Climate change and human activities can influence vegetation net primary productivity (NPP), a key component of natural ecosystems. The Qinghai-Tibet Plateau of China, in spite of its significant natural and cultural values, is one of the most susceptible regions to climate change and human disturbancesin the world. To assess the impact of climate change and human activities on vegetation dynamics in the grassland ecosystems ofthe northeastern Qinghai-Tibet Plateau, we applied a time-series trend analysis to normalized difference vegetation index (NDVI) datasets from 2000 to 2015 and compared these spatiotemporal variations with trends in climatic variables over the same time period. The constrained ordination approach (redundancy analysis) was used to determine which climatic variables or human-related factors mostly in?uenced the variation of NDVI. Furthermore, in order to determine whether current conservation measures and programs are effectivein ecological protection and reconstruction, we divided the northeastern Qinghai-Tibet Plateau into two parts: the Three-River Headwater conservation area (TRH zone) in the south and the non-conservation area (NTRH zone) in the north. The results indicatedan overall (73.32%)increasing trend of vegetation NPP in grasslands throughout the study area. During the period 2000-2015, NDVI in the TRH and NTRH zones increased at the rates of 0.0015/aand 0.0020/a, respectively.Specifically, precipitation accounted for 9.2% of the total variation in NDVI, while temperature accounted for 13.4%. In addition, variation in vegetation NPP of grasslands responded not only to long- and short-term changes in climate, as conceptualized in non-equilibrium theory, but also to the impact of human activities and their associated perturbations. The redundancy analysis successfully separated the relative contributions of climate change and human activities, of whichvillage populationand agricultural gross domestic product were the two most important contributors to the NDVI changes, explaining 17.8% and 17.1% of the total variationof NDVI (with the total contribution >30.0%), respectively. The total contributionpercentages of climate change and human activitiesto the NDVI variation were27.5% and 34.9%, respectively, inthe northeastern Qinghai-Tibet Plateau. Finally, our study shows that the grassland restoration in the study area was enhanced by protection measures and programs in the TRH zone, which explained 7.6% of the total variation in NDVI.

Drought is one of the most significant environmental disasters, especially in arid and semi-arid regions. Drought indices as a tool for management practices seeking to deal with the drought phenomenon are widely used around the world. One of these indicators is the Palmer drought severity index (PDSI), which is used in many parts of the world to assess the drought situation and continuation. In this study, the drought state of Fars Province in Iran was evaluated by using the PDSI over 1995-2014 according to meteorological data from six weather stations in the province. A statistical downscaling model (SDSM) was used to apply the output results of the general circulation model in Fars Province. To implement data processing and prediction of climate data, a statistical period 1995-2014 was considered as the monitoring period, and a statistical period 2019-2048 was for the prediction period. The results revealed that there is a good agreement between the simulated precipitation (R²>0.63; R², determination coefficient; MAE<0.52; MAE, mean absolute error; RMSE<0.56; RMSE, Root Mean Squared Error) and temperature (R²>0.95, MAE<1.74, and RMSE<1.78) with the observed data from the stations. The results of the drought monitoring model presented that dry periods would increase over the next three decades as compared to the historical data. The studies showed the highest drought in the meteorological stations Abadeh and Lar during the prediction period under two future scenarios representative concentration pathways (RCP4.5 and RCP8.5). According to the results of the validation periods and efficiency criteria, we suggest that the SDSM is a proper tool for predicting drought in arid and semi-arid regions.

Drought was a severe recurring phenomenon in Iraq over the past two decades due to climate change despite the fact that Iraq has been one of the most water-rich countries in the Middle East in the past. The Iraqi Kurdistan Region (IKR) is located in the north of Iraq, which has also suffered from extreme drought. In this study, the drought severity status in Sulaimaniyah Province, one of four provinces of the IKR, was investigated for the years from 1998 to 2017. Thus, Landsat time series dataset, including 40 images, were downloaded and used in this study. The Normalized Difference Vegetation Index (NDVI) and the Normalized Difference Water Index (NDWI) were utilized as spectral-based drought indices and the Standardized Precipitation Index (SPI) was employed as a meteorological-based drought index, to assess the drought severity and analyse the changes of vegetative cover and water bodies. The study area experienced precipitation deficiency and severe drought in 1999, 2000, 2008, 2009, and 2012. Study findings also revealed a drop in the vegetative cover by 33.3% in the year 2000. Furthermore, the most significant shrinkage in water bodies was observed in the Lake Darbandikhan (LDK), which lost 40.5% of its total surface area in 2009. The statistical analyses revealed that precipitation was significantly positively correlated with the SPI and the surface area of the LDK (correlation coefficients of 0.92 and 0.72, respectively). The relationship between SPI and NDVI-based vegetation cover was positive but not significant. Low precipitation did not always correspond to vegetative drought; the delay of the effect of precipitation on NDVI was one year.

The vegetation ecosystem of the Qinghai-Tibet Plateau in China, considered to be the ′′natural laboratory′′ of climate change in the world, has undergone profound changes under the stress of global change. Herein, we analyzed and discussed the spatial-temporal change patterns and the driving mechanisms of net primary productivity (NPP) in the Qinghai-Tibet Plateau from 2000 to 2015 based on the gravity center and correlation coefficient models. Subsequently, we quantitatively distinguished the relative effects of climate change (such as precipitation, temperature and evapotranspiration) and human activities (such as grazing and ecological construction) on the NPP changes using scenario analysis and Miami model based on the MOD17A3 and meteorological data. The average annual NPP in the Qinghai-Tibet Plateau showed a decreasing trend from the southeast to the northwest during 2000-2015. With respect to the inter-annual changes, the average annual NPP exhibited a fluctuating upward trend from 2000 to 2015, with a steep increase observed in 2005 and a high fluctuation observed from 2005 to 2015. In the Qinghai-Tibet Plateau, the regions with the increase in NPP (change rate higher than 10%) were mainly concentrated in the Three-River Source Region, the northern Hengduan Mountains, the middle and lower reaches of the Yarlung Zangbo River, and the eastern parts of the North Tibet Plateau, whereas the regions with the decrease in NPP (change rate lower than -10%) were mainly concentrated in the upper reaches of the Yarlung Zangbo River and the Ali Plateau. The gravity center of NPP in the Qinghai-Tibet Plateau has moved southwestward during 2000-2015, indicating that the increment and growth rate of NPP in the southwestern part is greater than those of NPP in the northeastern part. Further, a significant correlation was observed between NPP and climate factors in the Qinghai-Tibet Plateau. The regions exhibiting a significant correlation between NPP and precipitation were mainly located in the central and eastern Qinghai-Tibet Plateau, and the regions exhibiting a significant correlation between NPP and temperature were mainly located in the southern and eastern Qinghai-Tibet Plateau. Furthermore, the relative effects of climate change and human activities on the NPP changes in the Qinghai-Tibet Plateau exhibited significant spatial differences in three types of zones, i.e., the climate change-dominant zone, the human activity-dominant zone, and the climate change and human activity interaction zone. These research results can provide theoretical and methodological supports to reveal the driving mechanisms of the regional ecosystems to the global change in the Qinghai-Tibet Plateau.

It is known that the exploitation of opencast coal mines has seriously damaged the environments in the semi-arid areas. Vegetation status can reliably reflect the ecological degeneration and restoration in the opencast mining areas in the semi-arid areas. Long-time series MODIS NDVI data are widely used to simulate the vegetation cover to reflect the disturbance and restoration of local ecosystems. In this study, both qualitative (linear regression method and coefficient of variation (CoV)) and quantitative (spatial buffer analysis, and change amplitude and the rate of change in the average NDVI) analyses were conducted to analyze the spatio-temporal dynamics of vegetation during 2000-2017 in Jungar Banner of Inner Mongolia Autonomous Region, China, at the large (Jungar Banner and three mine groups) and small (three types of functional areas: opencast coal mining excavation areas, reclamation areas and natural areas) scales. The results show that the rates of change in the average NDVI in the reclamation areas (20%-60%) and opencast coal mining excavation areas (10%-20%) were considerably higher than that in the natural areas (<7%). The vegetation in the reclamation areas experienced a trend of increase (3-5 a after reclamation)-decrease (the sixth year of reclamation)-stability. The vegetation in Jungar Banner has a spatial heterogeneity under the influences of mining and reclamation activities. The ratio of vegetation improvement area to vegetation degradation area in the west, southwest and east mine groups during 2000-2017 was 8:1, 20:1 and 33:1, respectively. The regions with the high CoV of NDVI above 0.45 were mainly distributed around the opencast coal mining excavation areas, and the regions with the CoV of NDVI above 0.25 were mostly located in areas with low (28.8%) and medium-low (10.2%) vegetation cover. The average disturbance distances of mining activities on vegetation in the three mine groups (west, southwest and east) were 800, 800 and 1000 m, respectively. The greater the scale of mining, the farther the disturbance distances of mining activities on vegetation. We conclude that vegetation reclamation will certainly compensate for the negative impacts of opencast coal mining activities on vegetation. Sufficient attention should be paid to the proportional allocation of plant species (herbs and shrubs) in the reclamation areas, and the restored vegetation in these areas needs to be protected for more than 6 a. Then, as the repair time increased, the vegetation condition of the reclamation areas would exceed that of the natural areas.

