Geographically, the Qinling-Daba Mountains serve as the main body of the north-south transitional zone of China. However, the transitional patterns of their plant species still need to be clarified. This study analyzed latitudinal variations of plant species richness, relative importance values (RIV), and plant species abundance based on plant community field survey data for 163 sample sites along three north-south transect lines in the eastern, middle, and western parts of the study areas. The difference in RIV between subtropical and temperate species (SND-RIV) was selected to reveal the latitudinal interlacing pattern of northern and southern plant species. Along the eastern (Sanmenxia-Yichang), middle (Xi’an-Dazhou), and western (Tianshui-Guangyuan) transects, the richness and RIV of subtropical plant species increased while those of temperate plant species decreased from north to south. In the eastern transect, temperate plant species richness and RIV were the highest at Shennongjia and Funiu Mountain, respectively, because of their high elevations. In the middle transect, subtropical plant species richness and RIV were the highest in the Daba Mountains. In the western transect, richness and RIV were higher for subtropical than temperate plant species from the south of Longnan. The crisscrossing areas of northern and southern plant species were ~180 km, ~100 km, and ~60 km wide for the eastern, middle, and western transects, respectively, showing a narrowing trend from east to west. For the eastern and western transects, decreases in subtropical plant species distribution from south to north could be attributed to a decrease in mean annual precipitation in the same direction. However, for the middle transect, mean annual temperature had a slightly greater influence on plant species’ latitudinal distribution than the moisture index. This study provides a more solid scientific basis for future investigations of this key geographical boundary in China.
The trend estimate of vegetation change is essential to understand the change rule of the ecosystem. Previous studies were mainly focused on quantifying trends or analyzing their spatial distribution characteristics. Nevertheless, the uncertainties of trend estimates caused by spatiotemporal scale effects have rarely been studied. In response to this challenge, this study aims to investigate spatiotemporal scale effects on trend estimates using Moderate-Resolution Imaging Spectroradiometer (MODIS) Normalized Difference Vegetation Index (NDVI) and Gross Primary Productivity (GPP) products from 2001 to 2019 in the Qinghai-Tibet Plateau (QTP). Moreover, the possible influencing factors on spatiotemporal scale effect, including spatial heterogeneity, topography, and vegetation types, were explored. The results indicate that the spatial scale effect depends more on the dataset with a coarser spatial resolution, and temporal scale effects depend on the time span of datasets. Unexpectedly, the trend estimates on the 8-day and yearly scale are much closer than that on the monthly scale. In addition, in areas with low spatial heterogeneity, low topography variability, and sparse vegetation, the spatiotemporal scale effect can be ignored, and vice versa. The results in this study help deepen the consciousness and understanding of spatiotemporal scale effects on trend detection.
The impacts of climate change on the relationship between fluvial processes and dune landform evolution have been studied. However, the chronology data used to examine this relationship are deficient. The Keriya River has a glacial origin in the Kunlun Mountains on the south margin of the Tarim Basin. The river flows into the Taklimakan Desert, the second largest shifting-dune desert in the world. The dry channels and shifting dunes in this area provide an ideal opportunity to investigate fluvial and aeolian landform evolution processes and their relationship with climate change. We investigated this area during 2008-2011 and obtained 18 fluvial sediment samples from 16 sections for optically stimulated luminescence (OSL) dating. The results show that the ages ranged from 3.4-44.1 ka. Most of the samples (13) were Holocene in age, around 11 ka, 8-9 ka, 5-6.5 ka, 4.6 ka, and 3.4-3.7 ka and were distributed along ancient river channels around sites of Yuansha and Karadun. Two samples close to the Hotan River (38-47 ka) fall within the Marine Isotope Stage 3 (MIS3). Three samples (from one section) were located near ancient channels flowing towards the Yuansha Site and had ages of around 14.5 ka, i.e., during the Last Glacial Maximum (LGM). The analyses of the sediment samples and OSL ages suggest that the Keriya River flooded in the Holocene, the LGM, and MIS3. Fluvial sediments provided the source material for the dunes, and fluvial processes affected the landform evolution in the lower Keriya River. Our results suggest that most of the dunes covered in fluvial sediments in the lower reaches and the area west of the Keriya River developed since the Holocene. This differs from the results of previous studies, which suggested that they developed since the Han (202BC-220AD) and Tang (618-907AD) dynasties. The OSL ages of the fluvial sediments are consistent with the reported deglaciation (after glacial advance) ages in the alpine mountains surrounding the Tarim Basin. This suggests that climate fluctuations may have affected the occurrence of floods and the formation of dunes in the Taklimakan Desert.
