecosystems, including carbon flux indices (e.g., Gross Primary Productivity (Ruehr et al., 2014) and Net Primary Productivity (Zhang et al., 2014)), biomass indices (e.g., Normal Differential Vegetation Index (Mutiibwa et al., 2013), Enhanced Vegetation Index (Aulia et al., 2016), and Leaf Area Index (Chakroun et al., 2014)), water utilization indices (e.g., Water Use Efficiency (Gang et al., 2016; da Silva et al., 2017)), indices used to measure the carbon sequestration and conversion capabilities (e.g., CUE (Esmaeilpour et al., 2016; Malone et al., 2016)), and certain models (Limousin et al., 2015; Sawada et al., 2014). These indices can be used to discuss the response of terrestrial ecosystems to drought. To some extent, this offsets the deficiencies of experimental and model simulations and opens a new door for studying the response of terrestrial ecosystems to drought.
Northeast China is characterized by a vast territory, a complex topography, a unique vegetation distribution, a distinctive temperature and humidity, and integrated geographical regions. Moreover, Northeast China is also located in a region sensitive to global climate changes (Zhao et al., 2013). Today, Northeast China has become a hot spot for the research on global climate changes and ecogeographic process. One of the main impacts of climate changes on Northeast China is manifested as drought in growing seasons. Research data shows that the spatial distribution of drought in Northeast China is characterized by “severe in the west and slight in the east” (Xi et al., 2016). For over nearly 50 years, the arid and semi-arid boundary overall tends to extend eastwards and southwards (Wang et al., 2013), and drought is likely to be more severe in the forthcoming 30 years (Lu et al., 2016; Ma et al., 2013). Therefore, studying the response of the natural vegetation in Northeast China to drought is of great importance because it will provide a certain reference for formulating a regional sustainable development strategy, which involves issues, such as natural vegetation recovery, rational utilization of regional natural resources, and biodiversity protection. Many studies were performed in this regard. Yang Liu (2015) studied the influence of drought on the water parameters of Larix gmelinii by simulating the drought conditions through a water controlling experiment. Li et al. (2012) studied the biochemical changes of Taxus cuspidata in drought and rehydration conditions. By virtue of remote sensing technologies, Wang et al. (2016) studied the correlation between the standardized precipitation evapotranspiration index and the normalized difference vegetation index and interpreted the influence of drought on the vegetation growth from a meteorological perspective. However, their study did not consider the changes in the transpiration of plants in drought conditions, thereby ignoring the adaptability of plants to drought.
Drought inhibits the carbohydrate cycle process of ecosystems. As the productivity index of plants in certain water conditions, the WUE reflects the capability and the sensitivity of ecosystems to maintain normal production in certain water conditions. Furthermore, the WUE connects a biological process (e.g., photosynthesis and respiration) and a physical process (e.g., evapotranspiration) during the carbohydrate cycle of ecosystems (Schoo et al., 2017; Anower et al., 2017). In view of this, the WUE has a unique advantage in studying the response of ecosystems to drought. This study addresses the above-mentioned problem by summarizing previous research findings and acquiring the research data newly added in the last decade or so. The study object is the natural vegetation of Northeast China. This study uses the average WUE during many years based on MODIS data products and the Self-calibrating Palmer Drought Severity Index (scPDSI) to evaluate the response to drought, analyze the spatio-temporal evolution law of productivity of the natural vegetation in Northeast China since 2000, and study the quantitative influence of drought events on the productivity of natural vegetation and the response of natural vegetation to drought events. This study intends to improve the ability of evaluating the influence of drought on natural ecosystems.

2 The study area

Northeast China comprises three provinces of Heilongjiang, Jilin, and Liaoning, and some regions, including cities of Chifeng, Tongliao, Hulun Buir, and Xing’an League, in the east of Inner Mongolia Autonomous Region (Wang et al., 2006). Northeast China stretches from 111.15°E to 135.09°E and from 37.95°N to 53.56°N. Its north borders on Russia; its south is adjacent to the Yellow Sea and Bohai Sea; its east and north are surrounded by the Yalu River, Tumen River, Wusuli River, and Heilongjiang River; and its west borders on Mongolia. Northeast China covers a total area of 787,000 km2. The Northeast China territory includes the Greater Khingan Mountains, Lesser Khingan Mountains, Changbai Mountains, and Northeast China Plain (comprising the Songliao Plain, Liaohe Plain, and Three River Plain). Its altitude ranges from 2 to 2,667 m. Northeast China is dominated by a temperate monsoon climate, specifically cold and dry in winter and hot and rainy in summer. The annual average temperature over the past 30 years (1981 to 2010) has ranged from -4.08 °C to 11.34 °C. The annual rainfall ranges from 199.53 mm to 1170.60 mm. The daily average sunshine duration ranges from 5.26 h to 9.21 h.
Northeast China falls in the Eurasian forest-grass plant subregion and the China-Japan forest plant subregion. Cold-temperate coniferous forests, temperate pine-broad-leaved mixed forests, warm-temperate deciduous broad-leaved forests, and vast grasslands can be found in the Northeast China territory (Xu et al., 1986). Cold-temperate coniferous forests comprise frost-resisting evergreen or deciduous coniferous trees, and are a type of zonal vegetation in the north of the Greater Khingan Mountains. They are distributed from 50°N to 53°N and at the altitude of 300 to 1,400 m. Temperate pine-broad-leaved mixed forests comprise evergreen needle-leaved trees, deciduous needle-leaved trees, and deciduous broad-leaved trees. They are distributed in the south of the Greater Khingan Mountains, and most parts of the Lesser Khingan Mountains and Changbai Mountains, or specifically distributed from 40°N to 50° and at the altitude of 100 to 1600 m. Meanwhile, warm-temperate deciduous broad-leaved forests are distributed within a small area, specifically only in the regions south of the Changbai Mountains below an altitude of 500 m. Grasslands are mainly distributed inside Inner Mongolia located in the west of Northeast China, or specifically from 41°N to 50°N and from 110°E to 125°E at the altitude of 150 to 1300 m.

3 Data source and processing

3.1 Geographic information data

The digital elevation model (DEM) is a Shuttle Radar Topography Mission product (V4.1) with a spatial resolution of 90 m. The DEM data is available from the international scientific data mirroring website (http://www.gscloud.cn) of the Computer Network Information Center in the Chinese Academy of Sciences.
The data on the prefecture/provincial-level administrative division is available from the basic geographic information (scale: 1:250,000) released by the China Meteorological Administration. A topological check was conducted for all data.

3.2 Remote sensing data

The remote sensing data used in this study, except for the MOD16A2 data product available from the Numerical Terradynamic Simulation Group of The University of Montana, are all available from the DOS data center of The Land Processes Distributed Active Archive Center in NASA. All remote sensing data (e.g., image stitching, resampling (unified spatial resolution: 500 m), and projection transformation (sinusoidal projection is transformed into longitude-latitude projection)) were preprocessed using the MODIS Reprojection Tool (MRT 4.0) software. The time range was from 2002 to 2013, and the spatial range was Northeast China.
3.2.1 MCD12Q1 data product
This study extracted the data on the natural vegetation distribution in the study area based on the classification results under the IGBP Global Vegetation Classification Scheme specified by the MCD12Q1 data product (land cover classification product) of the MODIS. The original spatial resolution of the data was 500 m.
The MCD12Q1 data product tried to control the errors in classification by various means (e.g., introducing the surface temperature variable and using the data of the recent three years), but it still encountered certain difficulties in distinguishing the similar land types (Friedl et al., 2010). In this study, the classification results of the MCD12Q1 data were combined into forests, shrublands, grasslands, wetlands, and non-natural vegetation, including urban lands, farmlands, waters, and snowfields, because the changes in the vegetation types will largely influence the degree of response of plants to climate changes. Therefore, this study only retained the natural ecosystems, in which the vegetation types were not changed during 2002 to 2013, for use as the study object. Figure 1 shows the distribution of the study object. The shrublands and the wetlands in the study area accounted for a very small proportion (≤0.01%). Therefore, this study only discussed the response of forests and grasslands to drought. The forests of Northeast China were classified herein into cold-temperate coniferous forests, temperate pine-broad-leaved mixed forests, and warm-temperate deciduous broad-leaved forests according to the Chinese vegetation regionalization data available from the Data Center for Resources and Environmental Sciences in the Chinese Academy of Sciences (http://www.resdc.cn).
Figure 1 Natural vegetation distribution in Northeast China.
The white areas represent the non-vegetation regions and the regions with changes in land cover types from 2000 to 2013. The vegetation regions are arranged in a descending order as follows: grasslands (accounting for 20.45% of the total area of Northeast China), temperate pine-broad-leaved mixed forests (11.19%), cold-temperate coniferous forests (7.22%), warm-temperate deciduous broad-leaved forests (0.32%), and wetlands (<0.01).
3.2.2 MOD15A2H data product
The leaf area index (LAI) is an important characteristic index of vegetation growth (Iio et al., 2014). This study selects the MOD15A2H data product (MODIS/Terra Leaf Area Index/FPAR) to analyze the response of the vegetation growth to climate changes. The data is an eight-day synthetic product with an original spatial resolution of 500 m. The LAI is a dimensionless index. The data incorporates the fraction of photosynthetically active radiation (FPAR) and the LAI. Compared with the MOD15A2, the MOD15A2H uses the L2G-lite apparent reflectance instead of the MODIS apparent reflectance product (MODAGAGG). In the calculation process, the MOD15A2H cites the improved multi-year land cover classification product, thereby improving the spatial resolution from 1 km to 500 m (Myneni et al., 2015).
3.2.3 MOD16A2 data product
Evapotranspiration (EI) is an important link in the water migration in the soil-plant-atmosphere continuum (Hobbins et al., 2001). This study used the MOD16A2_MONTHLY data product (MODIS Global Evapotranspiration Project) to analyze the response of the EI of ecosystems to climate changes. The data is a part of the NASA/EOS program and intends to monitor the EI of the global land surface through satellite remote sensing. The data is a monthly synthetic product with an original spatial resolution of 1 km (unit: 0.1 mm/month). The data product uses an improved ET algorithm (Mu et al., 2011) to synthesize monthly data based on the calculated eight-day data.
3.2.4 MOD17A3H data product
The net primary productivity (NPP) is an important part of the carbon cycle on the Earth surface. It is also a characteristic index of the quality of terrestrial ecosystems (Murray et al., 2016). This study used the MOD17A3H data product (MODIS/Terra Net Primary Production) to analyze the spatio-temporal characteristics of the carbon allocation of ecosystems. The data is an annual synthetic product with an original spatial resolution of 500 m (unit: kg·C/m2). Compared with the MOD17A3, the MOD17A3H updates the Biome Property Look Up Tables, uses the meteorological data of the updated Global Modeling and Assimilation Office, and improves the spatial resolution from 1 km to 500 m (Running et al., 2015). The data passed the test conducted by the eddy covariance flux towers distributed in different climate zones and biozones (Malone et al., 2016; Turner et al., 2006; Cracknell et al., 2015). Therefore, this study no longer verified the accuracy of the MOD17A3H data product.

3.3 scPDSI data

This study selected the global monthly scPDSI data product to analyze the drought-wet changes of the study area and their influence on ecosystems during 2002 to 2013. The data is available from the rainfall and drought data set (http://www.cru.uea.ac.uk/) edited by the Climatic Research Unit in the University of East Anglia (UEA). Its spatial resolution is 50 km, and the data is dimensionless. The value range is [-5.0, 5.0]. Compared with the early PDSI, the scPDSI is more suitable for diverse climatic types, and has a greater advantage in detecting extremely dry or wet conditions (Wells et al., 2004). With reference to the scPDSI-based drought-wet grading criteria specified by Wang Zhaoli (2016), this study combined the drought-wet grades as needed and specified the following drought-wet grades: drought (scPDSI < -1), normal (-1.0 ≤ scPDSI < 1.0), and wet (1.0 ≤ scPDSI).

3.4 WUE

The WUE is an index used to reflect the water absorption and utilization efficiency of plants. It is also an important variable that connects the carbon cycle and the water cycle of the vegetation ecosystem (Niu et al., 2011). This study used the model proposed by Rosenberg et al. (1983) to calculate the WUE, thereby measuring the amount of organic carbon that can be sequestrated by evapotranspiring 1 kg water on a plant per unit area (gC/kg-1H2O). The WUE can be expressed as follows:
WUE = NPP/ET
where NPP refers to the net primary productivity, and ET refers to evapotranspiration.

4 Result analysis

4.1 scPDSI distribution characteristics of Northeast China

Figure 2a shows that drought frequently occurs in the regions stretching from 114.55°E to 120.90°E (i.e., at least nine times) (regions inside the red frame). In contrast, drought seldom occurs in the mountainous regions in the north and east and coastal regions in the south (i.e., zero to 3 years). According to Figure 2a combined with Figure 1, the regions with a high drought frequency are mostly grasslands, whereas those with a low drought frequency are mainly forests.
Figure 2b presents the percentage of the droughty forest/grassland areas to the total area of forests/grasslands. The changing trend in the percentage of the area of drought forests to the total area of forests was basically consistent with that in the percentage of the area of drought grasslands to the total area of grasslands. The droughty forest/grassland area of was very large in 2002 and 2008, but very small in 2004 and 2013. This finding was consistent with the analysis results of Figure 4a. Specifically, the droughty forest area of accounted for a small proportion, whereas the area of drought grasslands accounted for a large proportion during these years.
Figure 2 Spatial distribution of drought frequency (a) and the annual change in the percentage of drought regions (b) in Northeast China from 2002 to 2013
The droughty forest/grassland areas accounted for the largest proportion in 2008. Taking 2008 as an example, this study discussed the response of the WUE of forests and grasslands to drought in Northeast China.