Glaciers are a critical freshwater resource of river recharge in arid areas around the world. In recent decades, glaciers have shown evidence of retreat due to climate change, and the accelerated ablation of glaciers and associated impacts on water resources have received widespread attention. Glacier variations result from climate change, so they can serve as an indicator of climate change. Considering the climatic differences in different elevation ranges, it is worthwhile to explore whether different responses exist between glacier area and air temperature in each elevation zone. In this study, we selected a typical arid inland river basin (Sugan Lake Basin) in the western Qilian Mountains of Northwest China to analyze the glacier variations and their response to climate change. The glacier area data from 1989 to 2016 were delineated using Landsat Thematic Mapper (TM), Enhanced TM+ (ETM+) and Operational Land Imager (OLI) images. We compared the relationships between glacier area and air temperature at seven meteorological stations in the glacier-covered areas and in the Sugan Lake Basin, and further analyzed the relationship between glacier area and mean air temperature of the glacier surfaces in July-August in the elevation range of 4700-5500 m a.s.l. by the linear regression method and correlation analysis. In addition, based on the linear regression relationship established between glacier area and air temperature in each elevation zone, we predicted glacier areas under future climate scenarios during the periods of 2046-2065 and 2081-2100. The results indicate that the glaciers experienced a remarkable shrinkage from 1989 to 2016 with a shrinkage rate of -1.61 km²/a (-0.5%/a), and the rising temperature is the decisive factor dominating glacial retreat; there is a significant negative linear correlation between glacier area and mean air temperature of the glacier surfaces in July-August in each elevation zone from 1989 to 2016. The variations in glaciers are far less sensitive to changes in precipitation than to changes in air temperature. Due to the influence of climate and topographic conditions, the distribution of glacier area and the rate of glacier ablation first increased and then decreased in different elevation zones. The trend in glacier shrinkage will continue because air temperature will continue to increase in the future, and the result of glacier retreat in each elevation zone will be slightly slower than that in the entire study area. Quantitative glacier research can more accurately reflect the response of glacier variations to climate change, and the regression relationship can be used to predict the areas of glaciers under future climate scenarios. These conclusions can offer effective references for assessing glacier variations and their response to climate change in arid inland river basins in Northwest China as well as other similar regions in the world.

Land cover in the Chinese Loess Plateau has undergone dramatic changes since the late 1980s. Revealing the trend in land cover change and eco-environmental quality response of different geomorphic units in this stage is a realistic requirement for promoting sustainable development of the Chinese Loess Plateau. Based on the data of geomorphic units and land cover in 1990, 2000, 2010 and 2018 of the Chinese Loess Plateau, we studied the trend of land cover change and eco-environmental quality response of different geomorphic units by using a significance index of land cover change, a proportion index of land cover change and an eco-environmental response model. The results indicated that from 1990 to 2018, the areas of forestland and construction land substantially increased, whereas those of cropland, grassland, wetland and unused land considerably decreased. Land cover change exhibited large geomorphic differences, and the main conversion of land cover was from cropland into other land types. Unstable trend of land cover change in the loess tablelands and sandy loess hills declined, whereas the unstable trends in the other geomorphic units enhanced. Eco-environmental quality varied among different geomorphic units. The expansion of construction land and degradation of forestland, grassland and wetland resulted in the deterioration of eco-environmental quality. The conversion of cropland and unused land into forestland and grassland, and the conversion of grassland into forestland were the main factors that drove the improvement of eco-environmental quality. The findings of this study may provide theoretical reference and support decision making for the optimization of land use structure and the improvement of eco-environmental quality on the Chinese Loess Plateau.

Calotropis procera (Aiton) Dryand (Apocynaceae) is a native species in tropical and subtropical Africa and Asia. However, due to its fast growing and drought-tolerant, it has become an invasive species when it was introduced into Central and South America, as well as the Caribbean Islands. Currently, C. procera displays a wide distribution in the world. Invasiveness is important, in particular, because many invasive species exert a high reproductive pressure on the invaded communities or are highly productive in their new distributed areas. It has been suggested that a very deep root system and a high capacity to reduce stomatal conductance during water shortage could allow this species to maintain the water status required for a normal function. However, the true mechanism behind the successful distribution of C. procera across wet and dry environments is still unknown. C. procera leaves were collected from 12 natural populations in Brazil, Colombia and Mexico, ranging from wet to dry environments during 2014-2015. Many traits of morphology and anatomy from these distinct morphotypes were evaluated. We found that C. procera leaves had a considerable capacity to adjust their morphological, anatomical and physiological traits to different environments. The magnitude of acclimation responses, i.e., plasticity, had been hypothesized to reflect the specialized adaptation of plant species to a particular environment. However, allometric models for leaf area (LA) estimation cannot be grouped as a single model. Leaves are narrower and thicker with low amounts of air spaces inside the leaf parenchyma in wet environments, while they are broader and thinner with a small number of palisade cell layers in dry environments. Based on these, we argue that broader and thinner leaves of C. procera dissipate incident energy at the expense of a higher rate of transpiration to survive in environments in which water is the most limiting factor and to compete in favorable wet environments.

Tamarix taklamakanensis, a dominant species in the Taklimakan Desert of China, plays a crucial role in stabilizing sand dunes and maintaining regional ecosystem stability. This study aimed to determine the water use strategies of T. taklamakanensis in the Taklimakan Desert under a falling groundwater depth. Four typical T. taklamakanensis nabkha habitats (sandy desert of Tazhong site, saline desert-alluvial plain of Qiemo site, desert-oasis ecotone of Qira site and desert-oasis ecotone of Aral site) were selected with different climate, soil, groundwater and plant cover conditions. Stable isotope values of hydrogen and oxygen were measured for plant xylem water, soil water (soil depths within 0-500 cm), snowmelt water and groundwater in the different habitats. Four potential water sources for T. taklamakanensis, defined as shallow, middle and deep soil water, as well as groundwater, were investigated using a Bayesian isotope mixing model. It was found that groundwater in the Taklimakan Desert was not completely recharged by precipitation, but through the river runoff from snowmelt water in the nearby mountain ranges. The surface soil water content was quickly depleted by strong evaporation, groundwater depth was relatively shallow and the height of T. taklamakanensis nabkha was relatively low, thus T. taklamakanensis primarily utilized the middle (23%±1%) and deep (31%±5%) soil water and groundwater (36%±2%) within the sandy desert habitat. T. taklamakanensis mainly used the deep soil water (55%±4%) and a small amount of groundwater (25%±2%) within the saline desert-alluvial plain habitat, where the soil water content was relatively high and the groundwater depth was shallow. In contrast, within the desert-oasis ecotone in the Qira and Aral sites, T. taklamakanensis primarily utilized the deep soil water (35%±1% and 38%±2%, respectively) and may also use groundwater because the height of T. taklamakanensis nabkha was relatively high in these habitats and the soil water content was relatively low, which is associated with the reduced groundwater depth due to excessive water resource exploitation and utilization by surrounding cities. Consequently, T. taklamakanensis showed distinct water use strategies among the different habitats and primarily depended on the relatively stable water sources (deep soil water and groundwater), reflecting its adaptations to the different habitats in the arid desert environment. These findings improve our understanding on determining the water sources and water use strategies of T. taklamakanensis in the Taklimakan Desert.