Most terrestrial models synchronously calculate net primary productivity (NPP) using the input climate variable, without the consideration of time-lag effects, which may increase the uncertainty of NPP simulation. Based on Normalized Difference Vegetation Index (NDVI) and climate data, we used the time lag cross-correlation method to investigate the time-lag effects of temperature, precipitation, and solar radiation in different seasons on NDVI values. Then, we selected the Carnegie-Ames-Stanford approach (CASA) model to estimate the NPP of China from 2002 to 2017. The results showed that the response of vegetation growth to climate factors had an obvious lag effect, with the longest time lag in solar radiation and the shortest time lag in temperature. The time lag of vegetation to the climate variable showed great tempo-spatial heterogeneities among vegetation types, climate types, and vegetation growth periods. Based on the validation using eddy covariance data, the results showed that the simulation accuracy of the CASA model considering the time-lag effects was effectively improved. By considering the time-lag effects, the average total amount of NPP modeled by CASA during 2001-2017 in China was 3.977 PgC a-1, which is 11.37% higher than that of the original model. This study highlights the importance of considering the time lag for the simulation of vegetation growth, and provides a useful tool for the improvement of the vegetation productivity model.
The construction of channel regulation projects, reservoirs, and other human activities have led to significant changes in channel geometry and hydrodynamic conditions in mountainous macrotidal estuaries. However, their impact on the long-term evolution of the turbidity maximum zone (TMZ) in these estuaries is still unclear. Therefore, the Minjiang Estuary (ME) was selected as the study area and using the Gabor filter and surface suspended sediment concentration (SSSC) data retrieved from GF PMS/WFV and Landsat-TM/ETM+/ OLI images in the flood season from 1986 to 2020, the flow direction of Chuanshi Waterway, the spatiotemporal evolution characteristics of TMZ in the ME, and the influence of human activities on these were analyzed. The results indicate that during flood tides in the past 35 years, the TMZ was mainly distributed in sections from the Changmen to the Chuanshi and Meihua waterways. The construction of the Shuikou Reservoir caused the SSSC to decrease by 65 mg/L at the Chuanshi Tidal Gauge Station in the ME. The TMZ in the ME waterway channel notably migrated toward the sea due to the waterway regulation project, with the landward and seaward boundaries moving by 2.5 km and 3 km seaward, respectively. The main distribution area moved from Jinpaimen to the section from Chuanshi Waterway to the mouth of the ME. These variation characteristics were basically consistent with the annual average TMZ in the flood season. Through the interactions between nature and human interventions, the flow regime of the ME tended to converge in the flood season. Therefore, human activities have significantly impacted the long-term evolution of the TMZ in the ME.
Understanding the hydrological effects of the Three Gorges Dam operation in the entire reservoir area is significant to achieving optimal dam regulation. In this paper, a large-scale coupled hydrological-hydrodynamic-dam operation model is developed to comprehensively evaluate the hydrological effects of the river-type Three Gorges Reservoir. The results show that the coupled model is effective for hydrological, hydrodynamic regime and hydropower simulations in the reservoir area. Dam operation could have a notable positive effect on flood control and could reduce the maximum daily flood peak by up to 26.2%. It also contributes a large amount of hydropower, approximately 94.27 TWh/year, and a water supply increase for the downstream area of up to 22% during the dry season. In the flood season, the water level at Cuntan would increase under the condition that the water level of the dam is higher than approximately 158 m due to dam operation. In the dry season, attention should be paid to the low flow velocity near the dam in the reservoir area.