4.2 Spatio-temporal characteristics of the WUE of natural vegetation

A great spatial variation was observed in the annual average WUE (in the range of 0.68 to 3.30, Figure 3a) of the natural vegetation in Northeast China. The annual average WUE was very high in the north of the Greater Khingan Mountains and east of the Changbai Mountains, where the main vegetation types were cold-temperate coniferous forests and temperate pine-broad-leaved mixed forests. The annual average WUE was very low in the central and western parts of Northeast China, where the main vegetation type is grasslands. Figure 3b shows the temporal characteristics of the annual average WUE. The interannual difference in the annual average WUE of the forests or grasslands was very small. The annual average WUE of the cold-temperate coniferous forests was in the range of 0.93 to 1.08, which was slightly higher than that of the temperate pine-broad-leaved mixed forests (0.85 to 0.97), warm-temperate deciduous broad-leaved forests (0.82 to 0.94), and grasslands (0.84 to 0.99). The standard deviation of the WUE of the cold-temperate coniferous or temperate pine-broad-leaved mixed forests was in the range of 0.17 to 0.33, which was smaller than that of the warm-temperate deciduous broad-leaved forests or grasslands (i.e., in the 0.36 to 0.67 range). The WUE of the cold-temperate coniferous forests was higher than that of the grasslands and other types of forests. Moreover, a small regional difference in their WUE was observed. The WUE of the temperate pine-broad-leaved mixed forests was a little lower than that of the cold-temperate coniferous forests. A small regional difference in their WUE was also observed. The WUE of the warm-temperate deciduous broad-leaved forests or grasslands was very low, and a large regional difference in their WUE was found.
Figure 3 Spatial distribution (a) and interannual changes (b) of the WUE of the natural vegetation in Northeast China from 2002 to 2013

4.3 Response of the WUE of the natural vegetation to drought

4.3.1 Changes in the WUE of the natural vegetation in drought conditions
This study provides a comparison between the average WUE in the drought areas in 2008 (1.08 for the cold-temperate coniferous forests, 0.95 for the temperate pine-broad-leaved mixed forests, 0.97 for the warm-temperate deciduous broad-leaved forests, and 1.10 for grasslands) and that in normal conditions in other years (the baseline value of the WUE is denoted as WUEBaseline; 0.99 for the cold-temperate coniferous forests; 0.86 for the temperate pine-broad-leaved mixed forests; 0.87 for the warm-temperate deciduous broad-leaved forests; and 0.86 for grasslands) (Figure 4 and Table 1). The regions with ∆WUE > 0 accounted for 86.11% of the total area of the natural vegetation. The warm-temperate deciduous broad-leaved forests with ∆WUE > 0 accounted for 97.16% of their total area. Meanwhile, the grasslands with ∆WUE > 0 accounted for 94.25% of their total area. The proportion of the grasslands with ∆WUE > 0 in the regions stretching from 114°E to 119° was higher than that in other regions. The cold-temperate coniferous forests with ∆WUE > 0 were mainly distributed in the north of the Greater Khingan Mountains. In contrast, the temperate pine-broad-leaved mixed forests with ∆WUE > 0 were very scattered and alternately distributed with the temperate pine-broad-leaved mixed forests with ∆WUE < 0 and ∆WUE = 0. Drought will evidently result in a little increase in the WUE of the natural vegetation in most parts of Northeast China.
Figure 4 Comparison of the average WUE between drought conditions and normal water conditions in 2008
This study extracted the baseline WUE values (WUEBaseline) at the interval of 0.1° longitude and 0.1° latitude, the scPDSI values in drought conditions in 2008 (scPDSI_2008), the WUE values in drought conditions in 2008 (WUEDrought), and the differences between WUE and WUEBaseline (∆WUE) according to Figures 4a and 4c. This study analyzed the correlation between different variables (Figure 5: “1”, “2”, “3”, and “4” stand for the cold-temperate coniferous forests, temperate pine-broad-leaved mixed forests, warm-temperate deciduous broad-leaved forests, and grasslands, respectively).
Table 1 Changes in the WUE of the natural vegetation in drought conditions in Northeast China in 2008
Vegetation type ∆WUE < 0 ∆WUE > 0 ∆WUE = 0
Cold-temperate coniferous forests (%) 4.56 80.00 15.44
Temperate pine-broad-leaved mixed forests (%) 6.13 73.95 19.93
Warm-temperate deciduous broad-leaved forests (%) 0.78 97.16 2.06
Grasslands (%) 1.09 94.25 4.66
Total (%) 3.10 86.11 10.79
Regarding the four vegetation types, a significant linear positive correlation was always found between WUEDrought and WUEBaseline (sig < 0.001) (Figures 5a2 to 5a4), but no significant correlation between WUEDrought and scPDSI_2008 (Figures 5b1 to 5b4) and between ∆WUE and scPDSI_2008 (Figures 5c1 to 5c4). This study analyzed the correlation between ∆WUE and WUEBaseline and found the following phenomena: 1) a rise in the WUE of all cold-temperate coniferous forests with WUEBaseline < 0.69 was found in drought conditions (Figure 5d1); 2) a rise in the WUE of most of the cold-temperate coniferous forests with WUEBaseline ≥ 0.69 and a decrease in the WUE of only a few of the cold-temperate coniferous forests with WUEBaseline ≥ 0.69 were observed; and 3) no obvious linear correlation existed between ∆WUE and WUEBaseline and between ∆WUE and DEM (Figure 5e1). Drought will result in an increase in the WUE of some of the temperate pine-broad-leaved mixed forests and warm-temperate deciduous broad-leaved forests. No obvious boundary line or changing trend was found (Figures 5d2 and 5d3). However, a significant positive correlation between the altitudes and the ∆WUE of the temperate pine-broad-leaved mixed forests was observed. A rise in the WUE of all temperate pine-broad-leaved mixed forests with DEM ≥ 964 can be seen in drought conditions (Figure 5e2). Meanwhile, the grasslands exhibited a significant linear negative correlation (sig < 0.001) between ∆WUE and WUEBaseline. In drought conditions, a rise in the WUE of all the grasslands with WUEBaseline < 0.61, an increase in the WUE of most of the grasslands with WUEBaseline ≥ 0.61, and a decline in the WUE of a few of grasslands were observed. A significant positive correlation was also found between the altitudes and the ∆WUE of grasslands. Furthermore, an increase in the WUE of all grasslands with DEM ≥ 1440 existed (Figure 5e4).
4.3.2 Influence of drought on ET, NPP, and LAI
This study analyzed the distribution of ET, NPP, and LAI in normal water conditions with regard to the vegetation with a rise or decline in the WUE in drought conditions in 2008 (Figure 4a) to further explore the reasons for the changes in the WUE (Figure 6 and Table 2). The average ET of the pixels of the cold-temperate coniferous forests with ∆WUE > 0 was 482.21. Their peak of the distribution frequency (percentage) was approximately at 460. Meanwhile, the average ET of the pixels of the cold-temperate coniferous forests with ∆WUE < 0 was 440.20, and their peak of distribution frequency (percentage) was approximately at 400. No significant difference was found in the average values between the two groups of data (sig < 0.001). The average NPP of the pixels of the cold-temperate coniferous forests with ∆WUE > 0 was 454.23, and their peak of distribution frequency (percentage) was approximately at 500. In addition, the average NPP of the pixels of the cold-temperate coniferous forests with ∆WUE < 0 was 520, and their peak of distribution frequency (percentage) was approximately at 420. A significant difference was found in the average values between the two groups of data (sig < 0.001). The average LAI of the pixels for the cold-temperate coniferous forests with ∆WUE > 0 was 1.47, and their peak of distribution frequency (percentage) was approximately at 1.5. Meanwhile, the average LAI of the pixels of the cold-temperate coniferous forests with ∆WUE < 0 was 1.51, and their peak of distribution frequency (percentage) was approximately at 1.65. No difference was found in the average values between the two groups of data (sig = 0.001). Evidently, the WUE for the vegetation with high ET and low NPP was more likely to rise in drought conditions, while the LAI had no significant influence on the WUE.
Figure 5 Changes in the WUE of the natural vegetation in drought conditions in 2008
The WUE for the temperate pine-broad-leaved mixed forests with high ET and NPP was more likely to rise in drought conditions, while the LAI had no significant influence on the WUE. The WUE for the grasslands with low ET, NPP, and LAI was more likely to rise in drought conditions. Meanwhile, the ET, NPP, and LAI had no significant influence on the WUE in drought conditions of the warm-temperate deciduous broad-leaved forests. The reason for changes in the WUE was yet to be further explored.
Table 2 Indices of the natural vegetation in Northeast China in normal water conditions
Vegetation type ∆WUE ET (mm/year) NPP (kgC/m2) LAI
Mean Comparison of
the means		 Mean Comparison of
the means		 Mean Comparison of the means
Cold-temperate coniferous
forests		 ∆WUE > 0 482.21 sig < 0.001 454.23 sig < 0.001 1.47 sig = 0.101
∆WUE < 0 440.20 520.00 1.51
Temperate pine-broad-leaved
mixed forests		 ∆WUE > 0 526.99 sig < 0.001 468.76 sig < 0.001 1.40 sig = 0.131
∆WUE < 0 508.27 438.31 1.43
Warm-temperate deciduous
broad-leaved forests		 ∆WUE > 0 419.41 sig = 0.253 360.45 sig = 0.061 0.63 sig = 0.726
∆WUE < 0 394.14 353.08 0.62
Grasslands ∆WUE > 0 329.19 sig = 0.032 280.30 sig < 0.001 0.81 sig < 0.001
∆WUE < 0 318.84 299.40 0.80

Note: An independent-sample T test was used to compare the average values.

5 Discussion

5.1 Overall WUE of Northeast China at below-average level

The photosynthesis capability varies among different types of vegetation; hence, a certain difference can be found in the WUE and the carbon use efficiency between them (Stella et al., 2009; Jin et al., 2017). The findings herein support the above-mentioned conclusion. The average WUE of the forests in Northeast China in normal water conditions was in the range of 0.82 to 1.08 gC/kg-1H2O, which was higher than that of the grasslands in the range of 0.84 to 0.99 gC/kg-1H2O. Table 3 describes the value range of the WUE and the CUE of the forests in other studies across the world. The WUE of the forests in Northeast China was at a relatively low level, while the WUE of the grasslands in Northeast China was at a medium level. The difference in the WUE may be caused by the different climate conditions and vegetation types between the study areas.
Figure 6 Distribution of ET, NPP, and LAI of natural vegetation in drought conditions. The solid line is the distribution curve of the frequency (percentage) of ET, NPP, and LAI in normal water conditions with regard to the pixels with ∆WUE ≥ 0 specified in Figure 5d. The dotted line is the distribution curve of the frequency (percentage) of the corresponding indices in normal conditions with regard to the pixels with ∆WUE < 0 specified in Figure 5d.
Table 3 Comparison of the WUE data
Ecosystem class Study area Value (gC/kg-1H2O) Data source
Forests Yangtze River Delta, China 1.68-1.95 Wang et al. (2015)
Changbaishan temperate broad-leaved Korean pine mixed forest, China 9.43 Yu et al. (2008)
Qianyanzhou subtropical coniferous plantation, China 9.27 Yu et al. (2008)
Dinghushan subtropical evergreen broad-leaved forest, China 6.90 Yu et al. (2008)
Forest ecosystems, Europe 1.2-5.0 Kuglitsch et al. (2008)
Urban-forest reserve, China 2.6 ± 0.2 Xie et al. (2016)
Alpine area of Southwest China 0.83-1.46 Zhang et al. (2016)
California, USA 1.33 Malone et al. (2016)
Northeast China 0.82-1.08 This study
Grasslands Yangtze River Delta, China 1.66 Wang et al. (2015)
Xinjiang, China 0.26-0.69 Huang and Luo (2017)
Qinghai-Tibet Plateau, China 0.62 Mi et al. (2015)
Alpine area of Southwest China 0.84-1.14 Zhang et al. (2016)
California, USA 1.73 Malone et al. (2016)
Northeast China 0.84-0.99 This study

5.2 WUE as index for reflecting the responsiveness of plants to drought

The response to drought varies among different vegetation types (Reichstein et al., 2002; Lu et al., 2010). Some researchers contended that drought will result in a significant uptrend in the WUE of natural vegetation (Malone et al., 2016; Campos et al., 2013; Guillermo et al., 2013). Other researchers contended that drought will result in a decline in the WUE of most vegetation (Edwards et al., 2012; Gang et al., 2016) or ecosystems (Reichstein et al., 2002; Lu et al., 2010) possibly because their WUE has a remarkable internal variability and plasticity (Monclus et al., 2006; Ponton et al., 2002) caused by gene difference. This study supports the first view that drought will result in a little rise in the WUE of most forests and grasslands. The rise in the WUE means that plants acquire a high productivity under water stress (Guillermo et al., 2013), except that the rise is not linear (e.g., a decline in the WUE of very few plants is observed). Drought has evidently disrupted the ranking of the WUE of the forests and grasslands in Northeast China in normal water conditions. This result was also corroborated by the study on the relationship between the ∆WUE and the corresponding WUE in normal water conditions: 1) no significant linear correlation was found between ∆WUE and the corresponding WUE for the forests in normal water conditions and 2) a significant linear negative correlation between ∆WUE and the corresponding WUE was observed for the grasslands in normal water conditions. Drought will result in a little rise in the WUE of the plants with a low WUE in the normal water conditions, but in a linear downtrend in the WUE of the plants with a high WUE.

5.3 Deficiencies in the study

This study sensitively found the phenomenon that drought will result in a rise in the WUE of most vegetation in Northeast China, indicating that the forests and the grasslands in that area are resistant to drought. First, when this study used MODIS data products to study the natural vegetation status of the mountainous regions in the east of Northeast China, the spatial resolution (500 m) was indeed very low, thereby significantly affecting the accuracy. Second, the researchers had not accumulated sufficient historical data on ground-based verification and lacked sufficient experience in the ground-based verification of the MODIS data products. Third, this study did not definitely distinguish C3 plants from C4 plants (C4 plants have a great advantage over C3 plants in terms of high temperature resistance (Wilson et al., 2007; Killi et al., 2017)) and ignored the changes in the interior species of forests and grasslands. In addition, this study only considered the annual changes in the water conditions, and did not distinguish the occurrence time of drought. In the subsequent studies, the remote sensing data with a high spatial resolution must be selected, or a mode that allows the integration of high-resolution data and MODIS data products must be developed. Accumulating sufficient ground survey data and developing an effective data verification method are also necessary. The intent is to improve the spatial representativeness and accuracy of basic remote sensing data and make the research conclusions closer to the actual ones.

6 Conclusions

This work analyzed the spatio-temporal evolution law of productivity of natural vegetation in Northeast China based on the rainfall and scPDSI data from 2001 to 2013, land cover types, and the LAI, ET, and NPP data products specified by the MODIS. This work also studied the influence of drought events on the productivity of different vegetation types and their response to drought events. The research findings can be summarized as follows:
(1) In Northeast China, the regions with a high drought frequency are mostly grasslands, whereas the regions with a low drought frequency are mostly forests. Drought occurred on a large scale in 2002 and 2008, but on a small scale in 2004 and 2013.
(2) The annual average WUE of the natural vegetation ranged from 0.82 to 1.08 C/kg-1H2O. The WUE of the forests (0.82 to 1.08 C/kg-1H2O) was universally higher than that of the grasslands (0.84 to 0.99 C/kg-1H2O).
(3) In 2008, the regions with a rise in the WUE in drought conditions accounted for 86.11% of the study area. The warm-temperate deciduous broad-leaved forests with a rise in the WUE accounted for 97.16% of their total area. The grasslands with a rise in the WUE accounted for 94.25% of their total area.
(4) A significant linear positive correlation was found between the WUE in the drought conditions and the WUE in the normal water conditions. The degree of drought did not influence the WUE of the natural vegetation in an obviously linear manner. An increase in the WUE of all cold-temperate coniferous forests with WUE < 0.69 and all grasslands with WUE < 0.61 in the normal water conditions was observed in the drought conditions. An increase in the WUE of all temperate pine-broad-leaved mixed forests with DEM ≥ 964 and all grasslands with DEM ≥ 1447 was also observed in the drought conditions.
(5) The WUE of the cold-temperate coniferous forests and the temperate pine-broad- leaved mixed forests with a high ET or low NPP was more likely to rise in the drought conditions. The WUE for the grasslands with low ET, NPP, and LAI was also more likely to rise in the drought conditions. Meanwhile, the ET, NPP, and LAI of the warm-temperate deciduous broad-leaved forests had no significant influence on the WUE in the drought conditions, and the main reason for the changes in the WUE has yet to be further explored.

The authors have declared that no competing interests exist.

References

Publishing order | Descend order by publishing year | Descend order by cited within
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Aulia M R, Setiawan Y, Fatikhunnada A, 2016. Drought detection of west Java’s paddy field using MODIS EVI satellite images (Case Study: Rancaekek and Rancaekek Wetan).Procedia Environmental Sciences, 33: 646-653.Nowadays, drought phenomenon occurred in several area in Indonesia. The length of the dry season, especially in the southern equatorial allegedly caused by El Nino phenomenon. This causes crop failures in many center area of agriculture. West Java Province as one of the centers of agricultural activities in Indonesia experienced a severe drought within a period of 6 months (April-September in 2003). The monitoring of drought is useful to understanding the characterization of drought itself. In the next period, we can decide what should we do to decreased the impact of this phenomenon. The study aimed to implement Breaks for Additive Seasonal and Trend (BFAST) algorithm for detecting and monitoring paddy field areas experiencing drought in The West Java during the period 2000-2015. This study used remote sensing data to study the response of vegetation on the drought phenomenon. MODIS EVI time series were decomposed into seasonal, trend, and remainder component using BFAST which enables the detection of trend changes within the time series. The result of study shows that BFAST able to detect drought in MODIS EVI time series. The result also compared to a new drought index, called SPEI.