Soil surface mulching and planting density regulation are widely used for effective utilization of limited rainwater resources and improvement of crop productivity in dryland farming. However, the combined effects of mulching type and planting density on maize growth and yield have been seldom studied, especially in different hydrological years. A field experiment was conducted to evaluate the effects of mulching type and planting density on the soil temperature, growth, grain yield (GY), water use efficiency (WUE) and economic benefit of rainfed maize in the drylands of northern China during 2015-2017. Precipitation fluctuated over the three years. There were four mulching types (NM, flat cultivation with non-mulching; SM, flat cultivation with straw mulching; RP, plastic-mulched ridge plus bare furrow; RPFS, plastic-mulched ridge plus straw-mulched furrow) and three planting densities (LD, low planting density, 45.0×10³ plants/hm²; MD, medium planting density, 67.5×10³ plants/hm²; HD, high planting density, 90.0×10³ plants/hm²). Results showed that soil temperature was higher with RP and lower with SM compared with NM, but no significant difference was found between RPFS and NM. More soil water was retained by soil mulching at the early growth stage, but it significantly varied at the middle and late growth stages. Maize growth was significantly improved by soil mulching. With increasing planting density, stem diameter, net photosynthetic rate and chlorophyll content tended to decline, whereas a single-peak trend in biomass yield was observed. Mulching type and planting density did not have significant effect on evapotranspiration (ET), but GY and WUE were significantly affected. There were significant interacting effects of mulching type and planting density on biomass yield, GY, ET and WUE. Compared with NM, RPFS, RP and SM increased GY by 57.5%, 50.8% and 18.9%, and increased WUE by 66.6%, 54.3% and 18.1%, respectively. At MD, GY increased by 41.4% and 25.2%, and WUE increased by 38.6% and 22.4% compared with those of at LD and HD. The highest maize GY (7023.2 kg/hm²) was observed under MD+RPFS, but the value (6699.1 kg/hm²) was insignificant under MD+RP. Similar trends were observed for WUE under MD+RP and MD+RPFS, but no significant difference was observed between these two combinations. In terms of economic benefit, net income under MD+RP was the highest with a 9.8% increase compared with that of under MD+RPFS. Therefore, we concluded that RP cultivation pattern with a suitable planting density (67.5×10³ plants/hm²) is promising for rainwater resources utilization and maize production in the drylands of northern China.

The replacement of native dry forests by commercial (exotic) tree plantations could generate changes in rainfall partitioning, which further affects the water cycle. In this study, we determined (i) the rainfall partitioning into interception, throughfall and stemflow, (ii) the role of rainfall event size on rainfall partitioning, (iii) the pH of water channelized as throughfall and stemflow, and (iv) the runoff in Lithraea molleoides (a native species) and Pinus elliottii (an exotic species) stands in the dry Chaco mountain forests, central Argentina. On average, interception, throughfall and stemflow accounted for 19.3%, 79.5% and 1.2% of the gross rainfall in L. molleoides stand, and 32.6%, 66.7% and 0.7% of the gross rainfall in P. elliottii stand, respectively. Amounts of interception, throughfall and stemflow presented positive linear relationships with the increment of rainfall event size for both tree species (P<0.01 in all cases). Percentages of interception, throughfall and stemflow were all related to the increment of rainfall event size, showing different patterns. With increasing rainfall event size, interception exponentially decreased, throughfall asymptotically increased and stemflow linearly increased. Both P. elliottii and L. molleoides stands presented significant differences in the pH values of water channelized as throughfall (6.3 vs. 6.7, respectively; P<0.01) and stemflow (4.5 vs. 5.8, respectively; P<0.01). Runoff occupied only 0.3% of the gross rainfall in P. elliottii stand and was zero in L. molleoides stand. Our results showed that the native species L. molleoides presented 13.6% more water reaching the topsoil (i.e., net rainfall; net rainfall=gross rainfall-interception-runoff) than the exotic species P. elliottii. This study improves our understanding of the effects of native vegetation replacement on the local water balance in the dry forest ecosystems.

It is widely accepted that hydrogeochemistry of saline springs is extremely important to understand the water circulation and evolution of saline basins and to evaluate the potential of potassium-rich evaporites. The Kuqa Basin, located in the northern part of the Tarim Basin in Northwest China, is a saline basin regarded as the most potential potash-seeking area. However, the origin and water circulation processes of saline springs have yet to be fully characterized in this saline basin. In this study, a total of 30 saline spring samples and 11 river water samples were collected from the Qiulitage Structural Belt (QSB) of the Kuqa Basin. They were analyzed for major (K⁺, Ca²⁺, Na⁺, Mg²⁺, SO₄^2-, Cl^- and HCO₃^-) and trace (Sr²⁺ and Br^-) ion concentrations, stable H-O-Sr isotopes and tritium concentrations in combination with previously published hydrogeochemical and isotopic (H-O) data in the same area. It is found that the water chemical type of saline springs in the study area belonged to the Na-Cl type, and that of river water belonged to the Ca-Mg-HCO₃-SO₄ type. The total dissolved solid (TDS) of saline springs in the QSB ranged from 117.77 to 314.92 g/L, reaching the brine level. On the basis of the general chemical compositions and the characteristics of the stable H-O-Sr isotopes of saline springs, we infer that those saline springs mainly originated from precipitation following river water recharging. In addition, we found that saline springs were not formed by evapo-concentration because it is unlikely that the high chloride concentration of saline springs resulted in evapo-concentration and high salinity. Therefore, we conclude that saline spring water may have experienced intense evapo-concentration before dissolving the salty minerals or after returning to the surface. The results show that the origin of salinity was mainly dominated by dissolving salty minerals due to the river water and/or precipitation that passed through the halite-rich stratum. Moreover, there are two possible origins of saline springs in the QSB: one is the infiltration of the meteoric water (river water), which then circulates deep into the earth, wherein it dissolves salty minerals, travels along the fault and returns to the surface; another is the mixture of formation water, or the mixture of seawater or marine evaporate sources and its subsequent discharge to the surface under fault conditions. Our findings provide new insight into the possible saltwater circulation and evolution of saline basins in the Tarim Basin.

Coal mining has led to serious ecological damages in arid desert region of Northwest China. However, effects of climatic factor and mining activity on vegetation dynamics and plant diversity in this region remain unknown. Wuhai City located in the arid desert region of Northwest China is an industrial city and dominated by coal mining. Based on Landsat data and field investigation in Wuhai City, we analyzed the vegetation dynamics and the relationships with climate factors, coal mining activity and ecological restoration projects from 2000 to 2019. Results showed that vegetation in Wuhai City mostly consisted of desert plants, such as Caragana microphylla, Tetraena mongolica and Achnatherum splendens. And the vegetation fractional coverage (VFC) and greenness rate of change (GRC) showed that vegetation was slightly improved during the study period. Normalized difference vegetation index (NDVI) was positively correlated with annual mean precipitation, relative humidity and annual mean temperature, indicating that these climate factors might play important roles in the improved vegetation. Vegetation coverage and plant diversity around the coal mining area were reduced by coal mining, while the implementation of ecological restoration projects improved the vegetation coverage and plant diversity. Our results suggested that vegetation in the arid desert region was mainly affected by climate factors, and the implementation of ecological restoration projects could mitigate the impacts of coal mining on vegetation and ecological environment.

In recent year, desertification has become one of the most important environmental hazards all over the world, especially in developing countries such as Iran. Understanding the factors impactingon desertification and identifying the regionswith high desertification potential are essential to control this phenomenon (i.e., desertification). The life cycle assessment (LCA) method is essential in assessing the desertification of ecosystems, especially for susceptible ecosystems with high degradationrisks. The aim of the present study was to evaluate the desertification potential of Lorestan Province, Iran, based on the LCA method. We selected aridity, fire and dust as three indicators of desertification and collected data from 2000 to 2015. We divided the study area into 6 types of ecoregionsaccording to the climate types (arid, semi-arid and dry sub-humid) and dominant species (Quercus brantii and Astragalusadscendens), and calculated the characteristic factor (CF) of eachindicator (aridity, fire and dust) by combining the indicator layers and ecoregion layer of the study area. In a given ecoregion, the sum of CF values of aridity, dust and fire indicators represents the life cycle inventory (LCI) desertification value (the higher the LCI value, the greater the desertification potential).Then, we obtained the desertification potential map by combining and overlapping the ecoregions and the normalized indicators based on the LCA method. Aridity and fire exhibit significant impacts on desertification in the study area compared with dust. In the study area, semi-arid ecoregion with Quercus brantiias the dominant species is the largest ecoregion, while arid ecoregion withQuercus brantiias the dominant species is the smallest ecoregion.Arid ecoregion withAstragalusadscendensas the dominant species (LCI desertificationvalue of 1.99) and dry sub-humid ecoregion withQuercus brantiias the dominant species (LCI desertification value of0.79)show the highest and lowest desertification potentials, respectively. Furthermore, arid ecoregion with Quercus brantii as the dominant species also has a higher LCI desertification value (1.89), showing a high desertification potential. These results suggest the necessity of proper management and appropriate utilization in these ecoregions. In general, assessing desertification potential using the LCA method on a local and regional scale can possibly provide a new methodology for identifying and protecting areas with high degradation risks.

