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Journal of Geographical Sciences >

2018 , Vol. 28 >Issue 6: 833 - 844

DOI: https://doi.org/10.1007/s11442-018-1508-7

Research Articles

Long-term changes in the tree radial growth and intrinsic water-use efficiency of Chuanxi spruce (Picea likiangensis var. balfouriana) in southwestern China

  • WANG Yang , 1, 2 ,
  • ZHANG Yong 1 ,
  • FANG Ouya 3 ,
  • SHAO Xuemei , 1, *
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  • 1. Key Laboratory of Land Surface Pattern and Simulation, Institute of Geographic Sciences and Natural Resources Research, CAS, Beijing 100101, China
  • 2. University of Chinese Academy of Sciences, Beijing 100101, China
  • 3. State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, CAS, Beijing 100093, China
*Corresponding author: Shao Xuemei (1957-), Professor, specialized in dendrochronology and dendroclimatology. E-mail:

Author: Wang Yang (1984-), PhD, specialized in dendrochronology and dendroecology. E-mail:

Received date: 2017-04-18

  Accepted date: 2017-07-05

  Online published: 2018-06-20

Supported by

National Basic Research Program of China ‘973’, No.2012CB956201

Key Program of National Natural Science Foundation of China, No.41630529

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Journal of Geographical Sciences, All Rights Reserved
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Abstract

Elevated CO2 level in the atmosphere is expected to improve the tree growth rates and intrinsic water-use efficiency (iWUE). Although current results inferring from tree rings found the tree growth decline in water-limited area, it is still unclear whether spruce trees in humid southwestern China benefit from the increasing CO2. In this study, tree-ring width data were used to investigate the tree radial growth rate of Chuanxi spruce (Picea likiangensis var. balfouriana). Moreover, combining with the tree-ring carbon isotope date, we analyzed the physiological responses of Chuanxi spruce to rising CO2 concentrations in the atmosphere (Ca) associated with climatic change in southwestern China. From 1851 to 2009, iWUE of Chuanxi spruce rose by approximately 30.4%, and the ratio of atmospheric CO2 to leaf intercellular CO2 concentration (Ci/Ca) showed no significant trend in the study area. The result suggested that Chuanxi spruce used an active response strategy when Ca was significantly increased. iWUE showed a significant increasing trend in parallel with tree radial growth, indicating that the increasing iWUE resulted in an increase in radial growth. These results suggest that spruce forests in southwestern China have not shown declining trends under increasing Ca and climate change scenarios, in contrast to trees growing in water-limited areas. Therefore, spruce forests benefit from the increasing CO2 in the atmosphere in the humid areas of southwestern China.

Key words： tree ring; basel area increment; carbon isotope; intrinsic water-use efficiency; CO2 fertilization

Cite this article

WANG Yang , ZHANG Yong , FANG Ouya , SHAO Xuemei . Long-term changes in the tree radial growth and intrinsic water-use efficiency of Chuanxi spruce (Picea likiangensis var. balfouriana) in southwestern China[J]. Journal of Geographical Sciences, 2018 , 28(6) : 833 -844 . DOI: 10.1007/s11442-018-1508-7

1 Introduction

Atmospheric CO2 concentrations (Ca) have been rising at an unprecedented rate since the industrial revolution and are now greater than 390 ppm (IPCC, 2013). Increased Ca and associated with climate change are expected to improve the intrinsic water-use efficiency (iWUE) levels of trees, which is a useful physical indicator of the carbon assimilated in the leaf transpiration process (Ehleringer and Cerling, 1995; Norby and Zak, 2011; Kallarackal and Roby, 2012). As iWUE improved, the tree growth usually increased and its ability for carbon sequestration will enhance (Fang et al., 2001; Morén et al., 2001). According to simulations based on empirical analysis, additional increases in Ca will largely improve the iWUE in different forest types by the end of the 21st century (Huang et al., 2007). However, we know little about the effects of increasing Ca and associated with global change on natural tree growth in different regions on a long time scale (Liu et al., 2014) owing to lack of long-term observational data.
Stable carbon compositions (δ13C) in tree rings have been recommended as a useful tool for understanding past environment conditions and how plants respond to environmental change (Hietz et al., 2005; Linares et al., 2009). Previous studies have used tree ring δ13C to analyze how trees respond to climate change and increasing Ca in nature condition (Loader et al., 2008; Nock et al., 2011; Liu et al., 2014). Their results show that the responses are region-dependent, suggesting that trees use different adaptation strategies in response to environmental change. Increasing iWUE levels do not always enhance tree radial growth rates due to the interaction with different growth conditions and tree species (Newberry, 2010; Andreu-hayles et al., 2011; Gyenge et al., 2012; Silva et al., 2013; Brito et al., 2016)(Knapp, 2011 #328;Silva, 2013 #346). In China, studies have been performed in water-limited area (Wang et al., 2012; Xu et al., 2013; Wu et al., 2015), and their results show that the tree radial growth rates do not accelerate when iWUE increases significantly. However, the effects on trees growing in humid locations are not fully understood. Although some studies were carried out in humid area of China (Silva et al., 2016; Huang et al., 2017; Li et al., 2017), more research is needed to gain more knowledge on how different tree species growing in different environmental conditions respond to increasing Ca and climate change.
In southwestern China, Chuanxi spruce (Picea balfouriana) plays a key role in environmental protection and wood production (Li et al., 2014). It is crucial to evaluate how spruce trees respond to Ca increases and to climate change. In our study, long-term records of basal area increments (BAI) and iWUE were developed for Chuanxi spruce based on tree-ring width data and δ13C data, respectively. Further, we analyzed the tree growth rates and gas exchange responses to both rising Ca levels and changing climatic patterns. Our objectives were (1) to understand how BAI and iWUE have changed with increasing Ca levels and climatic changes from 1851 to 2009; (2) to determine the relationship between iWUE and BAI; and (3) to discuss how trees physiologically respond to increasing Ca.

2 Materials and methods

2.1 Description of the study site

The study site was located in the town of Jinchuan in Sichuan Province of Southwest China (101°42′E, 31°31′N). This area is located in Southwest China’s plateau area, and it is close to the Tibetan Plateau (Figure 1). The elevation exceeds 3500 m. The rainy season runs from May to September, and annual precipitation levels range from 500 mm to approximately 800 mm. June is the wettest month, and December is the driest. The annual mean temperature is 12.5°C. January is the coldest month, and July is the hottest month (Figure 2). The climate data used in this study was obtained from the Jinchuan Meteorological Station (102°0′E, 31°30′N). From 1961 to 2008, temperatures in this area did not show any increasing trend and annual precipitation increased slightly (by approximately 3.73 mm per year, Figure 3).
Figure 1 Location of the sample site and meteorological station
Figure 2 Mean monthly temperatures and precipitation at the Jinchuan Meteorological Station from 1960 to 2008
Figure 3 Trends in annual total precipitation (a) and annual mean temperature (b) from 1960 to 2008. The grey dash line is the result of the linear regression of precipitation.

2.2 Tree ring sampling and analysis

We sampled 56 increment cores from 25 healthy spruce trees (two or three cores per tree) in total using a 10-mm-diameter increment borer. The increment cores were treated by standard dendrochronological procedures at the tree ring lab. The tree-ring width was measured using the Lintab-6 platform. The resolution of the measurement was 0.01 mm. The COFECHA program was used to check the accuracy of the tree-ring width measurements and cross-dated results (Holmes, 1983). Then, the ARSTAN program was used to build the standard tree ring width chronology (Cook and Holmes, 1986). The negative exponential and linear regression curves were used to detrend the tree-ring width data. To evaluate long-term tree growth trends, we converted the tree-ring width data into BAI using equation 1 below:
BAI = π × ($R^{2}_{n}-R^{2}_{n-1}$) (1)
where R refers to the radius of a tree in a given year and n refers to the year that the ring formed (West, 1980; Johnson and Abrams, 2009).