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Chakroun H, Mouillot F, Nasr Zet al., 2014. Performance of LAI-MODIS and the influence on drought simulation in a Mediterranean forest.Ecohydrology, 7(3): 1014-1028.ABSTRACTThis study investigates the benefits and methodological issues to integrate weekly 165km Leaf Area Index (LAI) Moderate Resolution Imaging Spectroradiometer (LAI-MODIS) (MOD15A2) satellite product in a distributed water budget model over a Mediterranean forested ecosystem of about 255365km2 for drought assessment at regional scale. The high overestimation of LAI-MODIS compared to field measurements was corrected based on the calibration of high-resolution Satellites Pour l'Observation de la Terre (SPOT) images combined with ground measurements. The LAI-MODIS time series over the 2003 to 2009 period has been calibrated and integrated into a spatially explicit water budget model at the regional level using spatial information on vegetation, soil types and topoclimates. Actual LAI-MODIS scenario was tested against temporal and spatial null models to assess for the benefits of the regional heterogeneity from MODIS and its intra-annual and interannual variation in a water budget model. From water budget analyses at local and regional scales, we concluded that calibration of LAI-MODIS images was mandatory to enhance the correlation coefficient between measured and simulated daily actual transpiration. We estimated daily bias in the resulting ‘Water Stress Index’ to be slightly affected between constant and seasonally-varying LAI datasets. However, the interannual variability in LAI detected by MODIS followed LAI adjustments expected from the ecohydrological equilibrium hypothesis. This can significantly affect the simulated annual drought period features, so that integrating LAI-MODIS spatial and temporal variability into water budget models for evergreen Mediterranean vegetation can be a useful dataset when carbon allocation schemes in dynamic vegetation models are lacking, but should be carefully calibrated particularly in transitions towards semi-arid zones. Copyright 08 2013 John Wiley & Sons, Ltd.

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Cracknell A P, Kanniah K D, Tan K P , et al. 2015. Towards the development of a regional version of MOD17 for the determination of gross and net primary productivity of oil palm trees.International Journal of Remote Sensing, 36(1): 262-289.Conducting quantitative studies on the carbon balance or productivity of oil palm is important for understanding the role of this ecosystem in global climate change. The MOD17 algorithm is used for processing data from the Moderate Resolution Imaging Spectroradiometer (MODIS) to generate the values of gross primary productivity (GPP) and net primary productivity for input to global carbon cycle modelling. In view of the increasing importance of data on carbon sequestration at regional and national levels, we have studied one important factor affecting the accuracy of the implementation of MOD17 at the sub-global level, namely the database of MODIS land cover (MOD12Q1) used by MOD17. By using a study area of approximately 702km02×02702km (49 MODIS pixels) in semi-rural Johor in Peninsular Malaysia and using Google Earth 0.7502m resolution images as ground data, we found that the land-cover type for only 16 of these 49 MODIS pixels was correctly identified by MOD12Q1 using its 102km resolution land-cover database. This leads to errors of 24% to 50% in the maximum light use efficiency, leading to corresponding errors of 24% to 50% in the GPP. We show that by using the Finer Resolution Observation and Monitoring – Global Land Cover (FROM-GLC) land-cover database developed by Gong et al., this particular error can be essentially eliminated, but at the cost of using extra computing resources.

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[6]
da Silva J R, Patterson A E, Rodrigues W P , et al. 2017. Photosynthetic acclimation to elevated CO2 combined with partial rootzone drying results in improved water use efficiency, drought tolerance and leaf carbon balance of grapevines (Vitis labrusca). Environmental and Experimental Botany, 134: 82-95.We considered the interactive effects of elevated CO 2 concentration [CO 2 ] and reduced water availability (Partial Rootzone Drying – PRD and drought) on a variety of important physiological and growth traits in grapevine ( Vitis labrusca ). The following questions were addressed: (i) Will there be a down-regulation of photosynthesis at elevated [CO 2 ] and what role do leaf nitrogen (N) concentration and specific leaf weight (SLW) have in this process? (ii) What are the effects of PRD under elevated [CO 2 ]? (iii) Can elevated [CO 2 ] delay the negative effects of drought in grapevines? and (iv) What is the impact of leaf respiration in the light ( R light ) and in the dark ( R dark ) on leaf carbon balance (LCB) of grapevines? Three water management treatments ( WMT ) were applied [full-irrigated – both sides of the rootzone were irrigated to saturation; PRD – only one side of the rootzone was irrigated to saturation; and non-irrigated – irrigation was suspended on both sides of the rootzone] in each of two 1.402m 2 growth chambers, each one with different [CO 2 ] (40002ppm or 80002ppm). We found that: (i) net photosynthetic rate ( A net ) is down-regulated under elevated [CO 2 ], which may be caused by reductions in Rubisco content and/or total activity and by a reduction in the efficiency of the photochemical apparatus due to N dilution within leaves with higher SLW; (ii) Under elevated [CO 2 ], Rubisco carboxylation rate is increased when PRD is applied, leading to increased A net , thereby leading to greater water use efficiency ( WUE ) and intrinsic WUE ( iWUE ). However, these responses were not linked to photochemical or stomatal effects. (iii) Elevated [CO 2 ] delayed drought effects on both A net and Rubisco activity for four days, by reducing stomatal conductance, transpiration and stomatal density; (iv) Leaf respiration responses depend on drought level; and (v) Conditions that reduce N concentration, such as elevated [CO 2 ], also reduce R dark and R light . In addition, elevated [CO 2 ] intensifies light inhibition of respiration and reduces Rubisco oxygenation, as well as improves LCB and contributes for mitigating deleterious effects of drought on LCB in grapevines.

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[7]
Edwards C E, Ewers B E, McClung C Ret al., 2012. Quantitative variation in water-use efficiency across water regimes and its relationship with circadian, vegetative, reproductive, and leaf gas-exchange traits.Molecular Plant, 5(3): 653-668.Drought limits light harvesting,resulting in lower plant growth and reproduction.One trait important for plant drought response is water-use efficiency (WUE).We investigated (1) how the joint genetic architecture of WUE,reproductive characters,and vegetative traits changed across drought and well-watered conditions,(2) whether traits with distinct developmental bases (e.g.leaf gas exchange versus reproduction) differed in the environmental sensitivity of their genetic architecture,and (3) whether quantitative variation in circadian period was related to drought response in Brassica rapa.Overall,WUE increased in drought,primarily because stomatal conductance,and thus water loss,declined more than carbon fixation.Genotypes with the highest WUE in drought expressed the lowest WUE in well-watered conditions,and had the largest vegetative and floral organs in both treatments.Thus,large changes in WUE enabled some genotypes to approach vegetative and reproductive trait optima across environments.The genetic architecture differed for gas-exchange and vegetative traits across drought and well-watered conditions,but not for floral traits.Correlations between circadian and leaf gas-exchange traits were significant but did not vary across treatments,indicating that circadian period affects physiological function regardless of water availability.These results suggest that WUE is important for drought tolerance in Brassica rapa and that artificial selection for increased WUE in drought will not result in maladaptive expression of other traits that are correlated with WUE.

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[8]
Esmaeilpour A, Van L M C, Samson Ret al., 2016. Variation in biochemical characteristics, water status, stomata features, leaf carbon isotope composition and its relationship to water use efficiency in pistachio (Pistacia vera L.) cultivars under drought stress condition. Scientia Horticulturae, 211: 158-166.Pistachio (P. veraL., Anacardiaceae) is cultivated in regions where soil water deficits and salinity conditions are higher than usual. Despite adult pistachio trees having been documented as being drought tolerant; there is only a limited understanding of the physiological mechanisms pistachio cultivars use to survive drought. We therefore, carried out a greenhouse experiment to evaluate the effects of three osmotic drought stress treatments including; control conditions (610.1MPa), moderate (610.75MPa) and severe drought (611.5MPa) stress, using PEG 6000 for 14days with a subsequent two weeks recovery period. Carbohydrate contents, relative water content, water use efficiency, stomatal characteristics, and nitrogen and carbon isotope composition were evaluated in three Iranian pistachio cultivars, i.e. Akbari, Kaleghochi and Ohadi. Results revealed that the drought stress treatments induced osmotic adjustment by carbohydrate accumulation. Both drought stress treatments increased soluble carbohydrate and starch contents of the leaves. Relative water content was only affected by drought stress in Kaleghochi. Stomatal density and morphology varied with pistachio cultivars but was hardly affected by the stress treatments. Drought stress significantly increased the overall mean of water use efficiency (intrinsic and instantaneous WUE). There were no significant differences between the leaf carbon isotope compositions of all pistachio cultivars under stress. This indicates that this relation may not be used to determine pistachio cultivars with appropriate WUE via leaf carbon isotope composition within the time frame of the experiment. Leaf nitrogen isotope composition decreased under drought stress regardless off the cultivar.

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[9]
Friedl M A, Sulla-Menashe D, Tan B , et al. 2010. MODIS Collection 5 global land cover: Algorithm refinements and characterization of new datasets.Remote sensing of Environment, 114(1): 168-182.Information related to land cover is immensely important to global change science. In the past decade, data sources and methodologies for creating global land cover maps from remote sensing have evolved rapidly. Here we describe the datasets and algorithms used to create the Collection 5 MODIS Global Land Cover Type product, which is substantially changed relative to Collection 4. In addition to using updated input data, the algorithm and ancillary datasets used to produce the product have been refined. Most importantly, the Collection 5 product is generated at 500-m spatial resolution, providing a four-fold increase in spatial resolution relative to the previous version. In addition, many components of the classification algorithm have been changed. The training site database has been revised, land surface temperature is now included as an input feature, and ancillary datasets used in post-processing of ensemble decision tree results have been updated. Further, methods used to correct classifier results for bias imposed by training data properties have been refined, techniques used to fuse ancillary data based on spatially varying prior probabilities have been revised, and a variety of methods have been developed to address limitations of the algorithm for the urban, wetland, and deciduous needleleaf classes. Finally, techniques used to stabilize classification results across years have been developed and implemented to reduce year-to-year variation in land cover labels not associated with land cover change. Results from a cross-validation analysis indicate that the overall accuracy of the product is about 75% correctly classified, but that the range in class-specific accuracies is large. Comparison of Collection 5 maps with Collection 4 results show substantial differences arising from increased spatial resolution and changes in the input data and classification algorithm.

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[10]
Gang C, Wang Z, Chen Y , et al. 2016. Drought-induced dynamics of carbon and water use efficiency of global grasslands from 2000 to 2011.Ecological Indicators, 67: 788-797.Drought is frequently recorded as a result of climate warming and elevated concentration of greenhouse gases, which affect the carbon and water cycles in terrestrial ecosystems, particularly in arid and semi-arid regions. To identify the drought in grassland ecosystems and to determine how such drought affects grassland ecosystems in terms of carbon and water cycles across the globe, this study evaluated the drought conditions of global grassland ecosystems from 2000 to 2011 on the basis of the remotely sensed Drought Severity Index (DSI) data. The temporal dynamics of grassland carbon use efficiency (CUE) and water use efficiency (WUE), as well as their correlations with DSI, were also investigated at the global scale. Results showed that 57.04% of grassland ecosystems experienced a dry trend over this period. In general, most grassland ecosystems in the northern hemisphere (N.H.) were in near normal condition, whereas those in the southern hemisphere (S.H.) experienced a clear drying and wetting trend, with the year 2005 regarded as the turning point. Grassland CUE increased continually despite the varied drought conditions over this period. By contrast, WUE increased in the closed shrublands and woody savannas but decreased in all the other grassland types. The drought conditions affected the carbon and water use mainly by influencing the primary production and evapotranspiration of grass through photosynthesis and transpiration process. The CUE and WUE of savannas was most sensitive to droughts among all the grassland types. The areas of grassland DSI that showed significant correlations with CUE and WUE were 52.92% and 22.11% of the total grassland areas, respectively. Overall, droughts sufficiently explained the dynamics of grassland CUE, especially in the S.H. In comparison with grassland CUE, the grassland WUE was less sensitive to drought conditions at the global scale.

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[11]
Hasibeder R, Fuchslueger L, Richter A , et al. 2015. Summer drought alters carbon allocation to roots and root respiration in mountain grassland.New Phytologist, 205(3): 1117-1127.Drought affects the carbon (C) source and sink activities of plant organs, with potential consequences for belowground C allocation, a key process of the terrestrial C cycle. The responses of belowground C allocation dynamics to drought are so far poorly understood. We combined experimental rain exclusion with (13)C pulse labelling in a mountain meadow to analyse the effects of summer drought on the dynamics of belowground allocation of recently assimilated C and how it is partitioned among different carbohydrate pools and root respiration. Severe soil moisture deficit decreased the ecosystem C uptake and the amounts and velocity of C allocated from shoots to roots. However, the proportion of recently assimilated C translocated belowground remained unaffected by drought. Reduced root respiration, reflecting reduced C demand under drought, was increasingly sustained by C reserves, whilst recent assimilates were preferentially allocated to root storage and an enlarged pool of osmotically active compounds. Our results indicate that under drought conditions the usage of recent photosynthates is shifted from metabolic activity to osmotic adjustment and storage compounds.

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[12]
Hernandez S V, Rodriguez D C M, Fernandez L J E , et al. 2016. Role of leaf hydraulic conductance in the regulation of stomatal conductance in almond and olive in response to water stress.Tree Physiology, 36(6): 725-735.Abstract The decrease of stomatal conductance (g s ) is one of the prime responses to water shortage and the main determinant of yield limitation in fruit trees. Understanding the mechanisms related to stomatal closure in response to imposed water stress is crucial for correct irrigation management. The loss of leaf hydraulic functioning is considered as one of the major factors triggering stomatal closure. Thus, we conducted an experiment to quantify the dehydration response of leaf hydraulic conductance (K leaf ) and its impact on g s in two Mediterranean fruit tree species, one deciduous (almond) and one evergreen (olive). Our hypothesis was that a higher K leaf would be associated with a higher g s and that the reduction in K leaf would predict the reduction in g s in both species. We measured K leaf in olive and almond during a cycle of irrigation withholding. We also compared the results of two methods to measure K leaf : dynamic rehydration kinetics and evaporative flux methods. In addition, determined g s , leaf water potential (Ψ leaf ), vein density, photosynthetic capacity and turgor loss point. Results showed that g s was higher in almond than in olive and so was K leaf (K max 61=614.70 and 3.4261mmol61s -1 61MPa -1 61m -2 , in almond and olive, respectively) for Ψ leaf 61>61-1.261MPa. At greater water stress levels than -1.261MPa, however, K leaf decreased exponentially, being similar for both species, while g s was still higher in almond than in olive. We conclude that although the K leaf decrease with increasing water stress does not drive unequivocally the g s response to water stress, K leaf is the variable most strongly related to the g s response to water stress, especially in olive. Other variables such as the increase in abscisic acid (ABA) may be playing an important role in g s regulation, although in our study the g s -ABA relationship did not show a clear pattern. 08 The Author 2016. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.

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[13]
Hobbins M T, Ramirez J A, Brown T C, 2001. The complementary relationship in estimation of regional evapotranspiration: An enhanced advection-aridity model.Water Resources Research, 37(5): 1367-1387.Long-term monthly evapotranspiration estimates from Brutsaert and Stricker Advection-Aridity model were compared with independent estimates of evapotranspiration derived from long-term water balances for 139 undisturbed basins across the conterminous United States. On an average annual basis for the period 1962-1988 the original model, which uses a Penman wind function, underestimated evapotranspiration by 7.9% of precipitation compared with the water balance estimates. Model accuracy increased with basin humidity. An improved formulation of the model is presented in which the wind function and the Priestley-Taylor coefficient are modified. The wind function was reparameterized on a seasonal, regional basis to replicate independent proxy potential evapotranspiration surfaces. This led to significant differences from the original Penman wind function. The reparameterized wind function, together with a recalibrated Priestley-Taylor coefficient in the wet environment evapotranspiration formulation, reduced the underestimation of annual average evapotranspiration to only 1.15% of precipitation on an independent set of validation basins. The results offered here lend further support for Bouchet hypothesis as it applies to large-scale, long-term evapotranspiration.