Forest recovery may be influenced by several factors, of which fire is the most critical. However, moderate- and long-term effects of fire on forest recovery are less researched in Northwest China. Thus, the effects of different forest recovery time after fire (1917 (served as the control), 1974, 1983 and 1995) and fire severities (low, moderate and high) on larch (Larix sibirica Ledeb.) forest were investigated in the Kanas National Nature Reserve (KNNR), Northwest China in 2017. This paper analyzed post-fire changes in stand density, total basal area (TBA), litter mass, soil organic carbon (SOC) and soil nutrients (total nitrogen, total phosphorus and total potassium) with one-way analyses of variance. Results indicate that litter mass, TBA, SOC and soil nutrients increased with increasing recovery time after fire and decreasing fire severity, while the stand density showed an opposite response. The effects of fire disturbance on SOC and soil nutrients decreased with increasing soil depth. Moreover, we found that the time of more than 43 a is needed to recover the litter mass, TBA, SOC and soil nutrients to the pre-fire level. In conclusion, high-severity fire caused the greatest variations in stand structure and soil of larch forest, and low-severity fire was more advantageous for post-fire forest stand structure and soil recovery in the KNNR. Therefore, low-severity fire can be an efficient management mean through reducing the accumulation of forest floor fuel of post-fire forests in the KNNR, Northwest China.

Soil moisture is critical for vegetation growth in deserts. However, detailed dataregarding the soil moisture distribution in space and time in the Gurbantunggut Desert have not yet been reported. In this study, we conducted a series ofinsitu observation experiments in a fixed sand dune at the southern edge of the GurbantunggutDesert from February 2014 to October 2016, to explore the spatio-temporal variation of soil moisture content, investigate the impact of Haloxylonammodendron (C. A. Mey.) Bungeon soil moisture content in its root zone, and examine the factors influencing the soil moisture spatial pattern. One-way analysis of variance,least significant difference testsand correlation analysis were used to analyze the data. The results revealed that the soil moisture content exhibited annual periodicity and the temporal variation of soil moisture content throughout a year could be divided into three periods, namely, a moisture-gaining period, a moisture-losing period and a moisture-stable period.According to the temporal and spatial variability, the 0-400 cm soil profile could be divided into two layers: an active layer with moderate variability and a stable layer with weak variability.The temporal variability was larger than the spatial variability in the active layer,and the mean profile soil moisture content at different slope positions displayed the trend of decreasing with increasing relative heightand mainly followed the order of interdunearea>westand east slopes>slope top.The mean profile soil moisture content in the root zone of dead H. ammodendronindividuals was significantly higher than that in the root zones of adult and young individuals, while the soil moisture content in the root zone of adult individuals was slightly higher than that in the root zone of young individuals with no significant difference.The spatial pattern of soil moisture was attributable to the combined effects of snowfall, vegetation and soil texture, whereas the effects of rainfall and evaporation were not significant. The findings may offer a foundation for the management of sandy soil moisture and vegetation restoration in arid areas.

Shrubby seablite or lani (Suaeda fruticosa Forssk) is a perennial euhalophyte with succulent leaves, which could be planted on arid-saline lands for restoration and cultivated as a non-conventional edible or cash crop. Knowledge about the impacts of maternal saline environment on seed attributes of this important euhalophyte is lacking. This study investigated the effects of maternal salinity on yield, size and stress tolerance of S. fruticosa seeds. Seedlings of S. fruticosa were grown in a green net house under increasing maternal salinity levels (0, 300, 600 and 900 mM NaCl) until seed production. Total yield, size, stress tolerance and germination of the descended seeds under different maternal saline conditions were examined. Plants grown under saline conditions (300, 600 and 900 mM NaCl) produce a substantially higher quantity of seeds than plants grown under non-saline condition (0 mM NaCl). Low maternal salinity (300 mM NaCl) improves seed size. Seeds produced under all maternal salinity levels display a higher tolerance to low temperature (night/day thermoperiod of 10°C/20°C), whereas seeds produced under 300 mM NaCl maternal saline condition show a better tolerance to high temperature (night/day thermoperiod of 25°C/35°C) during germination. Seeds from all maternal saline conditions germinate better in the 12 h photoperiod (12 h light/12 h dark) than in the dark (24 h dark); however, seeds produced from low and moderate maternal saline conditions (300 and 600 mM NaCl) show a higher germination in the dark than those from control and high maternal saline conditions (0 and 900 mM NaCl). In general, maternal salinity is found to improve yield, size and stress tolerance of S. fruticosa seeds.

To date, much of research on revegetation has focused on soil microorganisms due to their contributions in the formation of soil and soil remediation process. However, little is known about the soil bacteria and their functions respond to the diverse vegetational types in the process of vegetation restoration. Effects of dominated vegetation, i.e., Artemisia halodendron Turcz Ex Bess, Caragana microphylla Lam., Hedysarum fruticosum Pall. and Pinus sylvestris L. on bacterial community structures and their potential functions in the Hulun Buir Sandy Land, China were determined using high-throughput 16S rRNA gene sequencing and phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt) in 2015. Although the dominant phyla of soil bacterial community among different types of vegetation, including Proteobacteria, Actinobacteria, Acidobacteria, Bacteroidetes and Firmicutes, were similar, the relative abundance of these dominant groups significantly differed, indicating that different types of vegetation might result in variations in the composition of soil bacterial community. In addition, functional genes of bacterial populations were similar among different types of vegetation, whereas its relative abundance was significantly differed. Most carbon fixation genes showed a high relative abundance in P. sylvestris, vs. recalcitrant carbon decomposition genes in A. halodendron, suggesting the variations in carbon cycling potential of different types of vegetation. Abundance of assimilatory nitrate reduction genes was the highest in P. sylvestris, vs. dissimilatory nitrate reduction and nitrate reductase genes in A. halodendron, indicating higher nitrogen gasification loss and lower nitrogen utilization gene functions in A. halodendron. The structures and functional genes of soil bacterial community showed marked sensitivities to different plant species, presenting the potentials for regulating soil carbon and nitrogen cycling.

Snow cover is an important water source for vegetation growth in arid and semi-arid areas, and grassland phenology provides valuable information on the response of terrestrial ecosystems to climate change. The Mongolian Plateau features both abundant snow cover resources and typical grassland ecosystems. In recent years, with the intensification of global climate change, the snow cover on the Mongolian Plateau has changed correspondingly, with resulting effects on vegetation growth. In this study, using MOD10A1 snow cover data and MOD13A1 Normalized Difference Vegetation Index (NDVI) data combined with remote sensing (RS) and geographic information system (GIS) techniques, we analyzed the spatiotemporal changes in snow cover and grassland phenology on the Mongolian Plateau from 2001 to 2018. The correlation analysis and grey relation analysis were used to determine the influence of snow cover parameters (snow cover fraction (SCF), snow cover duration (SCD), snow cover onset date (SCOD), and snow cover end date (SCED)) on different types of grassland vegetation. The results showed wide snow cover areas, an early start time, a late end time, and a long duration of snow cover over the northern Mongolian Plateau. Additionally, a late start, an early end, and a short duration were observed for grassland phenology, but the southern area showed the opposite trend. The SCF decreased at an annual rate of 0.33%. The SCD was shortened at an annual rate of 0.57 d. The SCOD and SCED in more than half of the study area advanced at annual rates of 5.33 and 5.74 DOY (day of year), respectively. For grassland phenology, the start of the growing season (SOS) advanced at an annual rate of 0.03 DOY, the end of the growing season (EOS) was delayed at an annual rate of 0.14 DOY, and the length of the growing season (LOS) was prolonged at an annual rate of 0.17 d. The SCF, SCD, and SCED in the snow season were significantly positively correlated with the SOS and negatively correlated with the EOS and LOS. The SCOD was significantly negatively correlated with the SOS and positively correlated with the EOS and LOS. The SCD and SCF can directly affect the SOS of grassland vegetation, while the EOS and LOS were obviously influenced by the SCOD and SCED. This study provides a scientific basis for exploring the response trends of alpine vegetation to global climate change.

The criteria used by International Union for Conservation of Nature (IUCN) for its Red List of Ecosystems (RLE) are the global standards for ecosystem-level risk assessment, and they have been increasingly used for biodiversity conservation. The changed distribution area of an ecosystem is one of the key criteria in such assessments. The Stipa bungeana grassland is one of the most widely distributed grasslands in the warm-temperate semi-arid regions of China. However, the total distribution area of this grassland was noted to have shrunk and become fragmented because of its conversion to cropland and grazing-induced degradation. Following the IUCN-RLE standards, here we analyzed changes in the geographical distribution of this degraded grassland, to evaluate its degradation and risk of collapse. Past (1950-1980) distribution areas were extracted from the Vegetation Map of China (1:1,000,000). Present realizable distribution areas were equated to these past areas minus any habitat area losses. We then predicted the grassland's present and future (under the Representative Concentration Pathway 8.5 scenario) potential distribution areas using maximum entropy algorithm (MaxEnt), based on field survey data and nine environmental layers. Our results showed that the S. bungeana grassland was mainly distributed in the Loess Plateau, Hexi Corridor, and low altitudes of the Qilian Mountains and Longshou Mountain. This ecosystem occurred mainly on loess soils, kastanozems, steppe aeolian soils and sierozems. Thermal and edaphic factors were the most important factors limiting the distribution of S. bungeana grassland across China. Since 56.1% of its past distribution area (4.9×10⁴ km²) disappeared in the last 50 a, the present realizable distribution area only amounts to 2.2×10⁴ km². But only 15.7% of its present potential distribution area (14.0×10⁴ km²) is actually occupied by the S. bungeana grassland. The future potential distribution of S. bungeana grassland was predicted to shift towards northwest, and the total area of this ecosystem will shrink by 12.4% over the next 50 a under the most pessimistic climate change scenario. Accordingly, following the IUCN-RLE criteria, we deemed the S. bungeana grassland ecosystem in China to be endangered (EN). Revegetation projects and the establishment of protected areas are recommended as effective ways to avert this looming crisis. This empirical modeling study provides an example of how IUCN-RLE categories and criteria may be valuably used for ecosystem assessments in China and abroad.