2.3 α-Cellulose extraction and isotope analysis

After cross-dating, eight increment cores with no obvious signs of damage from different trees were chosen for isotope analysis. We carefully split the increment cores at a one-year resolution by using knife blades under a microscope. The split rings of the same year were pooled together prior to α-cellulose extraction. The α-cellulose of tree rings was extracted in a standard method (Green, 1963; Loader et al., 1997). An elemental analyzer coupled with an isotope mass spectrometer was used to measure the tree-ring δ13C values. This process was carried out at the state key laboratory of vegetation and environmental change, Institute of Botany, the Chinese Academy of Science. The results were expressed as a ratio of tree ring samples to the Vienna Pee Dee Belemnite (VPDB) standard (Coplen, 1995). The standard deviation of the analysis was less than 0.3‰. iWUE values of Chuanxi spruce were calculated by the Farquhar’ method:
iWUE=A/g=(Ca-Ci)/1.6 (2)
where A is the photosynthetic assimilation rate and g is the stomatal conductance (Ehleringer et al., 1993). The Ci values were determined using the equation from Francey and Farquhar (1982):
Ci = (δ13Ca-δ13Cp) / (1+δ13Cp/1000) - a] *Ca/(b-a) (3)
where δ13Ca is the δ13C value of ambient air and δ13Cp is the δ13C value of the tree-ring ɑ-cellulose. The parameters a (4.4‰) and b (27‰) were considered constant (a was the isotopic discrimination when CO2 diffused from the atmosphere into the intercellular space of cells; b was the isotopic discrimination caused by discrimination of RuBP carboxylase against CO2).
Saurer et al. (2004) presented three scenarios to explain how plants respond to increasing CO2: (1) Ci remains constant and iWUE significantly increases, (2) Ci/Ca remains constant and iWUE slightly increases, (3) both Ca-Ci and iWUE remain constant. In this study, we use Saurer’s method to evaluate how trees physiologically respond to increasing Ca.

3 Results

3.1 The trends of tree-ring width, BAI and tree-ring δ13 C

Overall, the Chuanxi spruce tree-ring width exhibited a clearly increasing trend over the period of 1851-2009 (Figure 4). During the early phases of the study period (1851-1900), the tree-ring width showed a relatively flat phase followed by a continuously increasing phase until 1950. During the last portion of our study period (1951-2009), the tree-ring width data increased steadily, reaching values greater than the long-term mean. The BAI levels followed a similar trend as the tree-ring width. From 1851-1900, the BAI levels did not significantly change, with a mean value of 6.97 cm2×year-1. Then, the BAI levels rapidly increased from 1901 to 1950, reaching a peak value of 10.4 cm2×year-1 in 1933. After 1951, the BAI increased slightly, and its mean value reached peak level during the study period (1851-2009).
Figure 4 Trends of the tree-ring width (a) and basal area increment (BAI) (b) for Chuanxi spruce from 1851 to 2009. Horizontal lines represent the mean value for the study period. The grey line is the result of the linear regression of BAI.
The raw δ13C value for Chuanxi spruce decreased significantly from 1851 to 2009, with a mean value of -22.87‰. The δ13C changes from 1851 to 1950 were minor, ranging from -21.048 to -23.043 at a decreasing rate of 0.02‰ per year. After 1950, trees had accelerated rates of δ13C decline, reaching 0.04‰ per year, with the lowest value of -24.176‰ in 2008. These decreasing δ13C trends reflected changes in atmospheric δ13C. The raw δ13C values for Chuanxi spruce had more negative values after 1850 because more isotopically depleted CO2 was released into the atmosphere after the industrial revolution. It is necessary to remove this effect when using our δ13C series to extract climatic signals. To correct the raw δ13C values, we used a method presented in a previous study (McCarroll and Loader, 2004). After removing the atmospheric δ13C trends, the corrected series was smooth, with a mean value of -22.16‰ (Figure 5).
Figure 5 The raw (δ13Craw) and corrected (δ13Ccor) chronologies for Chuanxi spruce from 1851 to 2009

3.2 Climate correlation analysis

Based on the results of correlation analysis, the temperature in June presented significantly positive correlations with tree-ring width (r=0.495, p<0.01) and BAI (r=0.542, p<0.01), although the level of tree-ring width was slightly lower than BAI’s (Figures 6a and 6b). Meanwhile, correlations between precipitation and tree-ring width and BAI were very weak; none of these values reached significant levels.
Comparing with the climatic response of tree-ring width and BAI, the climatic response of the corrected δ13C (δ13Ccor) showed a different pattern. We found tree-ring δ13Ccor values presented a significantly positive correlation with temperature from June to August (Figure 6c). The temperature in February of the current year also showed a strong relationship with the δ13Ccor series. We also found a negative but non-significant correlation between the precipitant and δ13Ccor series for the growing season of the current year.
Figure 6 Correlations between tree-ring width (a), BAI (b), δ13Ccor (c) and climate factors from 1960-2008.
p indicates the previous year. Significant levels are shown above the bars: * P<0.05 and ** P<0.01.

3.3 Long-term Ci, Ci/Ca, Ca-Ci and iWUE trends

The Ci, iWUE, Ci/Ca, and Ca-Ci variations for the study period (1851-2009), which were inferred from tree-ring carbon isotopes, are shown in Figure 7. The Ci increased significantly from 1851 to 2009 and increased faster in recent years (from 1951 to 2009). Ca-Ci also showed a strong increasing trend from 1851 to 2009 (slope=0.25, P<0.001). During the early phases of the study period (1851-1900), Ci did not have an increasing or decreasing trend. However, Ci showed a clear increasing trend (slope=0.23, P<0.001) after the 1900s. Both Ci/Ca and Ca-Ci changed very slowly during the first 50 years of the study period (1851-1900), and Ci/Ca remained almost stable. However, the Ca-Ci levels changed faster than the Ci/Ca levels, which generated a slope of approximately 0.45 (p<0.001). The Ci/Ca levels remained relatively constant. Overall, throughout the study period, Ca-Ci and Ci followed a significantly increasing trend, but Ci/Ca remained almost stable.
Figure 7 Trends in iWUE (a), Ca-Ci (b), Ci/Ca (c) and Ci (d) for Chuanxi spruce from 1851 to 2009. The grey dash lines are the results of linear regression. iWUE in different scenarios was shown in (a).
The iWUE for Chuanxi spruce increased from 1851 to 2009. From 1851 to 1950, iWUE did not change rapidly (slope=0.02, p<0.001). After 1950, iWUE increased faster than it did over the previous 100 years, with a slope of 1.3 (p<0.001). iWUE reached a peak value of 120.75 μmol mol-1 in 2006. Compared with the mean value of iWUE from 1851 to 1950, it increased by approximately 30.4%.

4 Discussion

4.1 The climatic information recorded in the tree-ring width and δ13C data

The significantly positive effects of June temperatures on tree-ring width and BAI indicate that summer temperatures should control the growth rate of Chuanxi spruce. Meanwhile, the weak effect of precipitation on the tree-ring width, BAI and δ13C show that water is not as important as temperature in controlling tree radial growth. This difference is attributed to the fact that our study site is located in humid region, where annual precipitation levels reach nearly 800 mm. In addition, our study site is situated at a high elevation. Temperature should be an important factor controlling tree growth rates. Similar results were found on Western Sichuan Plateau (Duan et al., 2010; Li et al., 2014). Since tree growth was not limited by water condition, higher mean summer temperature could contribute to tree growth by stimulating photosynthesis.
From the results of correlation analysis, we found that tree-ring δ13C value was dominated by the growing season temperatures (from July to August). Because carbon isotope discrimination is controlled by photosynthesis and leaf stomatal conductance, the most important climatic controls for δ13C values influence these physiological processes (McCarroll and Loader, 2004; Saurer et al., 2004). Our study site positioned along an upper tree line, where temperature was an important factor affecting tree growth. Overall, the radial growth rate of Chuanxi spruce and its carbon isotope discrimination were controlled by temperature during the growing season in this study area (Ferrio and Voltas, 2005; Leavitt, 2010; Loader et al., 2013).

