Climate change is threatening natural ecosystems in the Earth, and arid regions of southern Africa are particularly exposed to further drying. Welwitschia mirabilis Hook. (Welwitschiaceae) is an unusual gymnosperm tree that is recognized as an icon of the Namib Desert, southern Africa. Many aspects of its biology were investigated in the past, with a special emphasis for its physiology and adaptations, but nothing is known about its potential sensitivity to current climate changes. In this study, we adopted an approach based on distribution data for W. mirabilis and ecological niche models for clarifying the species-climate interactions and for predicting the potential impacts of climate change on W. mirabilis populations in three well-separated sub-ranges (northern, southern and central) in northwestern Namibia, southern Africa. We evidenced that the populations occurring in the northern sub-range have peculiar climatic exigencies compared with those in the central and southern sub-ranges and are particularly exposed to the impact of climate change, which will consist of a substantial increase in temperature across the region. These impacts could be represented by demographic changes that should be detected and monitored detailedly to plan efficient measures for managing populations of this important species on the long-term scale.

Severe wind is a major natural hazard and a main driver of desertification on the Qinghai-Tibet Plateau. Generally, studies of Qinghai-Tibet Plateau's wind climatology focus on mean wind speeds and its gust speeds have been seldom investigated. Here, we used observed daily maximum gust speeds from a 95-station network over a 5-year period (2008-2012) to analyze the characteristics of extreme wind speeds and directions by fitting Weibull and Gumbel distributions. The results indicated the spatial distribution of extreme wind speeds and their direction on the Qinghai-Tibet Plateau is highly variable, with its western portion prone to greater mean speeds of extreme wind gusts than its eastern portion. Maximum extreme wind speeds of 30.9, 33.0, and 32.2 m/s were recorded at three stations along the Qinghai Tibet Railway. Severe winds occurred mostly from November to April, caused primarily by the westerly jet stream. Terrain greatly enhances the wind speeds. Our spatial analysis of wind speed data showed that the wind speeds increased exponentially with an increasing altitude. We also assessed the local wind hazard by calculating the return periods of maximum wind gusts from the observational data based on the statistical extreme value distributions of these wind speeds. Further attention should be given to those stations where the yearly maximum daily extreme wind speed increased at a rate greater than that of mean value of daily extreme wind speeds. Severe extreme wind events in these regions of the plateau are likely to become more frequent. Consequently, building structural designers working in these areas should use updated extreme wind data rather than relying on past data alone.

Hotan Prefecture is located at the southwestern edge of Taklimakan Desert,the world's largest shifting sand desert, of China. The desert is one of the main sources for frequent sand-dust weather events whichstrongly affectthe air quality of Hotan Prefecture.Although this region is characterized by the highest annual mean PM₁₀ concentration values that are routinely recorded by environmental monitoring stations across China, both this phenomenon and its underlying causes have not been adequately addressed in previous researches. Reliable pollutant PM₁₀ data are currently retrieved using a tapered element oscillating microbalance (TEOM) 1400a, a direct real-time monitor, while additional concentration values including for PM_2.5, sulfur dioxide (SO₂), nitrogen dioxide (NO₂), carbon monoxide (CO) and ozone (O₃) have been collected in recent years by the Hotan Environmental Monitoring Station. Based on these data, this paper presents a comparison of the influences of different kinds of sand-dust weather events on PM₁₀ (or PM_2.5) as well as the concentrations of other gaseous pollutants in Hotan Prefecture. It is revealed that the highest monthly average PM₁₀ concentrations are observed in the spring because of the frequent occurrence of three distinct kinds of sand-dust weather events at this time, including dust storms, blowing dust and floating dust. The floating dust makes the most significant contribution to PM₁₀ (or PM_2.5)concentration in this region, a result that differs from eastern Chinese cities where the heaviest PM₁₀ pollution occurs usually in winter and air pollution results from the excess emission of local anthropogenic pollutants. It is also shown that PM₁₀ concentration varies within typical dust storms. PM₁₀ concentrations vary among 20 dust storm events within Hotan Prefecture, and the hourly mean concentrations tend to sharply increase initially then slowly decreasing over time. Data collected from cities in eastern China show the opposite with the hourly mean PM₁₀ (or PM_2.5)concentration tending to slowly increase then sharply decrease during heavy air pollution due to the excess emission of local anthropogenic pollutants. It is also found that the concentration of gaseous pollutants during sand-dust weather events tends to be lower than those cases under clear sky conditions. This indicates that these dust events effectively remove and rapidly diffuse gaseous pollutants. The analysis also shows that the concentration of SO₂ decreases gradually at the onset of all three kinds of sand-dust weather events because of rapidly increasing wind velocity and the development of favorable atmospheric conditions for diffusion. In contrast, changes in O₃ and NO₂ concentrations conformed to the opposite pattern during all three kinds of sand-dust weather events within this region, implying the inter transformation of these gas species during these events.