The lack of clarity of how natural vegetation restoration influences soil organic carbon (SOC) content and SOC components in soil aggregate fractions limits the understanding of SOC sequestration and turnover in forest ecosystems. The aim of this study was to explore how natural vegetation restoration affects the SOC content and ratio of SOC components in soil macroaggregates (>250 μm), microaggregates (53-250 μm), and silt and clay (<53 μm) fractions in 30-, 60-, 90- and 120-year-old Liaodong oak (Quercus liaotungensis Koidz.) forests, Shaanxi, China in 2015. And the associated effects of biomasses of leaf litter and different sizes of roots (0-0.5, 0.5-1.0, 1.0-2.0 and >2.0 mm diameter) on SOC components were studied too. Results showed that the contents of high activated carbon (HAC), activated carbon (AC) and inert carbon (IC) in the macroaggregates, microaggregates and silt and clay fractions increased with restoration ages. Moreover, IC content in the microaggregates in topsoil (0-20 cm) rapidly increased; peaking in the 90-year-old restored forest, and was 5.74 times higher than AC content. In deep soil (20-80 cm), IC content was 3.58 times that of AC content. Biomasses of 0.5-1.0 mm diameter roots and leaf litter affected the content of aggregate fractions in topsoil, while the biomass of >2.0 mm diameter roots affected the content of aggregate fractions in deep soil. Across the soil profiles, macroaggregates had the highest capacity for HAC sequestration. The effects of restoration ages on soil aggregate fractions and SOC content were less in deep soil than in topsoil. In conclusion, natural vegetation restoration of Liaodong oak forests improved the contents of SOC, especially IC within topsoil and deep soil. The influence of IC on aggregate stability was greater than the other SOC components, and the aggregate stability was significantly affected by the biomasses of litter, 0.5-1.0 mm diameter roots in topsoil and >2.0 mm diameter roots in deep soil. Natural vegetation restoration of Liaodong oak forests promoted SOC sequestration by soil macroaggregates.

Many desert expressways are affected by the deposition of the wind-blown sand, which might block the movement of vehicles or cause accidents. W-beam central guardrails, which are used to improve the safety of desert expressways, are thought to influence the deposition of the wind-blown sand, but this has yet not to be studied adequately. To address this issue, we conducted a wind tunnel test to simulate and explore how the W-beam central guardrails affect the airflow, the wind-blown sand flux and the deposition of the wind-blown sand on desert expressways in sandy regions. The subgrade model is 3.5 cm high and 80.0 cm wide, with a bank slope ratio of 1:3. The W-beam central guardrails model is 3.7 cm high, which included a 1.4-cm-high W-beam and a 2.3-cm-high stand column. The wind velocity was measured by using pitot-static tubes placed at nine different heights (1, 2, 3, 5, 7, 10, 15, 30 and 50 cm) above the floor of the chamber. The vertical distribution of the wind-blown sand flux in the wind tunnel was measured by using the sand sampler, which was sectioned into 20 intervals. In addition, we measured the wind-blown sand flux in the field at K50 of the Bachu-Shache desert expressway in the Taklimakan Desert on 11 May 2016, by using a customized 78-cm-high gradient sand sampler for the sand flux structure test. Obstruction by the subgrade leads to the formation of two weak wind zones located at the foot of the windward slope and at the leeward slope of the subgrade, and the wind velocity on the leeward side weakens significantly. The W-beam central guardrails decrease the leeward wind velocity, whereas the velocity increases through the bottom gaps and over the top of the W-beam central guardrails. The vertical distribution of the wind-blown sand flux measured by wind tunnel follows neither a power-law nor an exponential function when affected by either the subgrade or the W-beam central guardrails. At 0.0H and 0.5H (where H=3.5 cm, which is the height of the subgrade), the sand transport is less at the 3 cm height from the subgrade surface than at the 1 and 5 cm heights as a result of obstruction by the W-beam central guardrails, and the maximum sand transportation occurs at the 5 cm height affected by the subgrade surface. The average saltation height in the presence of the W-beam central guardrails is greater than the subgrade height. The field test shows that the sand deposits on the overtaking lane leeward of the W-beam central guardrails and that the thickness of the deposited sand is determined by the difference in the sand mass transported between the inlet and outlet points, which is consistent with the position of the minimum wind velocity in the wind tunnel test. The results of this study could help us to understand the hazards of the wind-blown sand onto subgrade with the W-beam central guardrails.

Short-term climate reconstruction, i.e., the reproduction of short-term (several decades) historical climatic time series based on the relationship between observed data and available longer-term reference data in a certain area, can extend the length of climatic time series and offset the shortage of observations. This can be used to assess regional climate change over a much longer time scale. Based on monthly grid climate data from a Coupled Model Inter-comparison Project phase 5 (CMIP5) dataset for the period of 1850-2000, the Climatic Research Unit (CRU) dataset for the period of 1901-2000 and the observed data from 53 meteorological stations located in the Tianshan Mountains region (TMR) of China during the period of 1961-2011, we calibrated and validated monthly average temperature (MAT) and monthly accumulated precipitation (MAP) in the TMR using the delta, physical scaling (SP) and artificial neural network (ANN) methods. Performance and uncertainty during the calibration (1971-1999) and verification (1961-1970) periods were assessed and compared using traditional performance indices and a revised set pair analysis (RSPA) method. The calibration and verification processes were subjected to various sources of uncertainty due to the influence of different reconstructed variables, different data sources, and/or different methods used. According to traditional performance indices, both the CRU and CMIP5 datasets resulted in satisfactory calibrated and verified MAT time series at 53 meteorological stations and MAP time series at 20 meteorological stations using the delta and SP methods for the period of 1961-1999. However, the results differed from those obtained by the RSPA method. This showed that the CRU dataset produced a low degree of uncertainty (positive connection degree) during the calibration and verification of MAT using the delta and SP methods compared to the CMIP5 dataset. Overall, the calibrated and verified MAP had a high degree of uncertainty (negative connection degree) regardless of the dataset or reconstruction method used. Therefore, the reconstructed time series of MAT for the period of 1850 (or 1901)-1960 based on the CRU and CMIP5 datasets using the delta and SP methods could be used for further study. The results of this study will be useful for short-term (several decades) regional climate reconstruction and longer-term (100 a or more) assessments of regional climate change.

Ecosystems in high-altitude regions are more sensitive and respond more rapidly than other ecosystems to global climate warming. The Qinghai-Tibet Plateau (QTP) of China is an ecologically fragile zone that is sensitive to global climate warming. It is of great importance to study the changes in aboveground biomass and species diversity of alpine meadows on the QTP under predicted future climate warming. In this study, we selected an alpine meadow on the QTP as the study object and used infrared radiators as the warming device for a simulation experiment over eight years (2011-2018). We then analyzed the dynamic changes in aboveground biomass and species diversity of the alpine meadow at different time scales, including an early stage of warming (2011-2013) and a late stage of warming (2016-2018), in order to explore the response of alpine meadows to short-term (three years) and long-term warming (eight years). The results showed that the short-term warming increased air temperature by 0.31°C and decreased relative humidity by 2.54%, resulting in the air being warmer and drier. The long-term warming increased air temperature and relative humidity by 0.19°C and 1.47%, respectively, and the air tended to be warmer and wetter. The short-term warming increased soil temperature by 2.44°C and decreased soil moisture by 12.47%, whereas the long-term warming increased soil temperature by 1.76°C and decreased soil moisture by 9.90%. This caused the shallow soil layer to become warmer and drier under both short-term and long-term warming. Furthermore, the degree of soil drought was alleviated with increased warming duration. Under the long-term warming, the importance value and aboveground biomass of plants in different families changed. The importance values of grasses and sedges decreased by 47.56% and 3.67%, respectively, while the importance value of weeds increased by 1.37%. Aboveground biomass of grasses decreased by 36.55%, while those of sedges and weeds increased by 8.09% and 15.24%, respectively. The increase in temperature had a non-significant effect on species diversity. The species diversity indices increased at the early stage of warming and decreased at the late stage of warming, but none of them reached significant levels (P>0.05). Species diversity had no significant correlation with soil temperature and soil moisture under both short-term and long-term warming. Soil temperature and aboveground biomass were positively correlated in the control plots (P=0.014), but negatively correlated under the long-term warming (P=0.013). Therefore, eight years of warming aggravated drought in the shallow soil layer, which is beneficial for the growth of weeds but not for the growth of grasses. Warming changed the structure of alpine meadow communities and had a certain impact on the community species diversity. Our studies have great significance for the protection and effective utilization of alpine vegetation, as well as for the prevention of grassland degradation or desertification in high-altitude regions.