Given the challenges of re-creating complex bed load (BL) transport processes in rivers, models are preferred over gathering and examining field data. The highlight of the present research is to develop an approach to determine the ungauged bed load concentration (BLCu) utilizing the measured suspended sediment concentration (SSC) and hydraulic variables of the last four decades for the Mahanadi River Basin. This technique employs shear stress and SSC equations for turbulent open channel flow. Besides, the predicted BLCu is correlated with SSC using a power relation to estimate BLCu on the river and tributaries. Eventually, different BL functions (BLF) efficiency is assessed across stations. The model predicted BLCu is comparable with the published data for sandy rivers and falls within ± 20%. Outliers in hydraulic and sedimentological statistics significantly influence estimating the BL fraction apart from higher relative ratios and catchment geology. The constants of power functions are physically linked to sediment transport configuration, mechanism, and inflow to the stream. The stream power-based BLF best predicts the BL transport, followed by shear stress and unit discharge approaches. The disparity in the estimation of BLCu results from station-specific physical factors, sampling data dispersion, and associated uncertainties.
Scientific field management is an important path to realize ecological production and sustainable development of agriculture. As the main content of field management, nitrogen (N) management is the key to balance the economic and ecological benefits of agricultural production. In the loess hilly-gully region, for the fragile ecological and social system, ecologicalization of agricultural production is an important direction to promote sustainable agricultural development. However, irrational fertilization has been one of the main constraint factors, hindering the ecologicalization of local agriculture. In order to solve the problem and prove the practical significance of field management to ecologicalization of agriculture, this study aimed at evaluating the effects of different N fertilization rates and timing using Root Zone Water Quality Model (RZWQM) and then optimizing the N management. Experiments were conducted from 2018 to 2019 in Yangjuangou watershed, loess hilly-gully region, to calibrate and validate the model. The root mean square error (RMSE) of soil water content, nitrate N concentration, above-ground biomass, leaf area index ranged from 10.5-13.5 mm, 2.96-3.80 mg·kg–1, 730.3-1273.9 kg·ha–1 and 0.26-0.38, respectively, with the agreement index (d) between observed and simulated values ranging between 0.88 to 0.98. Simulation results showed that N leaching in semi-arid areas was also quite high due to concentrated rainfall and loose soil, which had previously been neglected. When the fertilization rate decreased by 35% (applying the chemical fertilizer at rate of 245.7 kg N ha–1) of typical fertilization (applying the chemical fertilizer at rate of 378.0 kg N ha–1), the leaching and residual N decreased by 72.2%-75.4% and 35.6%-50.9%, respectively, while NUE increased by 41.5%-45.2% with no reduction in maize yield. Additionally, compared with applying additional N at seedling stage in one batch, applying at seedling and jointing stages in two batches further decreased N leaching and improved NUE. Thus, a 35% reduction of typical fertilization rate combined with applying additional N at seedling and jointing stages is recommended. From the perspective of N management, this study demonstrated optimizing field management can play a positive role in the ecologicalization of agriculture, and more field management measures should be explored.