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[14]
Hoover D L, Duniway M C, Belnap J, 2017. Testing the apparent resistance of three dominant plants to chronic drought on the Colorado Plateau.Journal of Ecology, 105(1): 152-162.Many drylands, including the south-western United States, are projected to become more water-limited as these regions become warmer and drier with climate change. Such chronic drought may push individual species or plant functional types beyond key thresholds leading to reduced growth or even mortality. Indeed, recent observational and experimental evidence from the Colorado Plateau suggests that C3 grasses are the most vulnerable to chronic drought, while C4 grasses and C3 shrubs appear to have greater resistance.The effects of chronic, or press-drought are predicted to begin at the physiological level and translate up to higher hierarchical levels. To date, the drought resistance of C4grasses and C3 shrubs in this region has been only evaluated at the community level and thus we lack information on whether there are sensitivities to drought at lower hierarchical levels. In this study, we tested the apparent drought resistance of three dominant species (Pleuraphis jamesii, a C4 rhizomatous grass; Coleogyne ramosissima, a C3 drought-deciduous shrub; and Ephedra viridis, a C3 evergreen shrub) to an ongoing experimental press-drought (-35% precipitation) by comparing individual-level responses (ecophysiology and growth dynamics) to community-level responses (plant cover).For all three species, we observed consistent responses across all hierarchical levels:P. jamesii was sensitive to drought across all measured variables, while the shrubsC. ramosissima and E. viridis had little to no responses to the experimental press-drought at any given level.Synthesis. Our findings suggest that the apparent drought resistance at higher hierarchical levels, such as cover, may serve as good proxies for lower-level responses. Furthermore, it appears the shrubs are avoiding drought, possibly by utilizing moisture at deeper soil layers, while the grasses are limited to shallower layers and must endure the drought conditions. Give this differential sensitivity to drought, a future with less precipitation and higher temperatures may increase the dominance of shrubs on the Colorado Plateau, as grasses succumb to chronic water stress.

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[15]
Houghton J T, 1996.Climate Change 1995: The Science of Climate Change: Contribution of Working Group I to the Second Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University Press.

[16]
Huang X T, Luo G P, 2017. Spatio-temporal characteristics of evapotranspiration and water use efficiency in grasslands of Xinjiang.Chinese Journal of Plant Ecology, 41(5): 506-518. (in Chinese)Aims Xinjiang is located in the hinterland of the Eurasian arid areas, with grasslands widely distributed. Grasslands in Xinjiang provide significant economic and ecological benefits. However, research on evapotranspiration(ET) and water use efficiency(WUE) of the grasslands is still relatively weak. This study aimed to explore the spatio-temporal characteristics on ET and WUE in the grasslands of Xinjiang in the context of climate change. Methods The Biome-BGC model was used to determine the spatio-temporal characteristics of ET and WUE of the grasslands over the period 1979–2012 across different seasons, areas and grassland types in Xinjiang. Important findings The average annual ET in the grasslands of Xinjiang was estimated at 245.7 mm, with interannual variations generally consistent with that of precipitation. Overall, the value of ET was lower than that of precipitation. The higher values of ET mainly distributed in the Tianshan Mountains, Altai Mountains, Altun Mountains and the low mountain areas on the northern slope of Kunlun Mountains. The lower values of ETmainly distributed in the highland areas of Kunlun Mountains and the desert plains. Over the period 1979–2012, average annual ET was 183.2 mm in the grasslands of southern Xinjiang, 357.9 mm in the grasslands of the Tianshan Mountains, and 221.3 mm in grasslands of northern Xinjiang. In winter, ET in grasslands of northern Xinjiang was slightly higher than that of Tianshan Mountains. Average annual ET ranked among grassland types as: mid-mountain meadow swamp meadow typical grassland desert grassland alpine meadow saline meadow. The highest ET value occurred in summer, and the lowest ET value occurred in winter, with ET in spring being slightly higher than that in autumn. The higher WUE values mainly distributed in the areas of Tianshan Mountains and Altai Mountains. The lower WUE values mainly distributed in the highland areas of Kunlun Mountains and part of the desert plains. The average annual WUE in the grasslands of Xinjiang was 0.56 g·kg~(-1), with the seasonal values of 0.43 g·kg~(-1) in spring, 0.60 g·kg~(-1) in summer, and 0.48 g·kg~(-1) in autumn, respectively. Over the period 1979–2012, the values of WUE displayed significant regional differences: the average values were 0.73 g·kg~(-1) in northern Xinjiang, 0.26 g·kg~(-1) in southern Xinjiang, and 0.69 g·kg~(-1) in Tianshan Mountains. There were also significant differences in WUE among grassland types. The values of WUE ranked in the order of mid-mountain meadow typical grassland swamp meadow saline meadow alpine meadow desert grassland.

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[17]
Iio A, Hikosaka K, Anten N P Ret al., 2014. Global dependence of field-observed leaf area index in woody species on climate: A systematic review.Global Ecology and Biogeography, 23(3): 274-285.AimLeaf area index (LAI) is one of the key variables related to carbon, water and nutrient cycles in terrestrial ecosystems, but its global distribution patterns remain poorly understood. We evaluated the dependence of LAI on mean annual temperature (MAT) and wetness index (WI; a ratio of annual precipitation to potential evapotranspiration) for three plant functional types (PFTs: deciduous broadleaf, DB; evergreen conifer, EC; evergreen broadleaf, EB) at the global scale.LocationGlobal.MethodsWe developed a new global database of unprecedented size (2606 published values) of field-observed LAI (site-specific maximum) values for vegetation of woody species. To maximize the generic applicability of our analysis, we standardized the definition of LAI, and corrected or excluded potentially erroneous data obtained from indirect optical methods.ResultsThe global dependence of LAI on MAT showed a reverse S-shaped pattern, in which LAI peaked at around 8.9 and 25.0°C and was lowest at around 6110.0 and 18.8°C. The dependence on WI followed a saturation curve levelling off at around log WI65=650.30. LAI for evergreen forests increased linearly with increasing WI, but that for DB showed a curvilinear pattern saturating at log WI65=650.03. EC forests had higher LAI values than those of DB forests under cool conditions (MAT65≤658.9°C), but similar values under temperate conditions (MAT65=658.9–18.8°C).Main conclusionsThis analysis of global LAI61climate relationships supports the general belief that temperature limits LAI under cool conditions whereas water availability plays a predominant role under other conditions. We also found that these patterns differed significantly between PFTs, suggesting that the LAI of different PFTs may respond differently to climate change. Our study provides a broad empirical basis for predicting the global distribution of LAI and for analysing the effects of global climate change on vegetation structure and function.

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[18]
Jin J X, Zhan W F, Wang Y , et al. 2017. Water use efficiency in response to interannual variations in flux-based photosynthetic onset in temperate deciduous broadleaf forests.Ecological Indicators, 79: 122-127.Climate change has significantly influenced the productivity of terrestrial ecosystems through water cycles. Water use efficiency (WUE) is an important indicator for understanding how water couples with the carbon cycle. Abiotic factors such as climatic factors and CO 2 concentration have been investigated to understand the mechanisms involved in the coupled carbon-water cycle of terrestrial ecosystems in response to climate change. However, the effects of biotic factors on WUE are less clear. By analyzing 66 site-years of flux and meteorological data obtained from 8 temperate deciduous broadleaf forest sites across North America and Europe, we found that ecosystem-scale WUE (defined here as the ratio of gross primary production (GPP) to evapotranspiration (ET)) in the spring significantly increased with the advance of the flux-based photosynthetic onset (FPO), mainly because an earlier FPO could lead to a steeper increase in GPP than in ET. However, the advance of FPO probably reduced summer WUE as a result of the enhancement of water stress by ET in the spring in temperate deciduous broadleaf forest. Our results also implied that spring warming had an indirectly positive effect on WUE through advancing spring phenology, but such a positive effect will likely weaken once the sensitivity of spring phenology to warming decreases as reported. Here, we argue that phenology, which exerts critical biotic control over most ecological processes, plays a larger role than expected in the regulation of the seasonal WUE and cannot be ignored in earth system models.

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[19]
Joo E, Hussain M Z, Zeri M , et al. 2016. The influence of drought and heat stress on long-term carbon fluxes of bioenergy crops grown in the Midwestern USA. Plant, Cell & Environment, 39(9): 1928-1940.Abstract Perennial grasses are promising feedstocks for bioenergy production in the Midwestern US. Few experiments have addressed how drought influences their carbon fluxes and storage. This study provides a direct comparison of ecosystem-scale measurements of carbon fluxes associated with miscanthus (Miscanthus-iganteus), switchgrass (Panicum virgatum), restored native prairie and maize (Zea mays) / soybean (Glycine max) ecosystems. The main objective of this study was to assess the influence of a naturally occurring drought during 2012 on key components of the carbon cycle and plant development relative to non-extreme years. The perennials reached full maturity 3-5 years after establishment. Miscanthus had the highest GPP and lowest NEE in 2012 followed by similar values for switchgrass and prairie, and the row crops had the lowest GPP and highest NEE. A post-drought effect was observed for miscanthus. Over the duration of the experiment, perennial ecosystems were carbon sinks, as indicated by negative NECB, while maize/soybean was a net carbon source. Our observations suggest that perennial ecosystems, and in particular miscanthus, can provide high yield and large potential for CO 2 fixation even during drought, although drought may negatively influence carbon uptake in the following year, questioning the long-term consequence of its maintained productivity. This article is protected by copyright. All rights reserved.

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[20]
Killi D, Bussotti F, Raschi A , et al. 2017. Adaptation to high temperature mitigates the impact of water deficit during combined heat and drought stress in C3 sunflower and C4 maize varieties with contrasting drought tolerance.Physiologia Plantarum, 159(2): 130-147.Abstract Heat and drought stress frequently occur together, however their impact on plant growth and photosynthesis (P N ) is unclear. The frequency, duration and severity of heat and drought stress events are predicted to increase in the future, having severe implications for agricultural productivity and food security. To assess these impacts on plant gas exchange, physiology and morphology we grew drought tolerant and sensitive varieties of C3 sunflower (Helianthus annuus) and C4 maize (Zea mays) under conditions of elevated temperature for four weeks prior to the imposition of water deficit. The negative impact of temperature on P N was most apparent in sunflower. The drought tolerant sunflower retained ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) activity under heat stress to a greater extent than its drought sensitive counterpart. Maize exhibited no varietal difference in response to increased temperature. In contrast to previous studies, where a sudden rise in temperature induced an increase in stomatal conductance (G s ), we observed no change or a reduction in G s with elevated temperature, which alongside lower leaf area mitigated the impact of drought at the higher temperature. The drought tolerant sunflower and maize varieties exhibited greater investment in root-systems, allowing greater uptake of the available soil water. Elevated temperatures associated with heat-waves will have profound negative impacts on crop growth in both sunflower and maize, but the deleterious effect on P N was less apparent in the drought tolerant sunflower and both maize varieties. As C4 plants generally exhibit water use efficiency (WUE) and resistance to heat stress, selection on the basis of tolerance to heat and drought stress would be more beneficial to the yields of C3 crops cultivated in drought prone semi-arid regions. This article is protected by copyright. All rights reserved.

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[21]
Kuglitsch F G, Reichstein M, Beer C , et al. 2008. Characterisation of ecosystem water-use efficiency of European forests from eddy covariance measurements.Biogeosciences Discussions, 5(6): 4481-4519.Water-use efficiency (WUE) has been recognized as an important characteristic of vegetation productivity in various natural scientific disciplines for decades, but only recently at the ecosystem level, where different ways exist to characterize water-use efficiency. Hence, the objective of this research was (a) to systematically compare different ways of calculating ecosystem water-use efficiency (WUEe) from eddy-covariance measurements, (b) quantify the diurnal, seasonal and interannual variability of WUEe in relation to meteorological conditions, and (c) analyse between-site variability of WUEe as affected by vegetation type and climatic conditions, across sites in European forest ecosystems. Day-to-day variability of gross primary productivity (GPP) and evapotranspiration (ET) were more strongly coupled than net ecosystem production (NEP) and ET, obviously because NEP also depends on the respiration that is not heavily coupled to water fluxes. However, the slope of daytime NEP versus ET (mNEP) from half-hourly measurements of a single day may also be used as a WUEe-estimate giving very similar results to those of the GPP-ET slope (mGPP), since the diurnal variation is dominated by GPP. Since ET is the sum of transpiration (linked to GPP) and evaporation from wet vegetation and soil surfaces (not linked to GPP) we expected that WUEe is increasing when days after rain are excluded from the analysis. However only very minor changes were found, justifying an analysis of WUEe related to vegetation type. In most of the studied ecosystems the instantaneous WUEGPP was quite sensitive to diurnally varying meteorological conditions and tended to decline from the morning to the afternoon by more than 50% because of increasing vapour pressure deficits (VPD). Seasonally, WUEGPP increased with a rising monthly precipitation sum and rising average monthly temperatures up to a threshold of 11, 14 and 18C in boreal, temperate and Mediterranean ecosystems, respectively. Across all sites, the highest monthly WUEGPP-values were detected at times of positive anomalies of summer-precipitation. During drought periods with high temperatures, high VPD, little precipitation and low soil water content, the water-use efficiency of gross carbon uptake (WUEGPP) tended to decrease in all forest types because of a stronger decline of GPP compared to ET. However the largest variation of growing season WUEGPP was found betweensites and significantly related to vegetation type: WUEGPP was highest in ecosystems dominated by deciduous trees ranging from 5.0 gCO2 kgH2O 1 for temperate broadleaved deciduous forests (TD), to 4.5 for temperate mixed forests (TM), 3.5 for temperate evergreen conifers (TC), 3.4 for Mediterranean broad-leaved deciduous forests (MD), 3.3 for Mediterranean broad-leaved evergreen forests (Mbeg), 3.1 for Mediterranean evergreen conifers (MC), 2.9 for boreal evergreen conifers (BC) and only 1.2 g CO2 kgH2O 1 for a boreal wetland site (BT). Although vegetation type and meteorology co-vary, the WUEGPP variation was hardly related to meteorology, as we could show by comparing similar meteorological conditions only. Furthermore we compared across-site WUEGPP only under conditions when the 10% high GPP rates were exhibited. The between site differences remained, and at all sites ecosystem reached higher WUEGPP levels under this condition. This means when vegetation is most productive usually it also maximises the amount of carbon gained per water lost. Overall our results show that water-use efficiency exhibits a strong time-scale dependency in the sense that at longer time-scale meteorological conditions play a smaller role compared to shorter time scale. Moreover, we highlight the role of vegetation in determining carbon-water relation at ecosystem level. Consequently, all predictions of changing carbon-water cycle under changing climate should take into this role and the differences between vegetation types. These results show the strong time-scale dependency of water-use efficiency

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[22]
Ledger M E, Edwards F K, Brown L E , et al. 2011. Impact of simulated drought on ecosystem biomass production: An experimental test in stream mesocosms.Global Change Biology, 17(7): 2288-2297.Abstract Climate models predict widespread shifts in precipitation patterns and increases in the frequency of extreme events such as droughts, but consequences for key processes in affected ecosystems remains poorly understood. A 2-year manipulative experiment used a series of stream mesocosms to test the effect of recurrent drought disturbance on the composition and secondary production of macroinvertebrate consumer assemblages and functional groups. On average, secondary production in drought-disturbed communities (mean 4.5gm 612 yr 611 ) was less than half of that that in controls (mean 10.4gm 612 yr 611 ). The effects of the drought differed among functional feeding groups, with substantial declines for detritivore shredders (by 69%) and engulfing predators (by 94%). Contrasting responses were evident among taxa within most functional feeding groups, ranging from extirpation to irruptions in the case of several small midge larvae, but production of most species was suppressed. Taxon-specific responses were related to body mass and voltinism. The ratio of production to biomass (community P/B) increased under drought, reflecting a shift in production from large long-lived taxa to smaller taxa with faster life cycles. This research provides some of the first experimental evidence of the profound effects that droughts can have on both the structure and functioning of aquatic ecosystems.

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[23]
Li W, Zhao Y S, Zhou Z Q , et al. 2012. Effects of drought stress and rehydration on chlorophy II fluorescence characteristics and antioxidant enzyme activities in leaves ofTaxus cuspidate. Journal of Desert Research, 32(1): 112-116. (in Chinese)Three-year-old Taxus cuspidata seed seedling and cutting seedling were selected as the experimental material.The aims of this study were to explore the response and ecological adaptability of Taxus cuspidata seed seedling and cutting seedling to drought stress by investigating change of chlorophyll fluorescence characteristics and antioxidant enzyme activities.Result showed that minimum chlorophyll florescence(Fo) and non-photochemical quenching(NPQ) increased under the course of natural drought,Fo in cutting seedling increased faster than in seed seedling,and NPQ in seed seedling increased faster than in cutting seedling.After 15 days of drought stress,the cutting seedling showed significant decrease in maximum quantum efficiency of photosystemⅡphotochemistry(Fv/Fm),effective PSⅡquantum yield(ФPSⅡ) and apparent electron transport rate(ETR),compared with seed seedling.However,the alterations of seed seedling and cutting seedling could recover after rehydration.Under the drought stress,antioxidant enzyme activities of SOD,CAT and POD of seed seedling and cutting seedling increased.This indicated that the seed seedling had more drought adaptability than cutting seedling.