4.2 iWUE trends and their physiological implications

In recent decades, several studies were performed to investigate the long-term changes in iWUE in different regions. Most of the results showed that iWUE increased significantly, especially in recent years. Peñuelas et al. (2011) found that iWUE increased by approximately 20.5% worldwide. In tropical areas, Brienen et al. (2011) found that the iWUE increased by approximately 40%. In temperate areas, previous results show that iWUE increased from 19% to 48% among different tree species (Feng, 1999; Waterhouse et al., 2004). Frank et al. (2015) found that iWUE increased approximately 22% in Europe. According to our study, the iWUE values of Chuanxi spruce increased by 30.4% over the last 50 years, which generally agrees with the results in temperate areas. However, the increase found in our study is higher than the global average and the European average and lower than that of tropical areas.
The increasing Ca is the main cause of the strong increasing iWUE trends. Our regression analyses showed that increasing atmospheric Ca levels can explain 89.8% of the observed variation in iWUE (Figure 8). This result makes sense because the theoretical model that we used to calculate the iWUE (Ehleringer et al., 1993) was significantly affected by the concentration of CO2 (Silva and Horwath, 2013). Moreover, rising Ca limits leaf stomatal conductance but stimulates the photosynthesis of leaves, which may stimulate the iWUE of trees. FACE and open-top chamber experiments confirmed these physiological processes in trees (Eamus et al., 1993; Ainsworth and Rogers, 2007). Temporal variations in Ci and Ci/Ca may help us to know the response of spruce trees to increasing Ca and how their physiological processes have changed. For our study site, the Ci/Ca slope did not show any significant change, but both Ci and Ca-Ci showed a significant increasing trend. Based on the hypothesis of Saurer et al. (2004), our result was similar to scenario 2 (Figure 7 a). This scenario was the most common result of tree-ring carbon isotope analysis around the world (Feng, 1999). Constant Ci/Ca levels reflect proportional regulation of A and g in Chuanxi spruce. It also indicated that trees have already adapted to increased Ca. Trees should also have decreased leave stomatal conductance, and the photosynthetic rates of trees should also decrease. (Saurer et al., 2004).
Figure 8 Scatterplot of the annual values of iWUE versus Ca and the regression analysis results (grey line)

4.3 The relationship between iWUE and BAI

Generally, forests should theoretically show an increasing growth trend as Ca continues to rise (Bazzaz, 1990; Knapp et al., 2001). Increased Ca may directly stimulate photosynthesis through increased the RuBisCo enzyme’s reaction rate. Moreover, reduced CO2 requirements decrease stomatal conductance thereby increasing iWUE (Turnbull et al., 2001; Huang et al., 2007). This phenomenon has been found in controlled experiments (e.g., FACE and open-top chamber experiments). However, in recent decades, several studies have found that forests growing in arid and semi-arid areas did not exhibit CO2 fertilization effects. These studies attribute the decline in tree growth to more severe drought effects (Silva et al., 2010; Wang et al., 2012). Few reports have been conducted on humid areas in China. According to our results, increasing CO2 levels are the main cause of the increased iWUE levels among Chuanxi spruce trees (Figure 8). The regression analysis between iWUE and BAI (Figure 9a) shows a strong positive relationship (r2=0.38, p<0.05). For the first-difference data of iWUE and BAI (Figure 9b), they also have a positively relationship (r2=0.06, p<0.05). iWUE can explain 32% of BAI variation. Increasing iWUE levels may accelerate tree radial growth, and the trees can benefit from the rising CO2 levels, which indicates that the CO2 fertilization effect exists in the humid areas of southwestern China.
Figure 9 The relationship between BAI and iWUE for 1851-2009. (a) raw values; (b) first-difference values

5 Conclusion

In this study, annual-resolution δ13C and BAI chronologies were built for a temperate forest in southwestern China. The δ13C chronology responses to climatic variations showed that the δ13C was controlled by the current growing season temperature. Our results indicated that as Ca increased significantly, Ci/Ca remained constant, whereas Ci and iWUE showed a significantly increasing trend. Chuanxi spruce trees had a positive response to rising concentrations of CO2. iWUE levels increased by 30.4% in the last 50 years. Tree radial growth occurred in parallel with increased iWUE levels, and Ca was correlated with iWUE. The results showed that tree radial growth benefited from increasing Ca levels through the so-called CO2 fertilization effect. The spruce trees in our study site exhibited healthy growth rates and could fix a greater amount of CO2 in the future. Our results provide useful information for the local government to develop policies for forest management in this area.

The authors have declared that no competing interests exist.

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[14]
Feng X H, 1999. Trends in intrinsic water-use efficiency of natural trees for the past 100-200 years: A response to atmospheric CO2 concentration.Geochimica et Cosmochimica Acta, 63(13): 1891-1903.To evaluate how the land carbon reservoir has been acting as a sink to the anthropogenic CO 2 input to the atmosphere, it is important to study how plants in natural forests physiologically adjust to the changing atmospheric conditions. This has been studied intensively using controlled experiments, but it has been difficult to scale short-term observations to long-term ecosystem-level response. This paper derives variations of plant intrinsic water-use efficiency from natural trees for the past 100 200 years using carbon isotope chronologies. This parameter may potentially cause an increase in plant growth rate by improving the efficiency of plant water use, especially in arid environments. Attempts were made to isolate the variations of intrinsic water-use efficiency as a function of only the CO 2 concentration of the atmosphere. The intrinsic water-use efficiency of almost all trees increased with increasing atmospheric CO 2 concentration. This is caused by an increase in the carbon assimilation rate ( A ) and/or a decrease in the stomatal conductance ( g ). The increase in plant intrinsic water-use efficiency may imply an increase in plant transpiration efficiency which may have a direct connection with changes in plant biomass.

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[15]
Ferrio J P, Voltas J, 2005. Carbon and oxygen isotope ratios in wood constituents ofPinus halepensis as indicators of precipitation, temperature and vapour pressure deficit. Tellus B, 57(2): 164-173.ABSTRACT Carbon and oxygen isotope compositions (δ 13 C, δ 18 O) in tree rings have been shown to bear relevant climatic signals. However, little is known about the interrelationship between both isotopes in wood constituents for species from other than relatively wet climates. We hypothesized that in a species adapted to temporary droughts (e.g. Pinus halepensis Mill.) the signal derived from δ 18 O in precipitation would be hidden by the strong variability in leaf transpirative enrichment. To test this assumption, we compared the effect of precipitation, temperature and vapour pressure deficit (VPD) on δ 18 O and δ 13 C along 23 sites covering the ecological range for this species. We extracted the cores from the south side of four to six adult dominant trees per aspect (north/south) within each site. For each aspect and site, fragments of the period 1975–1999 were pooled and milled to a fine powder. To further test the postulated need for cellulose purification in the assessment of climatic information, we studied these relationships in whole and extracted wood, holocellulose and lignin. In all wood fractions, δ 13 C was related to annual precipitation [ r =610.58 ( P < 0.01) to 610.78 ( P < 0.001) ] and VPD [ r = 0.53 ( P < 0.01) to 0.57 ( P < 0.01) ]. In contrast, for δ 18 O only holocellulose showed consistent relationships with climatic data, being strongly significant for VPD [ r = 0.66 ( P < 0.001)] . However, it was unrelated to modelled δ 18 O in precipitation, confirming that transpirative enrichment (driven by VPD) dampened the source signal in P. halepensis . The relationships between δ 13 C and δ 18 O were generally poor, regardless of the wood constituent, suggesting that although both variables were somewhat related to transpirative demand, they were relatively independent. This was further confirmed by building stepwise models using both isotopes to predict annual and seasonal precipitation [ r 2 = 0.34 ( P < 0.01) to 0.68 ( P < 0.001) ], temperature [ r 2 = 0.15 ( P < 0.05) to 0.37 ( P < 0.01) ] and VPD [ r 2 = 0.31 ( P < 0.01) to 0.55 ( P < 0.001) ]. We concluded that, even when partially describing the same climate variables, the information underlying the two isotopes can be regarded as complementary.