Biological soil crusts (BSCs) are bio-sedimentary associations that play crucial ecological roles in arid and semi-arid regions. In the Gurbantunggut Desert of China, more than 27% of the land surface is characterized by a predominant cover of lichen-dominated BSCs that contribute to the stability of the desert. However, little is known about the major factors that limit the spatial distribution of BSCs at a macro scale. In this study, the cover of BSCs was investigated along a precipitation gradient from the margins to the center of the Gurbantunggut Desert. Environmental variables including precipitation, soil particle size, soil pH, electrical conductivity, soil organic carbon, total salt, total nitrogen, total phosphorus and total potassium were analyzed at a macro scale to determine their association with differing assemblages of BSCs (cyanobacteria crusts, lichen crusts and moss crusts) using constrained linear ordination redundancy analysis (RDA). A model of BSCs distribution correlated with environmental variables that dominated the first two axes of the RDA was constructed to clearly demonstrate the succession stages of BSCs. The study determined that soil particle size (represented by coarse sand content) and precipitation are the most significant drivers influencing the spatial distribution of BSCs at a macro scale in the Gurbantunggut Desert. The cover of lichen and moss crusts increased with increasing precipitation, while the cover of cyanobacteria crusts decreased with increasing precipitation. The cover of lichen and moss crusts was negatively associated with coarse sand content, whereas the cover of cyanobacteria crusts was positively correlated with coarse sand content. These findings highlight the need for both the availability of soil moisture and a relatively stable of soil matrix, not only for the growth of BSCs but more importantly, for the regeneration and rehabilitation of disturbed BSC communities in arid and semi-arid lands. Thereby, this study will provide a theory basis to effectively increase soil stability in desert regions.

Soil erosion on the Loess Plateau of China is effectively controlled due to the implementation of several ecological restoration projects that improve soil properties and reduce soil erodibility. However, few studies have examined the effects of vegetation restoration on soil properties and erodibility of gully head in the gully regions of the Loess Plateau. The objectives of this study were to quantify the effects of vegetation restoration on soil properties and erodibility in this region. Specifically, a control site in a slope cropland and 9 sites in 3 restored land-use types (5 sites in grassland, 3 in woodland and 1 in shrubland) in the Nanxiaohegou watershed of a typical gully region on the Loess Plateau were selected, and soil and root samples were collected to assess soil properties and root characteristics. Soil erodibility factor was calculated by the Erosion Productivity Impact Calculator method. Our results revealed that vegetation restoration increased soil sand content, soil saturated hydraulic conductivity, organic matter content and mean weight diameter of water-stable aggregate but decreased soil silt and clay contents and soil disintegration rate. A significant difference in soil erodibility was observed among different vegetation restoration patterns or land-use types. Compared with cropland, soil erodibility decreased in the restored lands by 3.99% to 21.43%. The restoration patterns of Cleistogenes caespitosa K. and Artemisia sacrorum L. in the grassland showed the lowest soil erodibility and can be considered as the optimal vegetation restoration pattern for improving soil anti-erodibility of the gully heads. Additionally, the negative linear change in soil erodibility for grassland with restoration time was faster than those of woodland and shrubland. Soil erodibility was significantly correlated with soil particle size distribution, soil disintegration rate, soil saturated hydraulic conductivity, water-stable aggregate stability, organic matter content and root characteristics (including root average diameter, root length density, root surface density and root biomass density), but it showed no association with soil bulk density and soil total porosity. These findings indicate that although vegetation destruction is a short-term process, returning the soil erodibility of cropland to the level of grassland, woodland and shrubland is a long-term process (8-50 years).