[24]
Limousin J M, Yepez E A, McDowell N G , et al. 2015. Convergence in resource use efficiency across trees with differing hydraulic strategies in response to ecosystem precipitation manipulation.Functional Ecology, 29(9): 1125-1136.Summary Plants are expected to respond to drought by maximizing the efficiency of the most limiting resource, the water use efficiency (WUE), at the expense of nitrogen and carbon use efficiencies (NUE and CUE). Therefore, plants resource use efficiencies are viewed as indicators of species drought tolerance. We tested these predictions by measuring leaf-level intrinsic WUE (WUEi, the ratio of net assimilation to stomatal conductance), photosynthetic NUE (PNUE, the ratio of daily maximum net assimilation to leaf nitrogen content) and leaf-scale CUE (approached by the ratio of night-time respiration to daytime net assimilation, R d /A n) in pi on pine and juniper, two tree species that differ in drought tolerance and vulnerability to drought-induced mortality. Variations in resource use efficiency in the two species were measured in response to seasonal drought and in response to an ecosystem-scale precipitation manipulation experiment comprising three precipitation treatments: ambient, irrigation (+30%) and partial rainfall exclusion ( 45%). Increasing water limitation, either seasonally or across treatments, resulted in increased WUE and decreased PNUE and CUE in both species. WUE, PNUE and CUE varied more strongly in response to water limitation than across species and converged to the same relationships against precipitation for pi on and juniper. Plasticity in WUE, PNUE and CUE in response to water limitation was associated, in both species, with low carbon acquisition during drought. Our results exhibited a convergence in resource use efficiency across pi on and juniper which contradicts the paradigm that resource use efficiencies are indicators of species drought tolerance and ecological strategy.

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[25]
Lu H J, Mo X G, Meng D J , et al. 2016. Analyzing spatiotemporal patterns of meteorological drought and its responses to climate change across Northeast China.Scientia Geographica Sinica, 35(8): 1051-1059. (in Chinese)In order to capture the spatiotemporal patterns of drought vulnerability and predict its response characteristics to climate change in Northeast China(NEC), observed precipitation data sets of 98 meteorological stations in 1961-2010 and output datasets of the WCRP coupled multi-model(CMIP3) for IPCC SRES A1 B,A2 and B1 climate change scenarios during 2011-2060 were engaged to calculate the commonly used Standardized Precipitation Index, which analyzed with the popular methods of Mann-Kendall detection, Empirical Orthogonal Function and Wavelet transformation. The main results can be concluded as following. Historical drought-wetness evolution in NEC could be properly detected by the SPI series, and drought severity in most areas significantly increased during last 50 years especially on the 12-month timescale. While drought conditions in southern part and Liao River basin in NEC are always most serious, the first four EOF loadings on different time steps show a similar distribution pattern with some local differences. The drought spatial coverage took on an obviously increasing trend accompanied with drastic inter-annual and inter-decadal fluctuations,and the dominant period were differently identified for the south(11a) and the north(3.5 a). Under the three climate change scenarios, the period of the early 30 years would share most of the mega droughts during2011-2060, and the A2 scenario may own more frequently and severe drought events than others; more importantly, the drought core may migrate northward to some extent.

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[26]
Lu X, Zhuang Q, 2010. Evaluating evapotranspiration and water use efficiency of terrestrial ecosystems in the conterminous United States using MODIS and America Flux data.Remote Sensing of Environment, 114: 1924-1939.In this study, we used the remotely-sensed data from the Moderate Resolution Imaging Spectrometer (MODIS), meteorological and eddy flux data and an artificial neural networks (ANNs) technique to develop a daily evapotranspiration (ET) product for the period of 2004-2005 for the conterminous U.S. We then estimated and analyzed the regional water-use efficiency (WUE) based on the developed ET and MODIS gross primary production (GPP) for the region. We first trained the ANNs to predict evapotranspiration fraction (EF) based on the data at 28 AmeriFlux sites between 2003 and 2005. Five remotely-sensed variables including land surface temperature (LST), normalized difference vegetation index (NDVI), normalized difference water index (NDWI), leaf area index (LAI) and photosynthetically active radiation (PAR) and ground-measured air temperature and wind velocity were used. The daily ET was calculated by multiplying net radiation flux derived from remote sensing products with EF. We then evaluated the model performance by comparing modeled ET with the data at 24 AmeriFlux sites in 2006. We found that the ANNs predicted daily ET well ( R 2 = 0.52-0.86). The ANNs were applied to predict the spatial and temporal distributions of daily ET for the conterminous U.S. in 2004 and 2005. The ecosystem WUE for the conterminous U.S. from 2004 to 2005 was calculated using MODIS GPP products (MOD17) and the estimated ET. We found that all ecosystems' WUE-drought relationships showed a two-stage pattern. Specifically, WUE increased when the intensity of drought was moderate; WUE tended to decrease under severe drought. These findings are consistent with the observations that WUE does not monotonously increase in response to water stress. Our study suggests a new water-use efficiency mechanism should be considered in ecosystem modeling. In addition, this study provides a high spatial and temporal resolution ET dataset, an important product for climate change and hydrological cycling studies for the MODIS era.

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[27]
Ma J Y, Xu Y L, 2013. Temporal and spatial characteristics and circulation background of drought in crop growing season over Northeast China.Chinese Journal of Agrometeorology, 34(1): 81-87. (in Chinese)Based on observed data of mean temperature and precipitation from 71 meteorological stations in Northeast China from 1961 to 2009 in crop growing season(from May to September),authors validated Palmer Drought Severity Index(PDSI) to assess the actual drought level,and analyzed its characteristic by using Empirical Orthogonal Function(EOF) method,used the NCEP/NCAR reanalysis data to discuss the circulation background in drought years.The results showed that,(1)PDSI could assess the actual drought level with high ability in crop growing season,but it was easily to exaggerate the seriousness of drought in some years;(2)The accumulative variance contribution ratio of the first three feature vectors reached to 74.9%,and the spatial types of drought in Northeast China could be divided to three patterns:uniform,south-north difference,east-west difference,corresponding with the first three feature vectors individually;(3)During 1961 to 2009,the drought variation mainly existed 9 years and 23 years periodic oscillations in crop growing season,and the drought trend in the next few years might continue to increase;(4)In drought years(PDSI-1),the subtropical high presented weak performance in 500hPa height field,Northeast China was the center of positive geo-potential height anomaly,indicating that the region was controlled by high pressure,resulting in less rainfall.In terms of 850hPa water vapor flux field,warm-wet airflows from southwest and southeast were hardly transported to the Northeast China,and the anti-cyclonic circulation and positive divergence also could lead to less precipitation.

[28]
Malone S L, Tulbure M G, Perez-Luque A J , et al. 2016. Drought resistance across California ecosystems: Evaluating changes in carbon dynamics using satellite imagery.Ecosphere, 7(11): 1-19.Drought is a global issue that is exacerbated by climate change and increasing anthropogenic water demands. The recent occurrence of drought in California provides an important opportunity to examine drought response across ecosystem classes (forests, shrublands, grasslands, and wetlands), which is essential to understand how climate influences ecosystem structure and function. We quantified ecosystem resistance to drought by comparing changes in satellite-derived estimates of water-use efficiency (WUE02=02net primary productivity [NPP]/evapotranspiration [ET]) under normal (i.e., baseline) and drought conditions (ΔWUE02=02WUE2014026102baseline WUE). With this method, areas with increasing WUE under drought conditions are considered more resilient than systems with declining WUE. Baseline WUE varied across California (0.08 to 3.8502g02C/mm H2O) and WUE generally increased under severe drought conditions in 2014. Strong correlations between ΔWUE, precipitation, and leaf area index (LAI) indicate that ecosystems with a lower average LAI (i.e., grasslands) also had greater C-uptake rates when water was limiting and higher rates of carbon-uptake efficiency (CUE02=02NPP/LAI) under drought conditions. We also found that systems with a baseline WUE02≤020.4 exhibited a decline in WUE under drought conditions, suggesting that a baseline WUE02≤020.4 might be indicative of low drought resistance. Drought severity, precipitation, and WUE were identified as important drivers of shifts in ecosystem classes over the study period. These findings have important implications for understanding climate change effects on primary productivity and C sequestration across ecosystems and how this may influence ecosystem resistance in the future.

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[29]
Mi Z R, Chen L T, Zhang Z Het al., 2015. Alpine grassland water use efficiency based on annual precipitation, growing season precipitation and growing season evapotranspiration.Chinese Journal of Plant Ecology, 39(7): 649-660. (in Chinese)Aims Water use efficiency(WUE) is an important parameter to understand the coupling between the water,and carbon cycles of terrestrial ecosystems.Previous studies on the grassland ecosystem WUE on the Qinghai-Xizang Plateau mainly based on annual precipitation(AP).However,vegetation water use mainly occurs in growing season.Therefore,we aimed to explore the differences of ecosystem WUE between alpine meadow and alpine steppe,and the relationships between ecosystem WUE and environmental factors from 2000 to 2010,using annual precipitation use efficiency(PUEa),growing season precipitation use efficiency(PUEgs),growing season water use efficiency(WUEgs) based on AP,growing season precipitation(GSP) and growing season evapotranspiration(ETgs) respectively.Methods Combining satellite-derived above-ground net primary productivity(ANPP),satellite-derived evapotranspiration and meteorological data from 2000 to 2010,we calculated PUEa(ANPP / AP),PUEgs(ANPP / GSP) and WUEgs(ANPP / ETgs) to find the differences of PUEa,PUEgs and WUEgs between alpine meadow and alpine steppe.Moreover,we explored the relationships between PUEa,PUEgs or WUEgs and precipitation(or evapotranspiration) or air temperature.Important findings We found that(1) the PUEa and PUEgs of alpine meadow were higher than that of alpinesteppe,but there were no significant difference between WUEgs of the two grassland types,indicating that there may be similar intrinsic water use efficiencies of the two grassland types.(2) The inter-annual variation of PUEa and PUEgs were similar while WUEgs showed a larger fluctuation,implying that ET-based WUEgs was more sensitive than precipitation-based PUEa and PUEgs,therefore WUEgs is a better indicator of ecosystem water use efficiency than PUEa or PUEgs.(3) The PUEa,PUEgs and WUEgs were negatively correlated with AP,GSP and ETgs respectively,reflecting a consistency of the three water use efficiency measurements.In the alpine steppe,only WUEgs was observed positively correlated with air temperature among the three measurements,but in the alpine meadow,no significant relationships between water use efficiency and air temperature was detected,suggesting that the WUEgs of alpine steppe was more sensitive to air temperature than that of alpine meadow.

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[30]
Monclus R, Dreyer E, Villar M , et al. 2006. Impact of drought on productivity and water use efficiency in 29 genotypes of Populus deltoides×Populus nigra. New Phytologist, 169(4): 765-777.

[31]
Mu Q, Zhao M, Running S W, 2011. Improvements to a MODIS global terrestrial evapotranspiration algorithm.Remote Sensing of Environment, 115(8): 1781-1800.78 Improving the MODIS ET algorithm (Mu et al., 2007a, old algorithm). 78 Global terrestrial annual total ET (62.8 × 10 3 km 3) agrees with reported 65.5 × 10 3 km 3. 78 MAE of 24.6% and 24.1% are in the 10–30% range of the accuracy of ET measurements.

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[32]
Murray T G, Friedlingstein P, Sitch S , et al. 2016. The dry season intensity as a key driver of NPP trends.Geophysical Research Letters, 43(6): 2632-2639.We analyze the impacts of changing dry season length and intensity on vegetation productivity and biomass. Our results show a wetness asymmetry in dry ecosystems, with dry seasons becoming drier and wet seasons becoming wetter, likely caused by climate change. The increasingly intense dry seasons were consistently correlated with a decreasing trend in net primary productivity (NPP) and biomass from different products and could potentially mean a reduction of 10-13% in NPP by 2100. We found that annual NPP in dry ecosystems is particularly sensitive to the intensity of the dry season, whereas an increase in precipitation during the wet season has a smaller effect. We conclude that changes in water availability over the dry season affect vegetation throughout the whole year, driving changes in regional NPP. Moreover, these results suggest that usage of seasonal water fluxes is necessary to improve our understanding of the link between water availability and the land carbon cycle.

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[33]
Mutiibwa D, Irmak S, 2013. AVHRR-NDVI-based crop coefficients for analyzing long-term trends in evapotranspiration in relation to changing climate in the US High Plains.Water Resources Research, 49(1): 231-244.Studies in regions of extensive irrigation practices have revealed a significant influence of evaporative cooling on regional temperatures as a result of surface energy redistribution during evaporation. In the U.S. High Plains, maximum temperatures during the last quarter of the 20th century have been decreasing. We investigated the trends in evapotranspiration (ET or latent heat) fluxes originating from increasing irrigation practices in the High Plains region from 1981 to 2008. We estimated actual ET (ETc) over the entire High Plains from the spatial crop coefficients (Kc) and spatial reference (potential) ET (ETref). We proposed and validated a global linear relation between Kc and advanced very high resolution radiometer-based normalized difference vegetation index. Our results show an increase in ETc trends over the region in the last three decades. The study shows that the increase in ETc flux was not in principal from increased atmospheric evaporative demand. Rather, the increase in ETc was due to significant increase in irrigated surfaces. The increase in ETc fluxes is likely a manifestation of increased redistribution of surface energy into latent heat and less partitioning into the sensible heat. We investigated the evolution of full canopy cover vegetation (normalized difference vegetation index >0.70) in relation to the maximum temperature anomalies during the study period. Results revealed a significant negative correlation between the two variables. These results appear to demonstrate that there is a regional evaporative cooling signal due to extensive irrigation practices, which impacts regional temperatures during the summer seasons.

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[34]
Myneni R, 2015. MOD15A2H MODIS/Terra Leaf Area Index/FPAR 8-Day L4 Global 500 m SIN Grid V006. NASA EOSDIS Land Processes DAAC. .

[35]
Niu S, Xing X, Zhang Z H E et al. , 2011. Water-use efficiency in response to climate change: From leaf to ecosystem in a temperate steppe.Global Change Biology, 17(2): 1073-1082.Water-use efficiency (WUE) has been recognized as an important characteristic of ecosystem productivity, which links carbon (C) and water cycling. However, little is known about how WUE responds to climate change at different scales. Here, we investigated WUE at leaf, canopy, and ecosystem levels under increased precipitation and warming from 2005 to 2008 in a temperate steppe in Northern China. We measured gross ecosystem productivity (GEP), net ecosystem CO2 exchange (NEE), evapotranspiration (ET), evaporation (E), canopy transpiration (Tc), as well as leaf photosynthesis (Pmax) and transpiration (Tl) of a dominant species to calculate canopy WUE (WUEc=GEP/T), ecosystem WUE (WUEgep=GEP/ET or WUEnee=NEE/ET) and leaf WUE (WUEl=Pmax/Tl). The results showed that increased precipitation stimulated WUEc, WUEgep and WUEnee by 17.1%, 10.2% and 12.6%, respectively, but decreased WUEl by 27.4%. Climate warming reduced canopy and ecosystem WUE over the 4 years but did not affect leaf level WUE. Across the 4 years and the measured plots, canopy and ecosystem WUE linearly increased, but leaf level WUE of the dominant species linearly decreased with increasing precipitation. The differential responses of canopy/ecosystem WUE and leaf WUE to climate change suggest that caution should be taken when upscaling WUE from leaf to larger scales. Our findings will also facilitate mechanistic understanding of the C-ater relationships across different organism levels and in projecting the effects of climate warming and shifting precipitation regimes on productivity in arid and semiarid ecosystems.