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[16]
Francey R J, Farquhar G D, 1982. An explanation of 13C/12C variations in tree rings.Nature, 297(5861): 28-31.Variations in the 13 C/ 12 C ratio in trees are examined in the light of a simple expression relating the relative isotope composition of plant material, δ p 13 to δ a 13 , the atmospheric isotope value, c a , the atmospheric CO 2 concentration and c i , the internal concentration of CO 2 in leaves. The expression gives good agreement with δ p 13 measurements where independent information on c i exists, such as seasonal growth, growth low in the canopy and in conditions of low humidity. The expression provides possible explanations for two previously unexplained phenomena: the absence of anticipated changes due to fossil fuel-induced changes in δ a 13 and regional differences in δ p 13 trends.

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[17]
Frank D C, Poulter B, Saurer M.et al, 2015. Water-use efficiency and transpiration across European forests during the Anthropocene.Nature Climate Change, 5(5): 579-583.article{b6aadca4-e8a1-464e-b2db-871937b481cf, author = {Frank, David. C. and Poulter, B. and Saurer, M. and Esper, J. and Huntingford, C. and Helle, G. and Treydte, K. and Zimmermann, N. E. and Schleser, G. H. and Ahlstr枚m, Anders and Ciais, P. and Friedlingstein, P. and Levis, S. and Lomas, M. and Sitch, S. and Viovy, N. and Andreu-Hayles, L. and Bednarz, Z. and Berninger, F. and Boettger, T. and D`Alessandro, C. M. and Daux, V. and Filot, M. and Grabner, M. and Gutierrez, E. and Haupt, M. and Hilasvuori, E. and Jungner, H. and Kalela-Brundin, M. and Krapiec, M. and Leuenberger, M. and Loader, N. J. and Marah, H. and Masson-Delmotte, V. and Pazdur, A. and Pawelczyk, S. and Pierre, M. and Planells, O. and Pukiene, R. and Reynolds-Henne, C. E. and Rinne, K. T. and Saracino, A. and Sonninen, E. and Stievenard, M. and Switsur, V. R. and Szczepanek, M. and Szychowska-Krapiec, E. and Todaro, L. and Waterhouse, J. S. and Weigl, M.}, issn = {1758-6798}, language = {eng}, number = {6}, pages = {579--579}, publisher = {Nature Research}, series = {Nature Climate Change}, title = {Water-use efficiency and transpiration across European forests during the Anthropocene}, url = {http://dx.doi.org/}, volume = {5}, year = {2015}, }

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[18]
Green J W, 1963. Wood Cellulose in Methods in Carbohydrate Chemistry. New York: Academic Press.

[19]
Gyenge J, Fernández M E, Varela S, 2012. Short- and long-term responses to seasonal drought in ponderosa pines growing at different plantation densities in Patagonia, South America.Trees, 26(6): 1905-1917.Abstract, ), transpiration (), photosynthesis (), wood 未C (as a proxy of intrinsic WUE), leaf to sapwood area ratio (:) and growth in the biggest () and the smallest () trees of high () and low () density stands. : was positively correlated with tree size and negatively correlated with competition level, increasing leaf hydraulic conductance in trees. Accordingly, higher and per unit were found in than in trees when soil water availability was high, but decreased abruptly during dry periods. BL trees maintained stable and values even during the summer drought. The functional adjustments observed in trees allow them to maintain their hydraulic integrity (no apparent losses), but their stem and leaf growth were severely affected by drought events. WUE was similar between all tree groups in a wet season, whereas it significantly decreased in SH trees in a dry season suggesting that when radiation and water are co-limiting gas exchange, functional adjustments not only affect absolute growth, but also WUE.

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[20]
Hietz P, Wanek W, Dünisch O, 2005. Long-term trends in cellulose δ13C and water-use efficiency of tropical Cedrela and Swietenia from Brazil.Tree Physiology, 25(6): 745-752.Abstract Elevated CO(2) concentrations ([CO(2)]) affect plant water relations and photosynthesis, and the increase in atmospheric [CO(2)] over the past 100-200 years has been related to changes in stomatal density and the carbon isotope ratio (delta(13)C) in tree rings and leaves from herbarium specimens. Because many tropical trees do not produce annual growth rings and their wood is therefore difficult to date, no trends in delta(13)C of tropical trees have been reported. Wood from Cedrela odorata L. (tropical cedar) and Swietenia macrophylla King (bigleaf mahogany), which do produce annual rings, was collected from a primary rain forest in Aripuan, Brazil (10 degrees 09' S, 59 degrees 26' W). We measured wood cellulose delta(13)C in 10-year growth increments from 37 Cedrela trees (between 11 and 151 years old in 2001) and 16 Swietenia trees (48-126 years old). A comparison of delta(13)C in cellulose of trees from different decades and of trees of different cambial ages showed that the amount of delta(13)C was largely related to the decade the wood was produced in, and not, or only to a minor extent, to tree age. Cellulose delta(13)C decreased from -26.0 to -27.3 per thousand in Cedrela and from -25.7 to -27.1 per thousand in Swietenia, with the largest changes occurring during the past 50 years. Based on these data and the trends in atmospheric [CO(2)] and delta(13)CO(2), we calculated that the internal [CO(2)] increased from about 220 to 260 ppm and that intrinsic water-use efficiency increased by 34% in Cedrela and by 52% in Swietenia. This may have implications for the water cycle and may explain the trend toward increased tree growth and turnover observed in some tropical forests.

DOI PMID

[21]
Holmes R L, 1983. Computer-assisted quality control in tree-ring dating and measurement.Tree-ring Bulletin, 43(1): 69-78.

[22]
Huang J G, Bergeron Y, Denneler B.et al, 2007. Response of forest trees to increased atmospheric CO2.Critical Reviews in Plant Sciences, 26(5/6): 265-283.