Identifying the spatiotemporal interaction pattern of agricultural product circulation (APC) is crucial for agricultural resource adjustment and food security. Current studies are mostly based on static statistical data over an entire year or a specific period, which cannot describe the spatial pattern of APC and its seasonal variation on a fine spatiotemporal scale. Thus, this study extracts an APC trip chain based on national truck trajectory data and constructs the flow network of the Beijing APC with the city as the spatial unit and the season as the temporal unit. The spatial interaction pattern and seasonal variation in APC are then analyzed from the network spatial form, city node function role, and transportation corridors. The results are as follows: (1) Compared with methods based on static statistical data, the proposed method provides a more complete and refined depiction of the spatiotemporal interaction pattern of APC. (2) The flow network of the Beijing APC involves 316 cities in China, of which 143 cities play a major role with typical seasonal characteristics. These cities can be divided into perennial core cities, perennial major cities, core cities in winter-spring, major cities in winter-spring, core cities in summer-autumn, and major cities in summer-autumn, contributing 2.6%-40.3% to the Beijing APC. (3) There are 6 transportation corridors for the Beijing APC. The Beijing-Tianjin-Hebei corridor and coastal corridor contribute 53.5% and 12.8% of the annual supply, respectively, with a balanced supply in all seasons. The Beijing-Kunming corridor and Beijing-Guangzhou corridor contribute 14.3% and 9.0%, respectively, with much higher supplies in winter and spring. The northeast and northwest corridors contribute 7.3% and 3.3%, respectively, mainly in the summer and autumn. These results help deepen the understanding of agricultural product supply patterns and provide a reference for the design and optimization of agricultural product transportation routes.
As a type of urban gentrification oriented by high-quality educational resources, jiaoyufication is a socio-spatial process that refers to the middle-class group that attaches importance to education realizes the agglomeration in famous school districts by purchasing high-price school district houses. Based on the theoretical analysis of jiaoyufication conducted by Chinese and foreign scholars, this paper takes Nanjing as a case city, applies multi-source data like POIs (points of interest), real estate market data, mobile-phone user portraits and questionnaires, and develops a composite measurement of jiaoyufication to identify jiaoyufied school district, jiaoyufied group & its spatial characteristics, and discusses the causal mechanisms and effects of jiaoyufication. With the GIS-entropy-TOPSIS model, this paper evaluates the jiaoyufication level of public primary school districts in the main urban area of Nanjing. The result show that 218 primary school districts are categorized into four types, i.e., high jiaoyufied school district, mid-high jiaoyufied school district, medium jiaoyufied school district, and low jiaoyufied school district. The high jiaoyufied school districts are closely associated with the institutional system. In the high jiaoyufied school districts, many middle-class families have abundant social, economic, and cultural capital. They purchase and move to houses with lower living quality in school districts to pursue high-quality education for their children. The strict school district system, soaring school district housing prices, and intense educational competition continuously solidify the jiaoyufication levels of famous school districts and lock the opportunities for high-quality education in specific school-district spaces and classes or groups. The phenomenon of this socio-spatial reconstruction process, which results from the unequal distribution of high-quality educational resources, tends to aggravate the rich-poor gap and social segregation in cities. It is suggested that equalization of compulsory education services should be effectively carried forward as soon as possible.
Urban shrinkage is the process of population loss, industrial and functional decline, and eco-environmental deterioration due to various natural and human factors that occur once a city’s development reaches a certain point. It is an external manifestation of positive or negative changes in an urban economy, society, culture, resource use, and sustainability. Urban shrinkage is a new feature of global urbanization as well as a frontier of international and Chinese research. It has attracted widespread attention from academics in China and overseas, becoming an emerging trend that has spread from developed countries to underdeveloped ones. Research on and responses to urban shrinkage have also become new tasks of China’s new urbanization strategy. Our review of the existing literature reveals that there are significant temporal and spatial differences in research by Chinese and overseas scholars on urban shrinkage. Urban shrinkage studies have paid scant attention to interactions with the eco-environment, with analyses of urban shrinkage mechanisms and factors largely focusing on economic and social development factors; and urban shrinkage index analyses have tended not to include eco-environmental quality and environmental pollution indicators. The key tasks for future urban shrinkage research include the following: to conduct in-depth research on the mechanisms and driving factors of urban shrinkage to reveal the essence of urban shrinkage; to discover the principles behind interactions between urban shrinkage and the eco-environment, as well as to analyze the eco-environmental effects of urban shrinkage; to construct an index system for evaluating the extent, monitoring and providing warnings of, and containing urban shrinkage; and to research urban shrinkage coping strategies under different circumstances and carry out experiments and demonstrations according to local conditions.