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[36]
PaiMazumder D, Done J M, 2016. Potential predictability sources of the 2012 US drought in observations and a regional model ensemble.Journal of Geophysical Research: Atmospheres, 121(21): 12581-12592.The 2012 drought was the most severe and extensive summertime U.S. drought in half a century with substantial economic loss and impacts on food security and commodity prices. A unique aspect of the 2012 drought was its rapid onset and intensification over the Southern Rockies, extending to the Great Plains during late spring and early summer, and the absence of known precursor large-scale patterns. Drought prediction therefore remains a major challenge. This study evaluates relationships among snow, soil moisture, and precipitation to identify sources of potential predictability of the 2012 summer drought using observations and a Weather Research and Forecasting model multiphysics ensemble experiment. Although underestimated in intensity, the drought signal is robust to the way atmospheric physical processes are represented in the model. For the Southern Rockies, soil moisture exhibits stronger persistence than precipitation in observations and the ensemble experiment. Correlations between winter/spring snowmelt and concurrent and following season soil moisture, and between soil moisture and concurrent and following season precipitation, in both observations and the model ensemble, suggest potential predictability beyond 1 and 2 month lead-time reside in the land surface conditions for apparent flash droughts such as the 2012 drought.

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[37]
Ponton S, Dupouey J L, Bréda N , et al. 2002. Comparison of water-use efficiency of seedlings from two sympatric oak species: Genotype × environment interactions. Tree Physiology, 22(6): 413-422.Abstract Seedlings of two sympatric oak species, Quercus robur L. and Quercus petraea (Matt.) Liebl., were grown in common garden conditions to test for potential interspecific differences in intrinsic water-use efficiency (WUE). Intrinsic water-use efficiency was estimated based on carbon isotope composition of shoots (delta13C) and on gas exchange measurements (ratio of net CO2 assimilation rate to stomatal conductance (A/g(sw))). In addition, genotype x environment interactions were tested by subjecting the seedlings to four irradiance treatments (8, 18, 48 and 100% of incident solar irradiance) imposed by neutral shading nets, and, in the 100% irradiance treatment, two watering regimes. In all treatments, initial growth of Q. robur was faster than that of Q. petraea. In both species, there was a tight correlation between delta13C and A/g(sw). Intrinsic water-use efficiency increased with increasing irradiance (almost doubling from 8 to 100% irradiance), and this effect paralleled the increase in A with increasing irradiance. In full sun, WUE of Q. petraea seedlings was 10-15% higher than in Q. robur seedlings, with the difference attributable to a difference between the species in g(sw). The interspecific difference in WUE was maintained during drought, despite the appreciable increase in WUE and decrease in growth imposed by drought. No interspecific differences in WUE were observed at low irradiances, suggesting a strong genotype x environment interaction for WUE. These findings confirm the existence of interspecific genetic differences in WUE, but also show that there is large intraspecific variability and plasticity in WUE. The initially greater height and biomass increments in Q. robur seedlings illustrate the ability of this species to out-compete Q. petraea in the early stages of forest regeneration. For adult trees growing in closed canopies, the high WUE of Q. petraea may contribute significantly to its survival during dry years, whereas the low WUE of Q. robur may account for the frequently observed declines in adult trees of this species following drought.

DOI PMID

[38]
Raich J W, Potter C S, Bhagawati D, 2002. Interannual variability in global soil respiration, 1980-94.Global Change Biology, 8(8): 800-812.We used a climate-driven regression model to develop spatially resolved estimates of soil-CO 2 emissions from the terrestrial land surface for each month from January 1980 to December 1994, to evaluate the effects of interannual variations in climate on global soil-to-atmosphere CO 2 fluxes. The mean annual global soil-CO 2 flux over this 15-y period was estimated to be 80.4 (range 79.3–81.8) Pg C. Monthly variations in global soil-CO 2 emissions followed closely the mean temperature cycle of the Northern Hemisphere. Globally, soil-CO 2 emissions reached their minima in February and peaked in July and August. Tropical and subtropical evergreen broad-leaved forests contributed more soil-derived CO 2 to the atmosphere than did any other vegetation type (6530% of the total) and exhibited a biannual cycle in their emissions. Soil-CO 2 emissions in other biomes exhibited a single annual cycle that paralleled the seasonal temperature cycle. Interannual variability in estimated global soil-CO 2 production is substantially less than is variability in net carbon uptake by plants (i.e., net primary productivity). Thus, soils appear to buffer atmospheric CO 2 concentrations against far more dramatic seasonal and interannual differences in plant growth. Within seasonally dry biomes (savannas, bushlands and deserts), interannual variability in soil-CO 2 emissions correlated significantly with interannual differences in precipitation. At the global scale, however, annual soil-CO 2 fluxes correlated with mean annual temperature, with a slope of 3.3 Pg C y 611 per °C. Although the distribution of precipitation influences seasonal and spatial patterns of soil-CO 2 emissions, global warming is likely to stimulate CO 2 emissions from soils.

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[39]
Reichstein M, Tenhunen J D, Roupsard O , et al. 2002. Severe drought effects on ecosystem CO2 and H2O fluxes at three Mediterranean evergreen sites: Revision of current hypotheses?Global Change Biology, 8: 999-1017.Eddy covariance and sapflow data from three Mediterranean ecosystems were analysed via top-down approaches in conjunction with a mechanistic ecosystem gas-exchange model to test current assumptions about drought effects on ecosystem respiration and canopy CO2/H2O exchange. The three sites include two nearly monospecific Quercus ilex L. forests - one on karstic limestone (Puechabon), the other on fluvial sand with access to ground water (Castelporziano) - and a typical mixed macchia on limestone (Arca di Noe). Estimates of ecosystem respiration were derived from light response curves of net ecosystem CO2 exchange. Subsequently, values of ecosystem gross carbon uptake were computed from eddy covariance CO2 fluxes and estimates of ecosystem respiration as a function of soil temperature and moisture. Bulk canopy conductance was calculated by inversion of the Penman-Monteith equation. In a top-down analysis, it was shown that all three sites exhibit similar behaviour in terms of their overall response to drought. In contrast to common assumptions, at all sites ecosystem respiration revealed a decreasing temperature sensitivity (Q(10)) in response to drought. Soil temperature and soil water content explained 70-80% of the seasonal variability of ecosystem respiration. During the drought, light-saturated ecosystem gross carbon uptake and day-time averaged canopy conductance declined by up to 90%. These changes were closely related to soil water content. Ecosystem water-use efficiency of gross carbon uptake decreased during the drought, regardless whether evapotranspiration from eddy covariance or transpiration from sapflow had been used for the calculation. We evidence that this clearly contrasts current models of canopy function which predict increasing ecosystem water-use efficiency (WUE) during the drought. Four potential explanations to those results were identified (patchy stomatal closure, changes in physiological capacities of photosynthesis, decreases in mesophyll

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[40]
Rosenberg N J, Blad B L, Verma S B, 1983.Microclimate: The Biological Environment. New York: John Wiley & Sons Press.

[41]
Ruehr N K, Law B E, Quandt D , et al. 2014. Effects of heat and drought on carbon and water dynamics in a regenerating semi-arid pine forest: A combined experimental and modeling approach.Biogeosciences, 11(15): 4139-4156.Increasing summer temperatures and a reduction in precipitation will enhance drought stress in Mediterranean and semi-arid ecosystems. Predicting the net effects on forests' carbon and water balance will depend on our ability to disentangle the sensitivity of component fluxes responding to increasing soil and atmospheric drought. Here we studied carbon and water dynamics in a semi-arid regenerating ponderosa pine forest using field observations and process based modeling. Field observations of two summer dry seasons were used to calibrate a soil-plant-atmosphere (SPA) model. In addition, the ecosystem's response to reduced soil drought was quantified based on a field watering experiment and evaluated with the model. Further, the SPA model was used to estimate the relative effects of increasing soil and atmospheric drought over time, by simulating temperature and precipitation scenarios for 2040 and 2080. The seasonality and drought response of ecosystem fluxes was well captured by the calibrated SPA model. Dramatic increases in summer water availability during seasonal drought had a small effect on pine physiology in both the watering experiment and the model. This clearly demonstrates that atmospheric drought induced a strong limitation on carbon uptake in young ponderosa pine due to tight regulation of stomatal conductance. Moreover, simulations showed that net ecosystem exchange (NEE) and gross primary productivity (GPP) were about three times more affected by summer heat and increased evaporative demand than by reductions in summer precipitation. Annual NEE decreased by 38% in response to extreme summer conditions as predicted to occur in 2080 (June-August: +4.5 °C), because of a strong decline in GPP (-17%) while heterotrophic respiration was relatively unaffected (-1%). Considering warming trends across all seasons (September-May: +3 °C and June-August: +4.5 °C), the negative drought effects were largely compensated by an earlier initiation of favorable growing conditions and bud break, enhancing early season GPP and needle biomass. An adverse effect, triggered by changes in early season allocation patterns, was the decline of wood and root biomass. This imbalance may increase water stress over the long-term to a threshold at which ponderosa pine may not survive, and highlights the need for an integrated process understanding of the combined effects of trends and extremes.

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[42]
Running S W, Mu Q, Zhao M, 2015. MOD17A3H MODIS/Terra Net Primary Production Yearly L4 Global 500m SIN Grid V006. NASA EOSDIS Land Processes DAAC. https://doi.org/10.5067/MODIS/MOD17A3H.006.

[43]
Sawada Y, Koike T, Jaranilla-Sanchez P A, 2014. Modeling hydrologic and ecologic responses using a new eco-hydrological model for identification of droughts.Water Resources Research, 50(7): 6214-6235.Abstract Drought severely damages water and agricultural resources, and both hydrological and ecological responses are important for its understanding. First, precipitation deficit induces soil moisture deficiency and high plant water stress causing agricultural droughts. Second, hydrological drought characterized by deficit of river discharge and groundwater follows agricultural drought. However, contributions of vegetation dynamics to these processes at basin scale have not been quantified. To address this issue, we develop an eco-hydrological model that can calculate river discharge, groundwater, energy flux, and vegetation dynamics as diagnostic variables at basin scale within a distributed hydrological modeling framework. The model is applied to drought analysis in the Medjerda River basin. From model inputs and outputs, we calculate drought indices for different drought types. The model shows reliable accuracy in reproducing observed river discharge in long-term (19 year) simulation. Moreover, the drought index calculated from the model-estimated annual peak of leaf area index correlates well (correlation coefficient r-=-0.89) with the drought index from nationwide annual crop production, which demonstrates that the modeled leaf area index is capable of representing agricultural droughts related to historical food shortages. We show that vegetation dynamics have a more rapid response to meteorological droughts than river discharge and groundwater dynamics in the Medjerda basin because vegetation dynamics are sensitive to soil moisture in surface layers, whereas soil moisture in deeper layers strongly contributes to streamflow and groundwater level. Our modeling framework can contribute to analyze drought progress, although analyses for other climate conditions are needed.

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[44]
Schoo B, Wittich K P, Bottcher Uet al., 2017. Drought tolerance and water-use efficiency of biogas crops: A comparison of cup plant, maize and lucerne-grass.Journal of Agronomy and Crop Science, 203(2): 117-130.The cup plant (L.) is discussed as an alternative energy crop for biogas production in Germany due to its ecological benefits over continuously grown maize. Moreover, a certain drought tolerance is assumed because of its intensive root growth and the dew water collection by the leaf cups, formed by fused leaf pairs. Therefore, the aim of this study was to estimate evapotranspiration (ET), water‐use efficiency (WUE) and the relevance of the leaf cups for the cup plant's water balance in a 2‐year field experiment. Parallel investigations were conducted for the two reference crops maize (high WUE) and lucerne‐grass (deep and intensive rooting) under rainfed and irrigated conditions. Root system performance was assessed by measuring water depletion at various soil depths. Transpiration‐use efficiency (TUE) was estimated using a model approach. Averaged over the 202years, drought‐related above‐ground dry matter reduction was higher for the cup plant (3302%) than for the maize (1802%) and lucerne‐grass (1402%). The WUE of the cup plant (3302kg ha02mm) was significantly lower than for maize (5002kg ha02mm). The cup plant had a lower water uptake capacity than lucerne‐grass. Cup plant dry matter yields as high as those of maize will only be attainable at sites that are well supplied with water, be it through a large soil water reserve, groundwater connection, high rainfall or supplemental irrigation.

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[45]
Shiogama H, Watanabe M, Imada Y , et al. 2013. An event attribution of the 2010 drought in the South Amazon region using the MIROC5 model.Atmospheric Science Letters, 14(3): 170-175.Abstract Top of page Abstract 1.Introduction 2.Simulations 3.Results 4.Summary and discussion Acknowledgements References We produced 100-member event attribution ensembles during 2009-2012 under all forcing conditions and in two different counterfactual worlds without anthropogenic forcing (mainly greenhouse gases and aerosols) and without aerosol emission changes using the MIROC5 atmospheric general circulation model. It seemed that both human influences and the sea surface temperature (SST) natural variability increased probabilities of the 2010 severe drought in the South Amazon region, and that changes in aerosols emissions had little effect on the drought. It should be noted that our assessments were sensitive to bias corrections according to the relationships between the SST natural variability and precipitation.

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[46]
Stella P, Lamaud E, Brunet Y , et al. 2009. Simultaneous measurements of CO2 and water exchanges over three agroecosystems in south-west France.Biogeosciences, 6(12): 2957-2971.

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[47]
Turner D P, Ritts W D, Cohen W B , et al. 2006. Evaluation of MODIS NPP and GPP products across multiple biomes.Remote Sensing of Environment, 102(3): 282-292.Estimates of daily gross primary production (GPP) and annual net primary production (NPP) at the 1 km spatial resolution are now produced operationally for the global terrestrial surface using imagery from the MODIS (Moderate Resolution Imaging Spectroradiometer) sensor. Ecosystem-level measurements of GPP at eddy covariance flux towers and plot-level measurements of NPP over the surrounding landscape offer opportunities for validating the MODIS NPP and GPP products, but these flux measurements must be scaled over areas on the order of 25 km 2 to make effective comparisons to the MODIS products. Here, we report results for such comparisons at 9 sites varying widely in biome type and land use. The sites included arctic tundra, boreal forest, temperate hardwood forest, temperate conifer forest, tropical rain forest, tallgrass prairie, desert grassland, and cropland. The ground-based NPP and GPP surfaces were generated by application of the Biome-BGC carbon cycle process model in a spatially-distributed mode. Model inputs of land cover and leaf area index were derived from Landsat data. The MODIS NPP and GPP products showed no overall bias. They tended to be overestimates at low productivity sites - often because of artificially high values of MODIS FPAR (fraction of photosynthetically active radiation absorbed by the canopy), a critical input to the MODIS GPP algorithm. In contrast, the MODIS products tended to be underestimates in high productivity sites - often a function of relatively low values for vegetation light use efficiency in the MODIS GPP algorithm. A global network of sites where both NPP and GPP are measured and scaled over the local landscape is needed to more comprehensively validate the MODIS NPP and GPP products and to potentially calibrate the MODIS NPP/GPP algorithm parameters.

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[48]
Vanlerberghe G C, Martyn G D, Dahal K, 2016. Alternative oxidase: A respiratory electron transport chain pathway essential for maintaining photosynthetic performance during drought stress.Physiologia Plantarum, 157(3): 322-337.Abstract Photosynthesis and respiration are the hubs of energy metabolism in plants. Drought strongly perturbs photosynthesis as a result of both diffusive limitations resulting from stomatal closure, and in some cases biochemical limitations that are associated with a reduced abundance of key photosynthetic components. The effects of drought on respiration, particularly respiration in the light (RL), are less understood. The plant mitochondrial electron transport chain includes a non-energy conserving terminal oxidase called alternative oxidase (AOX). Several studies have shown that drought increases AOX transcript, protein and maximum capacity. Here we review recent studies comparing wild-type (WT) tobacco to transgenic lines with altered AOX protein amount. Specifically during drought, RL was compromised in AOX knockdown plants and enhanced in AOX overexpression plants, compared to WT. Significantly, these differences in RL were accompanied by dramatic differences in photosynthetic performance. Knockdown of AOX increased the susceptibility of photosynthesis to drought-induced biochemical limitations, while overexpression of AOX delayed the development of such biochemical limitations, compared to WT. Overall, the results indicate that AOX is essential to maintaining RL during drought, and that this non-energy conserving respiration maintains photosynthesis during drought by promoting energy balance in the chloroplast. This review also outlines several areas for future research; including the possibility that enhancement of non-energy conserving respiratory electron sinks may be a useful biotechnological approach to increase plant performance during stress. This article is protected by copyright. All rights reserved.