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[23]
Huang R, Zhu H, Liu X.et al, 2017. Does increasing intrinsic water use efficiency (iWUE) stimulate tree growth at natural alpine timberline on the southeastern Tibetan Plateau?Global and Planetary Change, 148: 217-226.Little is known about whether increasing iWUE (intrinsic water use efficiency) can stimulate tree growth in the temperature-limited natural timberlines. Here, we presented the basal area increment (BAI) and iWUE chronologies of Smith fir ( Abies georgei var. smithii ) from 1900 to 2006 at a high-elevation (ca. 4400 m a.s.l.) timberline in the humid Sygera Mountains, southeastern Tibetan Plateau (TP). The commonality analysis model was applied to investigate the relationships among BAI, temperatures, atmospheric CO 2 concentration ( C a ) and iWUE during 1961 2006, taking into account of both pure and joint effects. As illustrated by the commonality analysis model, the pure effect of C a (39.15%) had more stronger influence on iWUE than that of the Tmean (annul mean temperature, 0.12%), but the joint effect between C a and Tmean (49.79%) on iWUE was stronger than any pure effect for the raw data with an increasing trend. For the first-difference data with year-to-year variations, the pure effect of C a (7.72%) on iWUE was stronger than that of Tmean (0.59%) and the joint effect between them (0.59%). All above imply the C a is the dominant factor for iWUE both for the 46-year trend and interannual variations. In addition, as showed by the commonality analysis model, the pure effect of iWUE (17.57%) played a much more important role on BAI than that of temperatures (smt, mean temperature during June, July, August of current year, 5.92%; amt, mean temperature during September, October, November of previous year, 3.04%), while joint effects of iWUE and temperatures contributed more (27.96%; 13.90%; 16.47%) to the BAI than their pure effects for the raw data with an increasing linear trend. For the first-difference data with interannual variations, the pure effect of smt (12.45%) had much more effect on BAI than that of iWUE (5.49%), at the same time the joint iWUE and temperatures contributed less (3.56%; 1.9%; 1.31%) to the BAI than their pure effects. These results suggest that an increasing iWUE could enhance 46-year increasing tree growth trend at humid and high-elevation timberlines, supporting the CO 2 fertilization hypothesis, while temperatures dominate the interannual variations of tree growth.

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[24]
Johnson S E, Abrams M D, 2009. Basal area increment trends across age classes for two long-lived tree species in the eastern US.TRACE, 7: 127-134.

[25]
Kallarackal J, Roby T J, 2012. Responses of trees to elevated carbon dioxide and climate change.Biodiversity and Conservation, 21(5): 1327-1342.Abstract and tries to draw conclusions based on different methods used for the study. It also discusses the possible functional variations in some tree species due to climate change.

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[26]
Knapp P A, Soulé P T, Grissino M.et al, 2001. Detecting potential regional effects of increased atmospheric CO2 on growth rates of western juniper.Global Change Biology, 7(8): 903-917.Abstract Evidence of an atmospheric CO 2 fertilization effect on radial growth rates was uncovered by examining climate–growth relationships for seven western juniper tree-ring chronologies in central Oregon using multiple regression models. Consistent upward trends of the residuals from dendroclimatic models indicated a decreased ability for climate parameters to predict growth with time. Additionally, an assessment was made of whether enhanced growth was detectable under drought conditions, because a major benefit of elevated atmospheric CO 2 is the reduction of water stress. Mean ring indices were compared between ecologically comparable drought years, when atmospheric CO 2 was lower (1896–1949), and more recent drought years that occurred under higher atmospheric CO 2 concentrations (1950–96/98). The results presented herein show that: (i) residuals from climate/growth models had a significant positive trend at six of seven sites, suggesting the presence of a nonclimatic factor causing increased growth during recent decades; (ii) overall growth was 23% greater in the latter half of the 20th century; (iii) growth indices during matched drought and matched wet years were 63% and 30% greater, respectively, in the later 20th century than the earlier 20th century; and (iv) harsher sites had greater responses during drought periods between early and late periods. While it is not possible to rule out other factors, these results are consistent with expectations for CO 2 fertilization effects.

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[27]
Leavitt S W, 2010. Tree-ring c-h-o isotope variability and sampling.Science of The Total Environment, 408(22): 5244-5253.In light of the proliferation of tree-ring isotope studies, the magnitude and cause of variability of tree-ring δ 13C, δ 18O and δ 2H within individual trees (circumferential) and among trees at a site is examined in reference to field and laboratory sampling requirements and strategies. Within this framework, this paper provides a state-of-knowledge summary of the influence of “juvenile” isotope effects, ageing effects, and genetic effects, as well as the interchangeability of species, choice of ring segment to analyze (whole ring, earlywood or latewood), and the option of sample pooling. The range of isotopic composition of the same ring among trees at a site is ca. 1–3‰ for δ 13C, 1–4‰ δ 18O, and 5–30‰ for δ 2H, whereas the circumferential variability within a tree is lower. A standard prescription for sampling and analysis does not exist because of differences in field environmental circumstances and mixed findings represented in relevant published literature. Decisions in this regard will usually be tightly constrained by goals of the study and project resources. Sampling 4–6 trees at a site while avoiding juvenile effects in rings near the pith seems to be the most commonly used methodology, and although there are some reasoned arguments for analyzing only latewood and developing separate isotope records from each tree, the existence of some contradictory findings together with efforts to reduce cost and effort have prompted alternate strategies (e.g., most years pooled with occasional analysis of rings in the sequence separately for each tree) that have produced useful results in many studies.

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[28]
Li D, Fang K, Li Y.et al, 2017. Climate, intrinsic water-use efficiency and tree growth over the past 150 years in humid subtropical China. Plos One, 2017, 12(2): e0172045.Influence of long-term changes in climate and CO2 concentration on intrinsic water-use efficiency (iWUE), defined as the ratio between net photosynthesis (A) and leaf conductance (g), and tree growth remain not fully revealed in humid subtropical China, which is distinct from other arid subtropical areas with dense coverage of broadleaf forests. This study presented the first tree-ring stable carbon isotope (13C) and iWUE series of Pinus massoniana from 1865 to 2013 in Fujian province, humid subtropical China, and the first tree-ring width standard chronology during the period of 1836 2013 for the Niumulin Nature Reserve (NML). Tree-ring width growth was limited by precipitation in July-August (r = 0.40, p < 0.01). The tree-ring carbon isotope discrimination (13C) was mainly controlled by the sunshine hours (r = -0.66, p < 0.001) and relative humidity (r = 0.58, p < 0.001) in September-October, a season with rapid latewood formation in this area. The iWUE increased by 42.6% and the atmospheric CO2 concentration (ca) explained 92.6% of the iWUE variance over the last 150 years. The steady increase in iWUE suggests an active response with a proportional increase in intercellular CO2 concentration (ci) in response to increase in ca. The contribution of iWUE to tree growth in the study region is not conspicuous, which points to influences of other factors such as climate.

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[29]
Li Q F, Zhang S G, Wang J H, 2014. Transcriptome analysis of callus from Picea balfouriana,BMC Genomics, 15(1): 553.Background Picea likiangensis var. balfouriana (Rehd. et Wils.) Hillier ex Slavin (also known as Picea balfouriana) is an ecologically and economically important conifer that grows rapidly under optimum conditions and produces high-quality wood. It has a wide geographic distribution and is prevalent in southwest and eastern regions of China. Under suboptimal conditions, P. balfouriana grows slowly, which restricts its cultivation. Somatic embryogenesis has been used in the mass propagation of commercial species. However, low initiation rates are a common problem and the mechanisms involved in the induction of somatic embryogenesis are not fully understood. To understand the molecular mechanisms regulating somatic embryogenesis in P. balfouriana, high-throughput RNA-seq technology was used to investigate the transcriptomes of embryogenic and non-embryogenic tissues from three P. balfouriana genotypes. We compared the genes expressed in these tissues to identify molecular markers with embryogenic potential. Results A total of 55,078,846 nucleotide sequence reads were obtained for the embryogenic and non-embryogenic tissues of P. balfouriana, and 49.56% of them uniquely matched 22,295 (84.3%) of the 26,437 genes in the Picea abies genome database (Nature 497: 579-584, 2013). Differential gene expression analysis identified 1,418 differentially expressed genes (false discovery rate <0.0001; fold change ???2) in the embryogenic tissues relative to the non-embryogenic tissues, including 431 significantly upregulated and 987 significantly downregulated genes. KEGG (Kyoto Encyclopedia of Genes and Genomes) pathway analysis revealed that the most significantly altered genes were involved in plant hormone signal transduction, metabolic pathways (starch and sucrose metabolism), and phenylalanine metabolism. Conclusions We found that the initiation of embryogenic tissues affected gene expression in many KEGG pathways, but predominantly in plant hormone signal transduction, plant-pathogen interaction, and starch and sucrose metabolism. The changes in multiple pathways related to induction in the P. balfouriana embryogenic tissues described here, will contribute to a more comprehensive understanding of the mechanisms involved in the initiation of somatic embryogenesis. Additionally, we found that somatic embryogenesis receptor kinase (SERK), arabinogalactan proteins, and members of the WUS-related homeobox protein family may play important roles and could act as molecular markers in the early stage of somatic embryogenesis, as reported previously.