Trees growing in a semi-arid sandy environment are often exposed to drought conditions due to seasonal variations in precipitation, low soil water retention and deep groundwater level. However, adaptability and plasticity of individuals to the changing drought conditions greatly vary among tree species. In this study, we estimated water use (T_s) of Mongolian Scots pine (MSP; Pinus sylvestris var. mongolica Litv.) based on sap flux density measurements over four successive years (2013-2016) that exhibited significant fluctuations in precipitation in a semi-arid sandy environment of northern China. The results showed that fluctuations in daily T_s synchronously varied with dry-wet cycles of soil moisture over the study period. The daily ratio of water use to reference evapotranspiration (T_s/ET₀) on sunny days in each year showed a negative linear relationship with the severity of drought in the upper soil layer (0-1 m; P<0.01). The decrease in Ts induced by erratic drought during the growing season recovered due to precipitation. However, this recovery ability failed under prolonged and severe droughts. The T_s/ET₀ ratio significantly declined with the progressive reduction in the groundwater level (g_w) over the study period (P<0.01). We concluded that the upper soil layer contributed the most to the T_s of MSP during the growing season. The severity and duration of droughts in this layer greatly reduced T_s. Nevertheless, g_w determined whether the T_s could completely recover after the alleviation of long-term soil drought. These results provide practical information for optimizing MSP management to stop ongoing degradation in the semi-arid sandy environments.

Excessive fertilization combined with unreasonable irrigation in farmland of the Hetao Irrigation Area (HIR), China, has resulted in a large amount of nitrogen (N) losses and agricultural non-point source pollution. Application of soil amendments has become one of the important strategies for reducing N losses of farmland. However, there is still no systematic study on the effects of various soil amendments on N losses in the HIR. In this study, three types of soil amendments (biochar, bentonite and polyacrylamide) were applied in a maize-wheat rotation system in the HIR during 2015-2017. Yields of maize and wheat, soil NH₃ volatilization, N₂O emission and NO₃^- leaching were determined and soil N balance was estimated. The results showed that applications of biochar, bentonite and polyacrylamide significantly increased yields of maize by 9.2%, 14.3% and 13.3%, respectively, and wheat by 9.2%, 16.6% and 12.3%, respectively, compared with the control (fertilization alone). Applications of biochar, bentonite and polyacrylamide significantly reduced soil N leaching by 23.1%, 35.5% and 27.1%, soil NH₃-N volatilization by 34.8%, 52.7% and 37.8%, and soil N surplus by 23.9%, 37.4% and 30.6%, respectively. Applications of bentonite and polyacrylamide significantly reduced N₂O-N emissions from soil by 37.3% and 35.8%, respectively, compared with the control. Compared with application of biochar, applications of bentonite and polyacrylamide increased yields of maize and wheat by 5.1% and 3.5%, respectively. Our results suggest that soil amendments (bentonite and polyacrylamide) can play important roles in reducing N losses and increasing yield for the maize-wheat rotation system in the HIR, China.

Central Asia is located in the hinterland of Eurasia, comprising Kazakhstan, Uzbekistan, Kyrgyzstan, Turkmenistan, and Tajikistan; over 93.00% of the total area is dryland. Temperature rise and human activities have severe impacts on the fragile ecosystems. Since the 1970s, nearly half the great lakes in Central Asia have shrunk and rivers are drying rapidly owing to climate changes and human activities. Water shortage and ecological crisis have attracted extensive international attention. In general, ecosystem services in Central Asia are declining, particularly with respect to biodiversity, water, and soil conservation. Furthermore, the annual average temperature and annual precipitation in Central Asia increased by 0.30°C/decade and 6.9 mm/decade in recent decades, respectively. Temperature rise significantly affected glacier retreat in the Tianshan Mountains and Pamir Mountains, which may intensify water shortage in the 21^st century. The increase in precipitation cannot counterbalance the aggravation of water shortage caused by the temperature rise and human activities in Central Asia. The population of Central Asia is growing gradually, and its economy is increasing steadily. Moreover, the agricultural land has not been expended in the last two decades. Thus, water and ecological crises, such as the Aral Sea shrinkage in the 21^st century, cannot be attributed to agriculture extension any longer. Unbalanced regional development and water interception/transfer have led to the irrational exploitation of water resources in some watersheds, inducing downstream water shortage and ecological degradation. In addition, accelerated industrialization and urbanization have intensified this process. Therefore, all Central Asian countries must urgently reach a consensus and adopt common measures for water and ecological protection.

Since the Grain for Green (GFG) program was implemented in 1999, most steeply sloping farmlands in the Loess Plateau of China have been returned to forestland and grassland. To understand its impact on the food production, this study analyzed the spatiotemporal changes of food crop production (FCP) in the plateau and quantified the contribution of sown area and yield changes to the total FCP during 1998-2014 using factor decomposition models, and then discussed the impact of GFG program on the FCP based on literature data. With the implementation of GFG program, total sown area in the Loess Plateau quickly deceased by 17.3% from 1998 to 2003, and then gradually restored to 1.03×10⁷ hm² in 2010. Thereafter, it slightly decreased to 1.02×10⁷ hm² (94.6% of the area in 1998) in 2014. By contrast, total FCP generally showed an apparent growth trend, averagely increased by 1.71% per year in the whole plateau during 1998-2014. This increase was jointly contributed by the improved yield of individual crops, and the adjustment of cropping structure, i.e., the expansion of high yield maize crop. The factor decomposition analysis results indicate that the sown area shrinkage only reduced the growth rate of total FCP by 0.29% per year during 1998-2014, although a significant impact was found for the early stage of 1999-2003. The results suggest that the implementation of GFG program would not induce an obvious risk of the food security. Therefore, it is suggested that the GFG program should be set as a long-term strategic policy, by not only supporting the conversion of slope farmlands, but also helping local farmers to seek sustainable ways of land use to improve the income and livelihood. It can be combined with the poverty eradication program, to simultaneously achieve the national goals of ecological civilization building and the livelihood improvement of rural people in the Loess Plateau. Considering rainfall limitation, the conversion of slope farmlands should be prioritized to grasslands.

Determining the mechanisms controlling the changes of wet and dry conditions will improve our understanding of climate change over the past hundred years, which is of great significance to the study of climate and environmental changes in the arid regions of Central Asia. Forest trees are ecologically significant in the local environment, and therefore the tree ring analysis can provide a clear record of regional historical climate. This study analyzed the correlation between the tree ring width chronology of Juniperus turkestanica Komarov and the standardized precipitation evapotranspiration index (SPEI) in Northwest Tajikistan, based on 56 tree ring samples collected from Shahristan in the Pamir region. Climate data including precipitation, temperature and the SPEI were downloaded from the Climate Research Unit (CRU) TS 4.00. The COFECHA program was used for cross-dating, and the ARSTAN program was used to remove the growth trend of the tree itself and the influence of non-climatic factors on the growth of the trees. A significant correlation was found between the radial growth of J. turkestanica trees and the monthly mean SPEI of February-April. The monthly mean SPEI sequence of February-April during the period of 1895-2016 was reconstructed, and the reconstruction equation explained 42.5% of the variance. During the past 122 a (1895-2016), the study area has experienced three wetter periods (precipitation above average): 1901-1919, 1945-1983 and 1995-2010, and four drier periods (precipitation below average): 1895-1900, 1920-1944, 1984-1994 and 2011-2016. The spatial correlation analysis revealed that the monthly mean SPEI reconstruction sequence of February-April could be used to characterize the large-scale dry-wet variations in Northwest Tajikistan during the period of 1895-2016. This study could provide comparative data for validating the projections of climate models and scientific basis for managing water resources in Tajikistan in the context of climate change.