DOI PMID

[49]
Wang F, Jiang H, Zhang X, 2015. Spatial-temporal dynamics of gross primary productivity, evapotranspiration, and water-use efficiency in the terrestrial ecosystems of the Yangtze River Delta region and their relations to climatic variables.International Journal of Remote Sensing, 36(10): 2654-2673.Moderate Resolution Imaging Spectroradiometer (MODIS) products and climate data collected from meteorological stations were used to characterize the spatial–temporal dynamics of gross primary productivity (GPP), evapotranspiration (ET), and water-use efficiency (WUE) in the Yangtze River Delta (YRD) region and the response of these three variables to meteorological factors. The seasonal patterns of GPP and WUE showed a bimodal distribution, with their peak values occurring in May and August, and April and October, respectively. By contrast, the seasonal variation of ET presented a unimodal pattern with its maximum in July or August. The spatial distribution of ET and GPP was similar to higher values occurring in the south. From 2001 to 2012, GPP in the eastern YRD decreased, while GPP in the western part increased. In comparison, over the 1202years, ET in the northern part of YRD decreased, while ET in the southern part increased. The spatial distribution and spatial variation of WUE were both similar to those of GPP. This implies that the changes in WUE are primarily controlled by the variations in GPP. The annual average WUE over vegetation types followed the order of: evergreen broadleaf forest (1.9502g02C02kg611 H2O) > deciduous broadleaf forest (1.8702g02C02kg611 H2O) > evergreen needle leaf forest (1.7002g02C02kg611 H2O) > deciduous needle leaf forest (1.6802g02C02kg611 H2O) > grassland (1.6602g02C02kg611 H2O) > cropland (1.6102g02C02kg611 H2O). Both GPP and ET increased with increasing annual mean temperature (Ta) and annual mean precipitation across all of the plant function types. WUE decreased as vapour pressure deficit (VPD) increased in all of the biomes. Interestingly, the relationship between WUE and VPD was the most significant in broadleaf forest. Whether this phenomenon is universal should be investigated in future studies.

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[50]
Wang J, Dong J, Yi Y , et al. 2017. Decreasing net primary production due to drought and slight decreases in solar radiation in China from 2000 to 2012.Journal of Geophysical Research: Biogeosciences, 122(1): 261-278.Terrestrial ecosystems have continued to provide the critical service of slowing the atmospheric CO2 growth rate. Terrestrial net primary productivity (NPP) is thought to be a major contributing factor to this trend. Yet our ability to estimate NPP at the regional scale remains limited due to large uncertainties in the response of NPP to multiple interacting climate factors and uncertainties in the driver data sets needed to estimate NPP. In this study, we introduced an improved NPP algorithm that used local driver data sets and parameters in China. We found that bias decreased by 30% for gross primary production (GPP) and 17% for NPP compared with the widely used global GPP and NPP products, respectively. From 2000 to 2012, a pixel-level analysis of our improved NPP for the region of China showed an overall decreasing NPP trend of 4.65TgCa(-1). Reductions in NPP were largest for the southern forests of China (-5.38TgCa(-1)), whereas minor increases in NPP were found for North China (0.65TgCa(-1)). Surprisingly, reductions in NPP were largely due to decreases in solar radiation (82%), rather than the more commonly expected effects of drought (18%). This was because for southern China, the interannual variability of NPP was more sensitive to solar radiation (R-2 in 0.29-0.59) relative to precipitation (R-2<0.13). These findings update our previous knowledge of carbon uptake responses to climate change in terrestrial ecosystems of China and highlight the importance of shortwave radiation in driving vegetation productivity for the region, especially for tropical forests.

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[51]
Wang S J, Song Y, 2006. Basic frame of the urban geography of Northeast China.Acta Geographica Sinica, 61(6): 574-584. (in Chinese)This paper discusses the basic frame of urban geography in Northeast China on the background of rebuilding northeast old industrial base, new-type industrialization and new-type urbanization. It is an overall summarization of the unban geographical system of northern China for the purpose of proving and practising urban geographical theory further. It is also a basic prerequisite for the analysis of the northeast economic development under the new situation of implementing the concrete strategy of "giving a new life to the region" and "the regional innovation" to meet the actual need of the region. Proceeding from the formation and development process of the urban geographical structure of Northeast China, this paper deals with such basic problems as the urban system frame, the key cities, the metropolitan district, the urban compact district and the urban agglomerations of Northeast China. It points out that the formation and development process of the urban geographical frame in Northeast China witnesses three periods, namely, forming period of the ancient cities and towns, forming period of the modern cities, and shaping and developing period of the modern urban system. In terms of the urban system frame, four relavant aspects are identified as: a top to bottom urbanization process that is fast first and slowing down afterwards, the structure of scale and grade of "weak pyramid" type, the "T-shaped" space structure distributed along railway lines, and the urban function structure developing from "single" to "new-type". The "four cities era" taking Harbin, Changchun, Shenyang and Dalian as key cities in Northeast China has come. The three provincial GDPs are highly concentrated on the four cities, taking shape and developing into four metropolitan districts of "Big Harbin", "Big Changchun", "Economic Zone of Shenyang" and "Big Dalian" relying on these four cities. In the process of rebuilding the northeast old industrial base, the four cities will choose different leading directions that rely on the original foundation and new opportunities for development. Harbin will continue to strengthen its position as the key city in the northern part of Northeast China. Changchun should fully take its basic advantage of automobile industry that has broad market and form an automobile industrial base. Shenyang will serve as the centre of Northeast China. Dalian will become an important passageway of Northeast China depending on its advantage while developing outward-oriented industries. As a result of certain degree of urbanization, the urban compact district and urban agglomerations of Northeast China have developed into a relatively complete and intensive frame, and has formed a basic pattern of "three groups" preliminarily, namely, urban agglomeration of "Central-southern Liaoning", "Central Jilin" and "Harbin-Daqing-Qiqihar".

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[52]
Wang W, Chang X L, Liu L X , et al. 2013. Trend of the eastern boundary of semiarid zone in Northeast.Journal of Desert Research, 33(2): 382-389. (in Chinese)

[53]
Wang Z L, Huang Z Q, Li Jet al., 2016. Assessing impacts of meteorological drought on vegetation at catchment scale in China based on SPEI and NDVI.Transactions of the Chinese Society of Agricultural Engineering, 32(14): 177-186. (in Chinese)Drought is a natural disaster caused by long-time water shortage and uneven water distribution, which is one of the most common disasters around the world. Under the background of global warming, droughts have become more and more frequent and devastating, causing negative impacts on agricultural production, ecological environment and social economy. For the protection of vegetation and crops, it would be of practical significance to investigate how vegetation develops under the influence of drought which varies in time and space. This research applied the standard precipitation evaporation index(SPEI) derived from high spatial resolution dataset of precipitation and evaporation, as well as the newly published normalized difference vegetation index(NDVI) which represents the growth condition of vegetation on land. Mann-Kendall(MK) test and correlation analysis were taken to study the spatial and temporal evolution of SPEI and NDVI at catchment scale with the help of ArcG IS 10.1 software. The results of the research illustrated that: 1) The MK value of SPEI at catchment scale showed great coincidence with that of the 4189 grid points both in time and space, indicating the feasibility to investigate the response of vegetation growth under the influence of meteorological drought at catchment scale. SPEI was getting increasing slightly at different time scales during 1962-2012, indicating that China as a whole was being more humid. However, taking the time series of 1982-2012 as the research object, China was getting drier slightly at different time scales. In space, both at annual and at seasonal scale, drought trends were mainly detected in catchments in the Loess Plateau area, Northeast and Southwest China, while catchments in Northwest China presented significant humidifying tendency. 2) The NDVI value of the whole China was upgrading with time during 1982-2012, among which the NDVI in spring, growing season and whole year had a significant increasing trend. In space, some catchments of northern Xinjiang and Northeast China demonstrated a significant downgrading tendency, while the upgrading tendency was different owing to different time scales. 3) At annual scale, 56 of the 209 catchments showed significant positive correlation(P0.05) between SPEI and NDVI, and most of them were located in north Xinjiang, and northern and northeastern China, which indicated that vegetation in arid and semiarid area was significantly affected by droughts. At the same time, negative correlation was mainly distributed in southern China(few of them passed the significant level of 0.95), indicating that droughts in southern China exerted little influence on vegetation growth. 4) Correlation analysis between seasonal NDVI and SPEI showed that in summer and autumn, catchments with positive relation mainly occurred in north area, while catchments with negative correlation mainly occurred in Southeast China and the middle and lower reaches of the Yangtze River plain. In spring and winter, negative relation came to dominate, which mainly occurred in Northeast China, the Yangtze River Basin and most catchments in Southeast China. Lag-relation analysis between seasonal NDVI and SPEI one or two seasons early showed similar correlation distribution as the result of relation analysis without lag. 5) For most of the catchments, air temperature, precipitation and evaporation exerted significant influence on NDVI of vegetation; among them, temperature played a leading role on the vegetation growth in catchments in southern China, while precipitation played a leading role on the vegetation growth in catchments in northern China. Our study on the relationship between meteorological drought and vegetation growth can provide a scientific basis for drought predication and precaution, and thus help to guide agricultural production and protect vulnerable ecosystems.

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[54]
Wang Z L, Li J, Huang Z Q , et al. 2016. Spatiotemporal variations analysis of meteorological drought in China based on scPDSI. Transactions of the Chinese Society of Agricultural Engineering, 32(2): 161-168. (in Chinese)In recent decades, frequent droughts have been detected in some places in China along with the global climate change, which cause great damage to the development of social economy. Thus, it's high time to investigate the spatio-temporal evolution of droughts. Based on the newly published scP DSI PM(self-calibrating Palmer drought severity index based on Penman-Monteith) dataset with high spatial resolution, the annual and seasonal variations of droughts in China from 1961 to 2009 were researched. Other research methods like linear regression, Mann-Kendall method, wavelet analysis as well as REOF(rotated empirical orthogonal function) were adopted to investigate the spatio-temporal pattern of droughts in China, and find out how droughts changed under the influence of the general circulation of atmosphere, in order to provide the foundation for drought control and drought resistance. The results indicated that: 1) In 1961-2009, China as a whole became moist significantly at annual and seasonal scale with the mutations of turning drought to wetness detected in early 1970 s. 2) Period analysis illustrated that the oscillation period of seasonal and annual drought remained generally unanimous, and the dominant period of annual drought was 6.2 years, while drought in spring, autumn and winter generally had the periods of 2.6 and 6.2 years with 6.2 years as the dominant period; otherwise, drought in summer presented a dominant period of 4.4 years. 3) In the light of the spatial modes disassembled from REOF, the entire country was divided into 8 regions with different characteristics of drought or wetness, which were Northwest China, Northeast- Inner Mongolia Plateau area, the Greater Khingan Range area, the northern Qinghai-Tibet Plateau area, the southern Qinghai-Tibet Plateau area, the Middle China(including Sichuan Basin, Hanzhong Basin and Loess Plateau), Huang-Huai-Hai Plain and Southeast China. It was found that the sub-regions of REOF could primely manifest the geographical features of different regions, and hence objectively testify the actual drought condition in China. Among the 8 regions, the Greater Khingan Range area, Southeast China, the southern Qinghai-Tibet Plateau area, and Northwest China were getting moist while the latter 2 areas had a significant trend(P0.05); Northeast- Inner Mongolia Plateau area, the northern Qinghai-Tibet Plateau area, the Middle China and Huang-Huai-Hai Plain were getting dry, and except Huang-Huai-Hai Plain, the former 3 regions were prominently getting dry. Take the regions as the objects, no matter at annual or seasonal scale, Northwest China was getting moist, while the Middle China was getting arid(P0.05). Although the geographical positions of the 8 regions resulted in the discrepancy of influence under different climate factors, the 8 regions generally had the oscillation period of 2-9 years. 4) M-K test(Mann-Kendall test) on the 8 regions showed that the seasonal MK values of different regions varied with each other, which indicated that drought-wet degree of different regions had obvious seasonal feature, and yet most regions had a tendency of getting drought in spring and getting moist in summer. 5) Good correlations were found between polar vortex index and drought in most areas. Correlation analysis indicated that droughts in Northwest China may be affected by the polar vortex, Indian Ocean Dipole(IOD) and Pacific Oscillation(PDO). Droughts in Northeast- Inner Mongolia Plateau area, the Qinghai-Tibet Plateau area and the Middle China were significantly influenced by IOD, and drought of Huang-Huai-Hai Plain was negatively correlated with PDO. Unlike the conventional PDSI index, scP DSI integrates solar radiation and air speed into account, which will be of scientific and practical importance to forecast and distinguish different drought conditions.

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[55]
Wells N, Goddard S, Hayes M J, 2004. A self-calibrating palmer drought severity index.Journal of Climate, 17: 2335-2351.

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[56]
Wilson S D, 2007. Competition, resources, and vegetation during 10 years in native grassland.Ecology, 88(12): 2951-2958.Abstract A 10-year experiment tested for variation in competition intensity over time in a natural grassland at the northern edge of the Great Plains. Growing-season precipitation varied fivefold during the study. All ecosystem-level variables varied significantly among years, and most covaried in expected ways. The covers of all common grasses possessing the C3 photosynthetic pathway varied significantly among years; in contrast, all common species with traits associated with drought tolerance (a C4 grass, a lichen, a spikemoss, and a subshrub) did not vary. Annual transplant experiments measured the competitive effects of neighbors on the growth of individuals of the native grass Bouteloua gracilis. A significant interaction between year and competition showed that competition intensity varied among years. The size of this effect, however, was small (eta2 = 0.074) relative to the size of the direct effect of competition (eta2 = 0.20) or the year in which the experiment was conducted (eta2 = 0.51). Further, competition intensity was not significantly related to any variable describing standing crop or resources, or species richness. Species richness was highest in years with high precipitation, standing crop, and individual growth, due to the recruitment of rare species that were absent from dry years. In summary, variation in competition intensity was statistically significant but had small effects relative to the direct effects of climate.

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[57]
Xi Z X, Yang X Y, Liu S , et al. 2016. The risk evaluation and division of the summer drought in Northeast China.Science Geographica Sinica, 33(6): 735-740. (in Chinese)Since the Northeast China is an important commodity grain base,the persistent drought events become a serious threat to food production in this area.Now the loss of grain caused by the drought is up to or even more than 10 billion YUAN each year.Due to the significant decrease of river runoff in the periods of drought and the over-exploitation of the groundwater,the situation of gradual deterioration of ecological environment in Northeast China has happened,the drought become a serious challenge in sustainable development of social economic and ecological environment.The study of the cause and its risk assessment of drought in Northeast China are of very important for the reduction of losses in drought disaster,and also important for providing scientific and technological support in the fight against droughts.Based on the daily mean temperature and precipitation data in Northeast China,and considering the influence of summer precipitation on food production,a summer drought index is defined by the use of K index,the space-time variation of summer drought is then investigated.The summer precipitation variation coefficient,the risk index and the risk probability of summer drought are calculated and analyzed.The comprehensive risk index is defined based on four indicators,i.e.the variation coefficient of summer precipitation,the frequency of summer drought,the risk index and risk probability of summer drought,the comprehensive risk division of summer drought is also conducted.Results show that: summer drought in Northeast China is more serious in the west than that in the east.The summer drought in Northeast China is get into the frequently-occurring stage since the 1990.The southwest of Heilongjiang Province,the west of Jilin Province and the west of Liaoning Province are the driest regions and they are the high-risk areas of summer drought,the north and east parts of Heilongjiang Province,the central of Jilin and Liaoning Province are the lower risks areas of summer drought,the south-central of Heilongjiang Province,the east of Jilin Province and the southeast of Liaoning Province are the low risk areas.Defensive measures should be taken in the high-risk and the higher risk areas,by promoting the agricultural drought resistant technology vigorously,taking grater efforts on the climate prediction,strengthing the construction of drought resistance so as to reduce the summer drought loss in Northeast China.