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[30]
Li Z S, Liu G H, Gong L.et al, 2015. Tree ring-based temperature reconstruction over the past 186 years for the Miyaluo Natural Reserve, western Sichuan Province of China.Theoretical and Applied Climatology, 120(3): 495-506.The mountain range of western Sichuan Province of China runs roughly north to south defining the eastern edge of the Tibetan Plateau, where high-resolution climate records are essential for understanding regional climatic phenomena. Unfortunately, instrumental records in this region are too short in duration to confidently gauge the long-term variability of climate change. This paper presented a temperature reconstruction for the western Sichuan Province based on a tree ring width chronology developed from a tree line site (4,15002m) of the Faxon fir ( Abies faxoniana ) at the Miyaluo Natural Reserve. This reconstruction, spanning the years from 1824 to 2009, could account for 46.702% of the actual variance of annual mean temperature during the calibration period from 1950 to 2002. The reconstruction could be essentially divided into two distinct subperiods: a relatively cold and stable period in the late nineteenth century and a relatively warm and unstable period in the twentieth century. Years 2001 and 1911 were the warmest (6.3202°C) and coldest (4.8702°C) years in the reconstruction, respectively, while 1960s (5.7702°C) and 1980s (5.0802°C) were the warmest and coldest ten consecutive years within the past 18602years. Close coupling observed with other temperature proxies (tree rings, ice cores, and glaciers) from surrounding areas emphasized the high degree of confidence in our reconstruction.

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[31]
Linares J C, Delgado H A, Camarero J J.et al, 2009. Competition and drought limit the response of water-use efficiency to rising atmospheric carbon dioxide in the Mediterranean fir Abies pinsapo.Oecologia, 161(3): 611-624.The gas-exchange and radial growth responses of conifer forests to climatic warming and increasing atmospheric CO60 have been widely studied. However, the modulating effects of variables related to stand structure (e.g., tree-to-tree competition) on those responses are poorly explored. The basal-area increment (BAI) and C isotope discrimination (C stable isotope ratio; δ0106C) in the Mediterranean fir Abies pinsapo were investigated to elucidate the influences of stand competition, atmospheric CO60 concentrations and climate on intrinsic water-use efficiency (WUEi). We assessed the variation in δ0106C of tree-rings from dominant or co-dominant trees subjected to different degrees of competition. A high-(H) and a lowelevation (L) population with contrasting climatic constraints were studied in southern Spain. Both populations showed an increase in long-term WUEi. However, this increase occurred more slowly at the L site, where a decline of BAI was also observed. Local warming and severe droughts have occurred in the study area over the past 30 years, which have reduced water availability more at lower elevations. Contrastingly, trees from the H site were able to maintain high BAI values at a lower cost in terms of water consumption. In each population, trees subjected to a higher degree of competition by neighboring trees showed lower BAI and WUEi than those subjected to less competition, although the slopes of the temporal trends in WUEi were independent of the competitive microenvironment experienced by the trees. The results are consistent with an increasing drought-induced limitation of BAI and a decreasing rate of WUEi improvement in lowelevation A. pinsapo forests. This relict species might not be able to mitigate the negative effects of a decrease in water availability through a reduction in stomatal conductance, thus leading to a growth decline in the more xeric sites. An intense and poorly asymmetric competitive environment at the stand level may also act as an important constraint on the adaptive capacity of these drought-sensitive forests to climatic warming.

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[32]
Liu X H, Wang W Z, Xu G B.et al, 2014. Tree growth and intrinsic water-use efficiency of inland riparian forests in northwestern China: evaluation via δ13C and δ18O analysis of tree rings.Tree Physiology, 34(6): 966-980.中国科学院机构知识库(CAS IR GRID)以发展机构知识能力和知识管理能力为目标，快速实现对本机构知识资产的收集、长期保存、合理传播利用，积极建设对知识内容进行捕获、转化、传播、利用和审计的能力，逐步建设包括知识内容分析、关系分析和能力审计在内的知识服务能力，开展综合知识管理。

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[33]
Loader N J, Robertson I, Barker A C.et al, 1997. An improved technique for the batch processing of small whole wood samples to α-cellulose.Chemical Geology, 136(3): 313-317.We present details of a modified technique for the extraction of α-cellulose from wood samples. The revised technique, based upon the sodium chlorite oxidation method of Green (1963) utilises an ultrasonic bath and small Soxhlet thimbles to prepare α-cellulose from slivers of wholewood. These modifications facilitate the rapid batch processing of small wholewood samples to α-cellulose and yield a material with sufficient homogeneity as required for palaeoclimatic reconstruction.

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[34]
Loader N J, Santillo P M, Woodman-Ralph J P.et al, 2008. Multiple stable isotopes from oak trees in southwestern Scotland and the potential for stable isotope dendroclimatology in maritime climatic regions.Chemical Geology, 252(1): 62-71.Across much of Europe, Eurasia and N. America there exist networks of long tree-ring chronologies which, under favourable circumstances, may be used to provide a record of palaeoclimate information. A proportion of these tree-ring archives, primarily those collected for archaeological dating purposes, represent a significant and largely untapped palaeoenvironmental archive. Such records may be unsuitable for palaeoclimatic reconstruction based solely upon their physical characteristics (ring width and density) owing to weak or poorly expressed climatic forcing. This is especially true of oak chronologies from maritime regions. This study explores the potential for extracting a climate signal from such chronologies by comparing the stable isotope ratios of C, H and O from the rings of common oak ( Quercus robur) trees in southwestern Scotland, with local and regional meteorological data. Summer (growing season) climate influences all three isotopes and the relationships identified are consistent with published empirical and mechanistic studies. The climate signal appears strongest in O and weakest in H. The C and O series, in combination, explain 31% of the variance in July–August mean temperature measured locally and 26% when compared with a homogenised gridded dataset for the period AD1957-2002. Over longer timescales the combination of C and O isotopes may also preserve a significant low-frequency signal (July–August r 2 = 0.57, 9-year running mean AD1879–1998). These findings demonstrate the potential of stable isotope dendroclimatology for investigating climatic change from oak chronologies in maritime regions.

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[35]
Loader N J, Young G H F, McCarroll D.et al, 2013. Quantifying uncertainty in isotope dendroclimatology.The Holocene, 23(9): 1221-1226.

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[36]
McCarroll D, Loader N J, 2004. Stable isotopes in tree rings.Quaternary Science Reviews, 23(7): 771-801.

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[37]
Newberry T L, 2010. Effect of climatic variability on δ13C and tree-ring growth in piñon pine (Pinus edulis). Trees, 24(3): 551-559.