Changes in ecological stoichiometry reflect nitrogen (N), phosphorus (P) and both N and P limitations in a plant community, which in turn affect plant diversity of the community. However, the relationship between plant community diversity and ecological stoichiometry has not yet been fully researched in arid and semi-arid regions. Ecological stoichiometry and plant community diversity indices of eighteen communities in the upper reaches of Tarim River, northwestern China, were analyzed by multivariate analysis of variance in 2016. The correlation between ecological stoichiometry and plant community diversity was assessed by redundancy analysis (RDA). Results indicated that the Margalef index was significantly correlated with carbon (C) and P concentrations, the Simpson index and Shannon-Weaner index were significantly correlated with plant C concentration, and the Pielou index was significantly correlated with plant C and N concentrations. Moreover, C:N and C:P ratios had significant impacts on plant community diversity. Our results highlight the importance of ecological stoichiometry in driving plant community diversity in the upper reaches of Tarim River, northwestern China.

Regulation of leaf gas exchange plays an important role in the survival of trees and shrubs under future climate change. However, the responses of leaf water potential and gas exchange of shrubs in semi-arid areas to the precipitation alteration are not clear. Here, we conducted a manipulated experiment with three levels of precipitation, i.e., a control with ambient precipitation, 50% above ambient precipitation (irrigation treatment), and 50% below ambient precipitation (drought treatment), with two common shrubs, Salix psammophila C. Wang & C. Y. Yang (isohydric plant, maintaining a constant leaf water potential by stomatal regulation) and Caragana korshinskii Kom. (anisohydric plant, having more variable leaf water potential), on the Chinese Loess Plateau in 2014 and 2015. We measured the seasonal variations of predawn and midday leaf water potential (Ψ_pd and Ψ_md), two parameters of gas exchange, i.e., light-saturated assimilation (A_n) and stomatal conductance (g_s), and other foliar and canopy traits. The isohydric S. psammophila had a similar A_n and a higher g_s than the anisohydric C. korshinskii under drought treatment in 2015, inconsistent with the view that photosynthetic capacity of anisohydric plants is higher than isohydric plants under severe drought. The two shrubs differently responded to precipitation manipulation. Ψ_pd, A_n and g_s were higher under irrigation treatment than control for S. psammophila, and these three variables and Ψ_md were significantly higher under irrigation treatment and lower under drought treatment than control for C. korshinskii. Leaf water potential and gas exchange responded to manipulated precipitation more strongly for C. korshinskii than for S. psammophila. However, precipitation manipulation did not alter the sensitivity of leaf gas exchange to vapor-pressure deficit and soil moisture in these two shrubs. Acclimation to long-term changes in soil moisture in these two shrubs was primarily attributed to the changes in leaf or canopy structure rather than leaf gas exchange. These findings will be useful for modeling canopy water-carbon exchange and elucidating the adaptive strategies of these two shrubs to future changes in precipitation.

In the arid and semi-arid areas of China, rainfall and drought affect the growth and photosynthetic activities of plants. Gross primary productivity (GPP) is one of the most important indices that measure the photosynthetic ability of plants. This paper focused on the GPP of two representative grassland species (Stipa krylovii Roshev. and Allium polyrhizum Turcz. ex Regel) to demonstrate the effect of a temporal rainfall on the two species. Our research was conducted in a temperate grassland in New Barag Right Banner, Hulun Buir City, Inner Mongolia Autonomous Region of China, in a dry year 2015. We measured net ecosystem productivity (NEP) and ecosystem respiration flux (ER) using a transparent chamber system and monitored the photosynthetically active radiation (PAR), air and soil temperature and humidity simultaneously. Based on the measured values of NEP and ER, we calculated the GPP of the two species before and after the rainfall. The saturated GPP per aboveground biomass (GPP_AGB) of A. polyrhizum remarkably increased from 0.033 (±0.018) to 0.185 (±0.055) μmol CO₂/(gdw?s) by 5.6-fold and that of S. krylovii decreased from 0.068 (±0.021) to 0.034 (±0.011) μmol CO₂/(gdw?s) by 0.5-fold on the 1^st and 2^nd d after a 9.1 mm rainfall event compared to the values before the rainfall at low temperatures below 35°C. However, on the 1^st and 2^nd d after the rainfall, both of the saturated GPP_AGB values of S. krylovii and A. polyrhizum were significantly lower at high temperatures above 35°C (0.018 (±0.007) and 0.110 (±0.061) μmol CO₂/(gdw?s), respectively) than at low temperatures below 35°C (0.034 (±0.011) and 0.185 (±0.055) μmol CO₂/(gdw?s), respectively). The results showed that the GPP responses to the temporal rainfall differed between S. krylovii and A. polyrhizum and strongly negative influenced by temperature. The temporal rainfall seems to be more effective on the GPP of A. polyrhizum than S. krylovii. These differences might be related to the different physiological and structural features, the coexistence of the species and their species-specific survival strategies.

Biomass allocation patterns among plant species are related to their adaptive ecological strategies. Ephemeral, ephemeroid and annual plant life forms represent three typical growth strategies of plants that grow in autumn and early spring in the cold deserts of China. These plants play an important role in reducing wind velocity in the desert areas. However, despite numerous studies, the strategies of biomass allocation among plant species with these three life forms remain contentious. In this study, we conducted a preliminary quadrat study during 2014-2016 in the southern part of the Gurbantunggut Desert, China, to investigate the allocation patterns of above-ground biomass (AGB) and below-ground biomass (BGB) at the individual level in 17 ephemeral, 3 ephemeroid and 4 annual plant species. Since ephemeral plants can germinate in autumn, we also compared biomass allocation patterns between plants that germinated in autumn 2015 and spring 2016 for 4 common ephemeral species. The healthy mature individual plants of each species were sampled and the AGB, BGB, total biomass (TB), leaf mass ratio (LMR) and root/shoot ratio (R/S) were calculated for 201 sample quadrats in the study area. We also studied the relationships between AGB and BGB of plants with the three different life forms (ephemeral, ephemeroid and annual). The mean AGB values of ephemeral, ephemeroid and annual plants were 0.806, 3.759 and 1.546 g/plant, respectively, and the mean BGB values were 0.106, 4.996 and 0.166 g/plant, respectively. The mean R/S value was significantly higher in ephemeroid plants (1.675) than in ephemeral (0.154) and annual (0.147) plants. The mean LMR was the highest in annual plants, followed by ephemeroid plants and ephemeral plants, reflecting the fact that annual plants allocate more biomass to leaves, associated with their longer life span. Biomass of ephemeral plants that germinated in autumn was significantly higher than those of corresponding plants that germinated in spring in terms of AGB, BGB and TB. However, the R/S value was similar in plants that germinated in autumn and spring. The slope of regression relationship between AGB and BGB differed significantly among the three plant life forms. These results support different biomass allocation hypotheses. Specifically, at the individual level, the AGB and BGB partitioning supports the allometric hypothesis for ephemeroid and annual plants and the isometric hypothesis for ephemeral plants.

Jizzakh Province in Uzbekistan is one of the largest irrigated areas in Central Asia without natural drainage. In combination with aridity, climate change and extensive irrigation practices, this has led to the widespread salinization of agricultural land. The aim of this study was to identify opportunities to improve the reclamation status of the irrigated area and how best to effectively use the water resources in Jizzakh Province based on investigations conducted between 1995 and 2016. A database of field measurements of groundwater levels, mineralization and soil salinity conducted by the provincial Hydro-Geological Reclamation Expeditions was used in the study. The total groundwater mineralization was determined using a portable electric conductometer (Progress 1T) and the chloride concentration was determined using the Mohr method. The soil salinity analyses were conducted by applying two different methods: (1) the extraction and assessment of the soluble salt content, and (2) using an SM-138 conductivity sensor applied to a 1:1 mixture of soil sample and water. The analyses of the monitoring results and the salt balance in the "irrigation water-soil-drainage water" system clearly demonstrated that the condition of the irrigated land in the province was not significantly improved. Under these conditions, the stability of crop yields is achieved mainly through the use of large volumes of fertilizer. However, excess amounts of mineral fertilizers can also cause the salinization of soils. The average groundwater salinization value in most of the irrigated land (75.3%) fluctuated between 1.1 and 5.0 g/L, while the values were less than 1.0 g/L in 13.1% of the land and in the range of 5.1-10.0 g/L in 10.5% of the land. During the period of 1995-2016 the salinization level of the irrigated land in Jizzakh Province increased slightly and the area could be divided into the following classes: no salinity (17.7% of the total area), low salinity (51.3%), moderate salinity (29.0%), and high salinity (2.0%). Detailed studies of the salt balance in irrigated land, the impact of climate change, increased fertilizer use, and repeated remediation leaching on the groundwater level and mineralization should be conducted in the future, due to the possibility of accelerated salinization, fertility decline, and reduced yields of agricultural crops.