[58]
Xie J, Zha T S, Zhou C X , et al. 2016. Seasonal variation in ecosystem water use efficiency in an urban-forest reserve affected by periodic drought.Agricultural and Forest Meteorology, 221: 142-151.The impact of extreme weather events on water-carbon coupling and ecosystem water use efficiency (WUE) in arid to semi-arid conditions is poorly understood. Evapotranspiration (ET) and gross ecosystem production (GEP) were based on continuously eddy-covariance measurements taken over an urban-forest reserve in Beijing, in a 3-year period (2012–2014) to calculate WUE (GEP:ET). Our objective was to investigate the seasonal response of WUE to changing environmental and drought conditions at different timescales. Annually, the forest produced new plant biomass at 2.602±020.202g02C per kg of water loss. Within each season, interactions of surface conductance (g c ) and normalized difference vegetation index (NDVI; i.e. , g c 02×02NDVI) in spring, net radiation (R n ) and air temperature (T a ; i.e. , R n 02×02T a ) in summer, and R n and vapor pressure deficit (D; i.e. , R n 02×02D) in autumn were found as the significant variables explaining seasonal variation in WUE. Daily WUE correlated positively with T a and NDVI during the growing season, but a negative relationship during excessively dry periods ( i.e. , 2014). Daily WUE decreased during warm and dry days or remained nearly constant at low levels due to proportional decreases in GEP and ET. An extreme drought during the leaf expansion led to a greater decline in GEP than in ET, causing WUE to be lower in 2012 and 2014 than that in 2013. In contrast, an extreme drought during the leaf coloration led to a greater decline in ET than in GEP, causing higher WUE in 2013 and 2014 than that in 2012. We concluded that: (i) high soil water content (SWC) during leaf expansion was more important than high SWC in mid-summer or autumn for maintaining a high seasonal WUE; and that (ii) seasonal water availability combined with variable drought severity and duration during periods of changing T a , caused seasonal ET and GEP to respond differently, introducing significant variation in seasonal WUE.

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[59]
Xu W D, 1986. The relation between the zonal distribution of types of vegetation and the climate in Northeast China.Acta Phytoecologica et Geobotanica Sinica, 10(4): 254-263. (in Chinese)The present paper is a study of the relation between the zonal distribution of types of vegetation and the climate in Northeast China, based on the method of Kira's temperatureindexes WI, (Warmth index), CI, (Coldness index) and the HI (Humidity index) proposed by the present author. The results are asfollows:1. The optimum range of warmth index and the value of warmth index, coldness index and humidity index of 10 types of zonal vegetation in the Northeast China have been determined. 2. The vertical distribution features in water-temperature conditions of zonal vegetation types on mountains in Northeast China have been studied. On the basis of the above studies, the paper has discussed the problems about the distribution of Betula ermanii forest zones on the eastern mountains in Northeast China and that of the mountain tundra on the Baigalashan mountain in the Da Hingganling and made a new attempt to divide the vegetation regions in Northeast China

[60]
Yang L, 2015. The influence of drought on water parameters of Larix gmelinii [D]. Harbin: Northeast Forestry University. (in Chinese)

[61]
Yu G R, Song X, Wang Q Fet al., 2008. Water-use efficiency of forest ecosystems in eastern China and its relations to climatic variables.New Phytologist, 177(4): 927-937.090004 Carbon (C) and water cycles of terrestrial ecosystems are two coupled ecological processes controlled partly by stomatal behavior. Water-use efficiency (WUE) reflects the coupling relationship to some extent. At stand and ecosystem levels, the variability of WUE results from the trade-off between water loss and C gain in the process of plant photosynthetic C assimilation. 090004 Continuous observations of C, water, and energy fluxes were made at three selected forest sites of ChinaFLUX with eddy covariance systems from 2003 to 2005. WUE at different temporal scales were defined and calculated with different C and water flux components. 090004 Variations in WUE were found among three sites. Average annual WUE was 9.43 mg CO 2 g 0908081 H 2 O at Changbaishan temperate broad-leaved Korean pine mixed forest, 9.27 mg CO 2 g 0908081 H 2 O at Qianyanzhou subtropical coniferous plantation, and 6.90 mg CO 2 g 0908081 H 2 O at Dinghushan subtropical evergreen broad-leaved forest. It was also found that temperate and subtropical forest ecosystems had different relationships between gross primary productivity (GPP) and evapotranspiration (ET). 090004 Variations in WUE indicated the difference in the coupling between C and water cycles. The asynchronous response of GPP and ET to climatic variables determined the coupling and decoupling between C and water cycles for the two regional forest ecosystems.

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[62]
Yu K, Okin G S, Ravi S , et al. 2016. Potential of grass invasions in desert shrublands to create novel ecosystem states under variable climate.Ecohydrology, 9(8): 1496-1506.ECOHYDROLOGY Ecohydrol. (2016) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/eco.1742 Potential of grass invasions in desert shrublands to create novel ecosystem states under variable climate Kailiang Yu, 1 * Gregory S. Okin, 2 Sujith Ravi 3 and Paolo D’Odorico 1,4 Department of Environmental Sciences, University of Virginia, Charlottesville, VI, USA Department of Geography, University of California, Los Angeles, CA, USA Department of Earth and Environmental Sciences, Temple University, Philadelphia, PA, USA National Socio-Environmental Synthesis Center, University of Maryland, Annapolis, MD, USA ABSTRACT The invasion of exotic grasses into shrublands is a major disturbance to dryland ecosystems. The presence of exotic grasses enhances the occurrence of wild03re in landscapes that had not evolved in the presence of 03re, leading to high rates of mortality of the native vegetation. Exotic grasses could be more prone to water stress and mortality than the shrubs they replaced and may not establish during drought, facts that are crucial in ecosystems undergoing increased climatic variability. Here, we develop a process-based modelling framework to investigate the complex dynamics resulting from the introduction of exotic grasses under variable climate. We 03nd that the system converges towards different steady states, depending on the magnitude of climatic variability. While in the absence of climate 04uctuations the shrubland state is replaced by an exotic grassland, interannual climate variability may inhibit grass invasion and stabilize the shrubland state. However, climatic variability also gives rise to a novel third, unvegetated state, with grass invasion being followed by drought, grass mortality and intense soil erosion. Most of the research on climate change effects on ecosystems has historically concentrated on the ecological impact of shifts in mean climate conditions. This study shows that changes in the variance are also important when shifts in vegetation composition (e.g. species invasions) result in different susceptibility to climatic variability. In the presence of random climate 04uctuations, ecosystems can display steady states that differ from those that would exist under a constant climate or with a climate trend. Copyright 08 2016 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this article. KEY WORDS invasive species; 03re cycle; climate variability; exotic grasses; unvegetated state Received 15 October 2015; Revised 18 March 2016; Accepted 28 March 2016 INTRODUCTION Biological invasions are recognized as major contributors to global environmental change (Vitousek et al., 1997; Mooney and Cleland, 2001). It has been observed that biological invasions affect ecosystem dynamics not only through their direct impact on resource competition and pool of available species (e.g. Olsson et al., 2012) but also indirectly through their ability to modify the disturbance regime. For example, invasive plants may change 03re intensity and frequency or alter the rate of abiotic processes such as soil erosion (D’Antonio and Vitousek, 1992; Ziska et al., 2005; Miller et al., 2010). This disturbance-mediated effect of species invasions on ecosystems is observed when the invader is functionally different from the native species, *Correspondence to: Kailiang Yu, Department of Environmental Sciences, University of Virginia, Charlottesville, VI, USA. E-mail: ky9hc@virginia.edu Copyright 08 2016 John Wiley & Sons, Ltd. i.e. when it exhibits some traits that (i) affect the disturbance regime and (ii) are missing in the native population (D’Antonio, 2000). For example, the invasion of desert shrublands by exotic annual and perennial grasses has been observed to lead to an increase in 03re frequency and intensity because of the increase in grass fuel and in connectivity of vegetation cover (Okin et al., 2009a). The introduction of 03res in shrubland ecosystems, where burning has not been historically a major selective force, results in an increase in the mortality rates of shrubland species, particularly if they are not adapted to 03re (e.g. Bond et al., 2005; Runyan et al., 2012). The loss of shrub biomass further enhances the establishment and spread of invasive grasses (D’Antonio, 2000). Known as ‘the 03re cycle’ (D’Antonio and Vitousek, 1992), this positive feedback leads to the replacement of 03re-intolerant native shrubs with exotic grasses (Figure 1). The grass-03re feedback may induce stable grass-dominated vegetated states in arid and semiarid environments (Grigulis et al., 2005; Keeley and Rundel, 2005), even when shrubs have

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[63]
Zhang B H, Zhang L,Guo H Det al., 2014. Drought impact on vegetation productivity in the Lower Mekong Basin.International Journal of Remote Sensing, 35(8): 2835-2856.The Lower Mekong Basin (LMB) has a typical monsoon climate, with high temperatures and an uneven distribution of precipitation throughout the year. This climate, combined with the geographic position of the LMB, has led to an increase in the frequency of extreme weather events over last decade. However, few previous studies have used remote-sensing data to investigate the impact of such weather events, particularly severe droughts, on biological productivity in the LMB. To address this, we assessed the impact of drought on vegetation productivity in the LMB during 2000-2011 using MOD17 products. Several drought events were identified during this period. Of these, the most severe occurred during 2005 and 2010, although the 2005 drought was both more extensive and more intense. Net primary productivity (NPP) exhibited considerable variation during 2000-2011: the droughts in 2005 and 2010 reduced NPP by 14.7% and 8.4%, respectively. The impact of drought on NPP in 2005 was much greater than that in 2010, likely owing to the longer duration and larger deficit of precipitation in 2005 (which lasted from winter 2004 to spring 2005). Our results demonstrate that severe drought had a greater impact on NPP than mild drought, especially for forests, woodlands, and shrublands. Comparatively, little variation in NPP was found for croplands, even under drought conditions, which were attributed to the wide use of irrigation and the exploitation of water sources during drought periods. Moreover, multi-season croplands in Vietnam experienced only a small reduction in gross primary productivity (GPP) in 2005 compared to one-season croplands in Cambodia, which can be related to the shorter growing periods of the former impacted by droughts.

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[64]
Zhang Y, Xie J B, Li Y, 2017. Effects of increasing root carbon investment on the mortality and resprouting of Haloxylon ammodendron seedlings under drought. Plant Biology, 19(2): 191-200.Abstract Tree mortality induced by drought is one of the most complex processes in ecology. Although two mechanisms associated with water and carbon balance respectively are proposed to explain tree mortality, outstanding problems still exist. In the greenhouse experiment, various water and carbon related physiological indicators (shoot water potential, photosynthesis, dark respiration, hydraulic conductance and non-structural carbohydrates) were examined on drought and control Haloxylon ammodendron seedlings, and how the root system benefited the survival and resprouting of seedlings was tested. The results showed that survival time of the seedling root system (died 70 d after drought) was double the survival time for the shoot (died at 35 d). Difference in survival time between shoot and root resulted from sustained root respiration supported by increased non-structural carbohydrates in root under drought. Furthermore, the investment into the root contributed to the resprouting following drought. Based on these results, a death criterion was proposed for this species. The time sequence of major events indicated that drought shifted the carbon allocation between shoot and root and altered the flux among different sinks (growth, respiration or storage). The interaction of water and carbon processes co-determine the death or survival of the droughted H. ammodendron seedlings. These findings revealed that the - oot protecting- strategy was critical in determining the survival and resprouting of this species, and provided insights into the effects of carbon and water dynamics on tree mortality. This article is protected by copyright. All rights reserved.

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[65]
Zhang Y D, Pang R, Gu F X , et al. 2016. Temporal-spatial variations of WUE and its response to climate change in alpine area of southwestern China.Acta Ecologica Sinica, 36(6): 1515-1525. (in Chinese)The various types of ecosystems and complex landforms found in the cold alpine area of southwestern China makethis region ideal for researching regional responses to global climate change. Therefore,to evaluate the responses of regional carbon and water cycles to climate change; it is of great importance to investigate the response of water use efficiency(WUE) to the climate in this region. A process-based ecosystem model,Carbon Exchange between Vegetation,Soil,and the Atmosphere( CEVSA),was used to estimate temporal and spatial variations of WUE in the terrestrial ecosystems in the alpine area of southwestern China during 1954-2010. First,we ran the model using the average climate data from 1954 to2010 until an ecological equilibrium was reached,then we conducted dynamic simulations with climate data at a time-step of 10 days during the same period. Moreover,the correlation coefficients between WUE and climate variables were calculated to analyze the relative effects of temperature and precipitation on variations of WUE. To achieve the results,various types of computer software were used,such as ANUSPLIN4. 1,Fortran 95,Arcgis9. 3,and SPSS18. 0. The results showed that the average WUE in the studied region was 1. 13 g C mm- 1m- 2during 1954-2010. The mean WUE of three main vegetation types included 1. 35 g C mm- 1m- 2for herbaceous cover,1. 14 g C mm- 1m- 2for evergreen needle-leaf tree cover,and 0. 99 g C mm- 1m- 2for evergreen broadleaf tree cover. In spatial distribution,significant positive correlations were found between the annual WUE and altitude( r = 0. 156,P 0. 05),and significant negative correlation was found between the annual WUE and annual mean temperature( r =- 0. 386,P 0. 01). Moreover,the annual mean WUE in the entire region showed a significantly decreasing trend at a rate of 0. 006 g C mm- 1m- 2a- 1( P 0. 01). Significant negative correlations were found between the annual mean WUE and annual mean temperature( r =- 0. 727,P 0. 01),and no significant correlations were found between the annual mean WUE and annual precipitation. The decrease in WUE resulting from an increase in evapotranspiration( ET) was more than that of net primary production( NPP) from the temperature increase during the study period. Furthermore,decreasing trends were highly significant in herbaceous cover at1. 37 10- 3g C mm- 1m- 2a- 1,evergreen needle-leaf tree cover at 6. 17 10- 3g C mm- 1m- 2a- 1,and evergreen broadleaf tree cover at 1. 03 10- 2g C mm- 1m- 2a- 1during the study period. The annual WUE showed significant negative correlations with temperature in 76. 3% of the study area( P 0. 05) and significant positive correlations with annual precipitation in 34. 1% of the study area( P 0. 05). Herbaceous and evergreen needle-leaf tree cover in the study area were both correlated negatively with temperature( r =- 0. 889,P 0. 01; r =- 0. 863,P 0. 01) and were not correlated with annual precipitation.

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[66]
Zhao D S, Wu S H, 2013. Responses of vulnerability for natural ecosystem to climate change in China.Acta Geographica Sinica, 68(5): 602-610. (in Chinese)Vulnerability of natural ecosystem to climate change is scientific basis for adapting and mitigating climate change,and is one of the important issues in the area of climate change and ecology.In this study,LPJ,a modified dynamical vegetation model according to features of China's natural ecosystems,was employed to simulate ecosystem dynamics under A2,B2 and A1B emission scenarios generated by PRECES(Providing Regional Climate for Impacts Studies) system.Using a vulnerability assessment model,vulnerability of natural ecosystem against climate change was evaluated in the future.Results suggest that vulnerability for China's natural ecosystems may strengthen in the east and weaken in the west,but the pattern of ecosystem vulnerability in China,decline from southeast to northeast,would not be altered under climate change.Ecosystem rising vulnerability may be mainly observed in temperate humid/sub-humid region and warm temperate humid/sub-humid region.Ecosystem decreasing vulnerability may be found in arid region of Northwest China and Tibetan Plateau region.In the near-term scale,natural ecosystem in China may be slightly affected by climate change.However,in mid-term and long-term scales,there may be severely adverse effect.Particularly,in the east with better water and thermal condition.

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