[38]
Nock C A, Baker P J, Wanek W.et al, 2011. Long-term increases in intrinsic water-use efficiency do not lead to increased stem growth in a tropical monsoon forest in western Thailand.Global Change Biology, 17(2): 1049-1063.Rising atmospheric carbon dioxide [CO2] can accelerate tree growth by stimulating photosynthesis and increasing intrinsic water-use efficiency (iWUE). Little evidence exists, however, for the long-term growth and gas-exchange responses of mature trees in tropical forests to the combined effects of rising [CO2] and other global changes such as warming. Using tree rings and stable isotopes of carbon and oxygen, we investigated long-term trends in the iWUE and stem growth (basal area increment, BAI) of three canopy tree species in a tropical monsoon forest in western Thailand (Chukrasia tabularis, Melia azedarach, and Toona ciliata). To do this, we modelled the contribution of ontogenetic effects (tree diameter or age) and calendar year to variation in iWUE, oxygen isotopes, and BAI using mixed-effects models. Although iWUE increased significantly with both tree diameter and calendar year in all species, BAI at a given tree diameter was lower in more recent years. For one species, C. tabularis, differences in crown dominance significantly influence stable isotopes and growth. Tree ring Δ18O increased with calendar year in all species, suggesting that increasing iWUE may have been driven by relatively greater reductions in stomatal conductance – leading to enrichment in Δ18O – than increases in photosynthetic capacity. Plausible explanations for the observed declines in growth include water stress resulting from rising temperatures and El Ni09o events, increased respiration, changes in allocation, or more likely, a combination of these factors.

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[39]
Norby R J, Zak D R, 2011. Ecological lessons from free-air CO2 enrichment (FACE) experiments. Annual Review of Ecology, Evolution, and Systematics, 42(1): 181.Free-air CO{sub 2} enrichment (FACE) experiments have provided novel insights into the ecological mechanisms controlling the cycling and storage of carbon in terrestrial ecosystems and contribute to our ability to project how ecosystems respond to increasing CO{sub 2} in the Earth's atmosphere. Important lessons emerge by evaluating a set of hypotheses that initially guided the design and longevity of forested FACE experiments. Net primary productivity is increased by elevated CO{sub 2}, but the response can diminish over time. Carbon accumulation is driven by the distribution of carbon among plant and soil components with differing turnover rates and by interactions between the carbon and nitrogen cycles. Plant community structure may change, but elevated CO{sub 2} has only minor effects on microbial community structure. FACE results provide a strong foundation for next-generation experiments in unexplored ecosystems and inform coupled climate-biogeochemical models of the ecological mechanisms controlling ecosystem response to the rising atmospheric CO{sub 2} concentration.

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[40]
Morén A S, Lindroth A, Grelle A, 2001. Water-use efficiency as a means of modelling net assimilation in boreal forests.Trees, 15(2): 67-74.Although the processes governing photosynthesis are well understood, scaling from shoot to canopy in coniferous forests is complex. Development of different sap-flow techniques has made it possible to measure transpiration of whole trees and thereby also of whole canopies. There is a strong link between photosynthesis and transpiration, for which reason it would be interesting to test whether measurements of canopy transpiration could also be used to estimate canopy photosynthesis. As a first step towards this, water-use efficiency (WUE) was studied at branch and canopy scales on the basis of branch gas-exchange measurements, with half-hourly and daily temporal resolution. Half-hourly and daily WUE at both branch and canopy scales showed a strong dependency on vapour-pressure deficit ( e) . Branch photosynthesis modelled from branch transpiration and e mimicked well measured branch photosynthesis. Also, modelled photosynthesis, scaled to canopy and compared to net forest CO 2 exchange measured by the eddy-covariance technique, occasionally showed good agreement. In spite of these seemingly promising results, there was a difference in the response to e between branches and between years, which needs to be better understood.

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[41]
Peñuelas J, Canadell J G, Ogaya R, 2011. Increased water-use efficiency during the 20th century did not translate into enhanced tree growth.Global Ecology and Biogeography, 20(4): 597-608.

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[42]
Saurer M, Siegwolf R, Schweingruber F H, 2004. Carbon isotope discrimination indicates improving water-use efficiency of trees in northern Eurasia over the last 100 years.Global Change Biology, 10(12): 2109-2120.

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[43]
Silva L C R, Anand M, Leithead M D, 2010. Recent widespread tree growth decline despite increasing atmospheric CO2.Plos One, 5(7): e11543.Abstract BACKGROUND: The synergetic effects of recent rising atmospheric CO(2) and temperature are expected to favor tree growth in boreal and temperate forests. However, recent dendrochronological studies have shown site-specific unprecedented growth enhancements or declines. The question of whether either of these trends is caused by changes in the atmosphere remains unanswered because dendrochronology alone has not been able to clarify the physiological basis of such trends. METHODOLOGY/PRINCIPAL FINDINGS: Here we combined standard dendrochronological methods with carbon isotopic analysis to investigate whether atmospheric changes enhanced water use efficiency (WUE) and growth of two deciduous and two coniferous tree species along a 9 degrees latitudinal gradient across temperate and boreal forests in Ontario, Canada. Our results show that although trees have had around 53% increases in WUE over the past century, growth decline (measured as a decrease in basal area increment--BAI) has been the prevalent response in recent decades irrespective of species identity and latitude. Since the 1950s, tree BAI was predominantly negatively correlated with warmer climates and/or positively correlated with precipitation, suggesting warming induced water stress. However, where growth declines were not explained by climate, WUE and BAI were linearly and positively correlated, showing that declines are not always attributable to warming induced stress and additional stressors may exist. CONCLUSIONS: Our results show an unexpected widespread tree growth decline in temperate and boreal forests due to warming induced stress but are also suggestive of additional stressors. Rising atmospheric CO2 levels during the past century resulted in consistent increases in water use efficiency, but this did not prevent growth decline. These findings challenge current predictions of increasing terrestrial carbon stocks under climate change scenarios.

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[44]
Silva L C R, Anand M, 2013. Probing for the influence of atmospheric CO2 and climate change on forest ecosystems across biomes.Global Ecology and Biogeography, 22(1): 83-92.AimRising atmospheric CO 2 and climate warming have induced changes in tree growth and intrinsic water-use efficiency (iWUE) world-wide, but the long-term impact of such changes on terrestrial productivity remains unknown. Based on a synthesis of the literature, here we investigate the net impact of recent atmospheric changes across forest biomes.LocationA range of sites covering major forest biomes.MethodsWe use dendrochronological and isotopic records to provide an integrated analysis of changes in growth and iWUE, evaluating the impacts of atmospheric changes in tree growth. In our analysis, positive relationships between changes in growth and iWUE reflect CO 2 stimulation, while neutral effects yield inflections in growth curves (plotted against iWUE), and negative relationships indicate the prevalence of stressors. To estimate net effects (since 1960) and compare responses across biomes, we use a response contrast (RC) index, based on the ratio between cumulative changes in growth and iWUE.ResultsIn 37 recently published case studies changes in iWUE were consistently positive, increasing by between 10 and 60%, but shifts in growth varied widely within and among forest biomes. Positive RC values were observed in high latitudes (>6540°N), while progressively lower (always negative) responses were observed toward lower latitudes. Growth rates declined between 15 and 55% in tropical forests. In subtropical sites growth declined by between 7 and 10%, while mixed responses occurred in other regions.Main conclusionsOver the past 50 years, tree growth decline has prevailed despite increasing atmospheric CO 2. The impact of atmospheric changes on forest productivity is latitude dependent (R 265=650.9, P65<650.05), but our results suggest that, globally, CO 2 stimulation of mature trees will not counteract emissions. In most surveyed case studies warming-induced stress was evoked to explain growth decline, but other factors, such as nutrient limitation, could have overridden the potential benefits of rising CO 2 levels.