Biochar addition can improve the physical and hydraulic characteristics of sandy soil. This study investigated the effects of biochar on water holding capacity and water movement in sandy soil under drip irrigation. By indoor simulation experiments, the effects of biochar application at five levels (0%, 1%, 2%, 4%and 6%) on the soil water retention curve, infiltration characteristics of drip irrigation and water distribution were tested and analyzed. The results showed thatbiochar addition rate was positively correlated with water holding capacity of sandy soil and soil available water. Within the same infiltration time, with an increasing amount of added biochar, the diffusion distance of the horizontal wetting front (HWF) tended to decrease, but the infiltration distance of vertical wetting front (VWF) initially declined and then rose. The features of wetted bodies changed from "broad-shallow" to "narrow-deep" type. The relationship between the transport distance of HWF and VWF and the infiltration time was described by a power function. At the same distance from the point source, the larger was the amount of added biochar, the higher was the soil water content. Biochar had a great influence on the water content of the layer with biochar (0-200mm) and had some effects at 200-250mm without biochar; but had less influence on the soil water content deeper than 250mm. For the application rate of biochar of 4%, most water was retained within 0-250mm soil layer. However, when biochar application amount was high (6%), it would be helpful for water infiltration. During the improvement of sandy soil, biochar application rate of 4% in the plow layer had the best effect.

Playing an important role in global warming and plant growth, relative humidity (RH) has profound impacts on production and living, and can be used as an integrated indicator for evaluating the wet-dry conditions in the arid and semi-arid area. However, information on the spatial-temporal variation and the influencing factors of RH in these regions is still limited. This study attempted to use daily meteorological data during 1966-2017 to reveal the spatial-temporal characteristics of RH in the arid region of Northwest China through rotated empirical orthogonal function and statistical analysis method, and the path analysis was used to clarify the impact of temperature (T), precipitation (P), actual evapotranspiration (ET_a), wind speed (W) and sunshine duration (S) on RH. The results demonstrated that climatic conditions in North Xinjiang (NXJ) was more humid than those in Hexi Corridor (HXC) and South Xinjiang (SXJ). RH had a less significant downtrend in NXJ than that in HXC, but an increasingly rising trend was observed in SXJ during the last five decades, implying that HXC and NXJ were under the process of droughts, while SXJ was getting wetter. There was a turning point for the trend of RH in Xinjiang, which occurred in 2000. Path analysis indicated that RH was negatively correlated to T, ET_a, W and S, but it increased with increase of P. S, T and W had the greatest direct effects on RH in HXC, NXJ and SXJ, respectively. ET_a was the factor which had the greatest indirect effect on RH in HXC and NXJ, while T was the dominant factor in SXJ.

Salinization is a gradual process that should be monitored. Modelling is a suitable alternative technique that saves time and cost for the field monitoring. But the performance of the models should be evaluated using the measured data. Therefore, the aim of this study was to evaluate and compare the SALTMED and HYDRUS-1D models using the measured soil water content, soil salinity and wheat yield data under different levels of saline irrigation water and groundwater depth. The field experiment was conducted in 2013 and in this research three controlled groundwater depths, i.e., 60 (CD60), 80 (CD80) and 100 (CD100) cm and two salinity levels of irrigation water, i.e., 4 (EC4) and 8 (EC8) dS/m were used in a complete randomized design with three replications. Soil water content and soil salinity were measured in soil profile and compared with the predicted values by the SALTMED and HYDRUS-1D models. Calibrations of the SALTMED and HYDRUS-1D models were carried out using the measured data under EC4-CD100 treatment and the data of the other treatments were used for validation. The statistical parameters including normalized root mean square error (NRMSE) and degree of agreement (d) showed that the values for predicting soil water content and soil salinity were more accurate in the HYDRUS-1D model than in the SALTMED model. The NRMSE and d values of the HYDRUS-1D model were 9.6% and 0.64 for the predicted soil water content and 6.2% and 0.98 for the predicted soil salinity, respectively. These indices of the SALTMED model were 10.6% and 0.81 for the predicted soil water content and 11.0% and 0.97 for the predicted soil salinity, respectively. According to the NRMSE and d values for the predicted wheat yield (9.8% and 0.91, respectively) and dry matter (2.9% and 0.99, respectively), we concluded that the SALTMED model predicted the wheat yield and dry matter accurately.

Water resources are critical for the existence and development of oases in endorheic basins. Thus, to enable sustainable development, it is fundamentally important to understand how to allocate and use these resources in a reasonable way. We therefore simulated and analyzed changes in water consumption pattern within the Dunhuang Oasis of China under three scenarios using a system dynamic model that corresponds to different water consumption pattern. This was done to assess the impacts of regional water resource planning (comprehensive planning of the rational use of water resource and protection of ecosystem services in the Dunhuang Basin) on water consumption pattern within the Dunhuang Oasis. The first of these, Scenario 1, is a baseline in which the status quo is maintained, while Scenario 2 incorporates the comprehensive effects of agricultural water-saving irrigation measures with an inter-basin water diversion project, and Scenario 3 focuses on ecological rehabilitation. In the baseline Scenario 1, the total water consumption within the Dunhuang Oasis increased progressively while agricultural water consumption remained extremely high and threatened overall ecological security. In contrast, Scenario 2 would decrease agricultural water consumption by almost 5.30×10⁷ m³ following the implementation of water-saving practices. The additional water allocated from an inter-basin water diversion project would play an important role in alleviating ecological strain on the oasis. Finally, in Scenario 3, the total irrigated land must be decreased to 20.6×10³ hm² by 2025 assuming that water supply for ecosystem restoration would be at least 50% of the total consumption. Although water resource planning plays a very important role in alleviating the ecological water crisis within the oasis, it is necessary to consider the suitable scale of oasis with regard to current water consumption pattern.

The seif dune field over the gravel desert surface in the eastern margin of the Kumtagh Desert is a valuable experimental site for the observation of dune formation and dynamics. We used high-resolution remote sensing and station observation approaches, combined with wind and grain size data, to study the characteristics of the aeolian environment and the morphologies of and dynamic changes in seif dunes. We observed the ratio of the resultant drift potential (RDP) to the drift potential (DP), which was 0.37, associated with an obtuse bimodal wind regime. The drift potentials in the west-northwest (WNW) and east-northeast (ENE) directions were dominant, and the angle between the two primary DP directions was 135.00°. The dune orientations ranged from 168.75°-213.75°, which were parallel to the resultant drift direction (186.15°). The dune lengths ranged from 51.68 to 1932.11 m with a mean value of 344.91 m. The spacings of the dunes ranged from 32.34 to 319.77 m with a mean value of 93.39 m. The mean grain size of the sediments became finer, and the sorting became better from upwind tail to downwind tip, which indicated that the sediment of the seif dunes in the study region may be transported from northward to southward. The rate of increase in the length, the mean longitudinal migration rate of the dune tail, and the mean longitudinal extension rate of the dune tip (also called elongation rate) were 4.93, 4.63, and 9.55 m/a, respectively. The mean lateral migration vector of the seif dunes was approximately 0.11 m/a towards the west (-0.11 m/a), while the mean amplitude of lateral migration was 0.53 m/a, ignoring the direction of lateral migration. We found that the seif dune field formed first beside seasonal rivers, which can provide sediment, and then expanded downwind.

Windblown sand transport is a leading factor in the geophysical evolution of arid and semi-arid regions. The evolution speed is usually indicated by the sand transport rate that is a function of launch velocity of sand particle, whichhasbeen investigated by the experimental measurement and numerical simulation. However, the obtained results in literatures are inconsistent. Some researchers have discovered a relation between average launch velocity and wind shear velocity, while some other researchers have suggested that average launch velocity is independent of wind shear velocity. The inconsistence of launch velocity leads to a controversy in the scaling law of the sand transport rate in the windblown case. On the contrary, in subaqueous case, the scaling law of the sand transport rate has been widely accepted as a cubic function of fluid shear velocity. In order to explain the debates surrounding the windblown case and the difference between windblown and subaquatic cases, this study reinvestigates the scaling law of the vertical launch velocity of windblown transported sand particles by using a dimensional analysis in consideration of the compatibility of the characteristic time of sand particle motion and that of air flow. Then a wind tunnel experiment is conducted to confirm the revisited scaling law, where the sand particle motion pictures are recorded by a high-speed camera and then the launch velocity is solved by the particle tracking velocimetry. By incorporating the results of dimensional analysis and wind tunnel experiment, it can be concluded that, the ratio of saltonsnumber to reptonsnumberdetermines the scaling law of sand particle launch velocity and that of sand transport rate, and using this ratio is able to explain the discrepancies among the classical models of steady sand transport. Moreover, the resulting scaling law can explain the sand sieving phenomenon: a greater fraction of large grains is observed as the distance to the wind tunnel entrance becomes larger.