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[45]
Silva L C R, Horwath W R, 2013. Explaining global increases in water use efficiency: why have we overestimated responses to rising atmospheric CO2 in natural forest ecosystems?PloS One, 8(1): e53089.The analysis of tree-ring carbon isotope composition (13C) has been widely used to estimate spatio-temporal variations in intrinsic water use efficiency (iWUE) of tree species. Numerous studies have reported widespread increases in iWUE coinciding with rising atmospheric CO2 over the past century. While this could represent a coherent global response, the fact that increases of similar magnitude were observed across biomes with no apparent effect on tree growth raises the question of whether iWUE calculations reflect actual physiological responses to elevated CO2 levels. Here we use Monte Carlo simulations to test if an artifact of calculation could explain observed increases in iWUE. We show that highly significant positive relationships between iWUE and CO2 occur even when simulated data (randomized 13C values spanning the observed range) are used in place of actual tree-ring 13C measurements. From simulated data sets we calculated non-physiological changes in iWUE from 1900 to present and across a 4000 m altitudinal range. This generated results strikingly similar to those reported in recent studies encompassing 22 species from tropical, subtropical, temperate, boreal and mediterranean ecosystems. Only 6 of 49 surveyed case studies showed increases in iWUE significantly higher than predicted from random values. Our results reveal that increases in iWUE estimated from tree-ring 13C occur independently of changes in 13C discrimination that characterize physiological responses to elevated CO2. Due to a correlation with CO2 concentration, which is used as an independent factor in the iWUE calculation, any tree-ring 13C data set would inevitably generate increasing iWUE over time. Therefore, although consistent, previously reported trends in iWUE do not necessarily reflect a coherent global response to rising atmospheric CO2. We discuss the significance of these findings and suggest ways to distinguish real from artificial responses in future studies.

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[46]
Silva L C R, Sun G, Xia Z B.et al, 2016. Tree growth acceleration and expansion of alpine forests: The synergistic effect of atmospheric and edaphic change.Science Advances, 2(8): e1501302.Soil-plant-atmosphere interactions regulate the impact of climate on forest ecosystems. Many forest ecosystems have experienced recent declines in productivity; however, in some alpine regions, tree growth and forest expansion are increasing at marked rates. Dendrochronological analyses at the upper limit of alpine forests in the Tibetan Plateau show a steady increase in tree growth since the early 1900s, which intensified during the 1930s and 1960s, and have reached unprecedented levels since 1760. This recent growth acceleration was observed in small/young and large/old trees and coincided with the establishment of trees outside the forest range, reflecting a connection between the physiological performance of dominant species and shifts in forest distribution. Measurements of stable isotopes (carbon, oxygen, and nitrogen) in tree rings indicate that tree growth has been stimulated by the synergistic effect of rising atmospheric CO2and a warming-induced increase in water and nutrient availability from thawing permafrost. These findings illustrate the importance of considering soil-plant-atmosphere interactions to understand current and anticipate future changes in productivity and distribution of forest ecosystems.

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[47]
Turnbull M H, Whitehead D, Tissue D T.et al, 2001. Responses of leaf respiration to temperature and leaf characteristics in three deciduous tree species vary with site water availability.Tree Physiology, 21(9): 571-578.

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[48]
Waterhouse J S, Switsur V R, Barker A C.et al, 2004. Northern European trees show a progressively diminishing response to increasing atmospheric carbon dioxide concentrations.Quaternary Science Reviews, 23(7): 803-810.In order to predict accurately how elevated atmospheric CO 2 concentrations will affect the global carbon cycle, it is necessary to know how trees respond to increasing CO 2 concentrations. In this paper, we examine the response over the period AD 1895 1994 of three tree species growing across northern Europe to increases in atmospheric CO 2 concentrations using parameters derived from stable carbon isotope ratios of trunk cellulose. Using the isotope data we calculate values of intrinsic water-use efficiency (IWUE) and intercellular CO 2 concentrations in the leaf ( c i). Our results show that trees have responded to higher levels of atmospheric CO 2 by increasing IWUE whilst generally maintaining constant c i values. However, the IWUE of most of the trees in this study has not continued to rise in line with increasing atmospheric CO 2. This behaviour has implications for estimations of future terrestrial carbon storage.

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[49]
Wang W Z, Liu X H, An W L.et al, 2012. Increased intrinsic water-use efficiency during a period with persistent decreased tree radial growth in northwestern China: Causes and implications.Forest Ecology and Management, 275(1): 14-22.To understand how tree growth responds to rising atmospheric CO2 concentration ([CO2]) and climate change in arid and semi-arid regions, it is important to determine how trees in natural forests adjust physiologically to the changing atmospheric environment. Here, we developed an annual-resolution tree-ring stable carbon isotope (13C) chronology from the dominant natural spruce (Picea crassifolia) trees in the Xinglong Mountains, in the eastern part of northwestern China, from 1800 to 2009. The climate response recorded in the tree-ring 13C residuals indicated that insufficient moisture during the previous autumn and the current growing season was the dominant climatic factor that controlled tree-ring carbon isotope discrimination in the semi-arid Xinglong Mountains. By combining the tree-ring 13C, the atmospheric [CO2] (Ca), and ring width data, and by calculating the variations in leaf intercellular [CO2] (Ci), their ratio (Ci/Ca), intrinsic water-use efficiency (iWUE), and basal area increment (BAI), we found that the Ci/Ca remained mostly constant during the last century whereas Ci and iWUE increased significantly. The increase in iWUE totaled about 40% by the end of the study period. However, after 1998, the trees switched from an active to a passive response to elevated atmospheric [CO2], leading to relatively stable iWUE. Since 1800, tree radial growth did not always parallel the increase in iWUE; the tree growth rate declined during drought periods, and especially during the severe droughts that occurred from 1923 to 1934 and recent decade. This response is a concern for future forest management and conservation under the current climate change scenario.

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[50]
West P W, 1980. Use of diameter increment and basal area increment in tree growth studies.Canadian Journal of Forest Research, 10(1): 71-77.The growth of trees was studied in pure and mixed stands of even-aged, regrowth forest of Eucalyptus regnans, E. obliqua and E. globulus, aged 6-80 yr, thinned and unthinned, in SE Tasmania. Weighted least squares regression equations were developed to relate diam. and b.a. increments, over 1- to 6-yr increment periods, to initial tree diam. in 29 growth plots. The correlation between b.a. incr...

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[51]
Wu G J, Liu X H, Chen T.et al, 2015. Long-term variation of tree growth and intrinsic water-use efficiency in Schrenk spruce with increasing CO2 concentration and climate warming in the western Tianshan Mountains, China.Acta Physiologiae Plantarum, 37(8): 150.

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[52]
Xu G B, Liu X H, Qin D H.et al, 2013. Climate warming and increasing atmospheric CO2 have contributed to increased intrinsic water-use efficiency on the northeastern Tibetan Plateau since 1850.Trees, 27(2): 465-475.We investigated the physiological responses of Tibetan juniper (Sabina tibetica) to changes in the atmospheric CO2 concentration (C (a)) and climate on the northeastern Tibetan Plateau based on annual tree-ring delta C-13 values since 1850. Intrinsic water-use efficiency (iWUE) increased, and the internal to ambient CO2 ratio (C (i) /C (a)) showed no significant trend from 1895 to 1974 in the study region, indicating an active response to changing C (a). The long-term trends in iWUE in the naturally occurring trees were mainly caused by the anthropogenic increase in C (a). However, from 1975 to 2002, iWUE increased rapidly at the study site (by 12.4 % compared with the overall mean from 1850 to 2002), which is greater than the expected increase due only to an active response to C (a). Our analysis showed that decreased water availability caused by greater evaporation due to decreased precipitation and a warming growth environment from 1975 to 2002 may have reduced stomatal conductance, leading to a higher iWUE. The warming climate and increased C (a) accounted for 83.6 % of the variance in iWUE of Tibetan juniper on the northeastern Tibetan Plateau from 1975 to 2002.

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