Soil acidification is a major global issue of sustainable development for ecosystems. The increasing soil acidity induced by excessive nitrogen (N) fertilization in farmlands has profoundly impacted the soil carbon dynamics. However, the way in which changes in soil pH regulating the soil carbon dynamics in a deep soil profile is still not well elucidated. In this study, through a 12-year field N fertilization experiment with three N fertilizer treatments (0, 120, and 240 kg N/(hm²·a)) in a dryland agroecosystem of China, we explored the soil pH changes over a soil profile up to a depth of 200 cm and determined the responses of soil organic carbon (SOC) and soil inorganic carbon (SIC) to the changed soil pH. Using a generalized additive model, we identified the soil depth intervals with the most powerful statistical relationships between changes in soil pH and soil carbon dynamics. Hierarchical responses of SOC and SIC dynamics to soil acidification were found. The results indicate that the changes in soil pH explained the SOC dynamics well by using a non-linear relationship at the soil depth of 0-80 cm (P=0.006), whereas the changes in soil pH were significantly linearly correlated with SIC dynamics at the 100-180 cm soil depth (P=0.015). After a long-term N fertilization in the experimental field, the soil pH value decreased in all three N fertilizer treatments. Furthermore, the declines in soil pH in the deep soil layer (100-200 cm) were significantly greater (P=0.035) than those in the upper soil layer (0-80 cm). These results indicate that soil acidification in the upper soil layer can transfer excess protons to the deep soil layer, and subsequently, the structural heterogeneous responses of SOC and SIC to soil acidification were identified because of different buffer capacities for the SOC and SIC. To better estimate the effects of soil acidification on soil carbon dynamics, we suggest that future investigations for soil acidification should be extended to a deeper soil depth, e.g., 200 cm.

Mixed or chloride salty ions dominate in saline soils, and exert wide-ranging adversely affect on soil biological processes and soil functions. The objectives of this study were to (1) explore the impacts of mixed (0, 3, 6, 10, 20 and 40 g Cl^-/SO₄^2- salt/kg dry soil) and chloride (0, 1.5, 3, 5, 8 and 15 g Cl^- salt/kg dry soil) salts on soil enzyme activities, soil physiological functional (Biolog) profiles and microbial community structure by using soil enzymatic, Biolog-Eco microplates as well as denaturing gradient gel electrophoresis (DEEG) methods, and (2) determine the threshold concentration of soil electronic conductivity (EC_1:5) on maintaining the functional and structural diversity of soil microbial community. The addition of either Cl^- or mixed Cl^-/SO₄^2- salt obviously increased soil EC, but adversely affected soil biological activities including soil invertase activity, soil microbial biomass carbon (MBC) and substrate-induced respiration (SIR). Cl^- salt showed a greater deleterious influence than mixed Cl^-/SO₄^2- salt on soil enzymes and MBC, e.g., the higher soil MBC consistently appeared with Cl^-/SO₄^2- instead of Cl^- treated soil. Meanwhile, we found that SIR was more reliable than soil basal respiration (SBR) on explaining the changes of soil biological activity responsive to salt disturbance. In addition, microbial community structures of the soil bacteria, fungi, and Bacillus were obviously affected by both salt types and soil EC levels, and its diversity increased with increasing of mixed Cl^-/SO₄^2- salt rates, and then sharply declined down after it reached critical point. Moreover, the diversity of fungal community was more sensitive to the mixed salt addition than other groups. The response of soil physiological profiles (Biolog) followed a dose-response pattern with Cl^- (R²=0.83) or mixed Cl^-/SO₄^2- (R²=0.89) salt. The critical threshold concentrations of salts for soil physiological function were 0.45 dS/m for Cl^- and 1.26 dS/m for Cl^-/SO₄^2-, and those for soil microbial community structural diversity were 0.70 dS/m for Cl^- and 1.75 dS/m for Cl^-/SO₄^2-.

Many riparian (Tugai) forests growing along rivers in arid and hyper-arid regions of Central Asia are dominated by the Euphrates poplar (Populus euphratica). Besides generative reproduction, which is only possible upon flooding events and at a distance to the groundwater of less than 2 m, this phreatophytic tree species also reproduces vegetatively by forming clones that can cover land surface areas of several hectares. Along a gradient of groundwater distances, we investigated whether the fraction of clones in P. euphratica stands (1) increases with increasing distance to the water table; (2) is higher if supplied with water via river cut-offs; and (3) approaches 100% at a short distance to the groundwater, but at high salt concentrations in the upper soil layers, which would prevent germination and establishment of seedlings. AFLP (Amplified Fragment Length Polymorphism) analyses were conducted on leaf samples taken from mature P. euphratica trees growing at the fringes of the Taklimakan Desert in stands with different distances (2-12 m) to the groundwater at two plots at the middle and the lower reaches of the Tarim River and in a stand close to Ebinur Lake, Xinjiang, China. Genetic diversity was large among plots, but considerably smaller within plots. We found the highest genetic diversity (caused by regeneration from seeds) at plots that have a short distance to the groundwater or are supplied with additional water. There was no significant relationship between groundwater distance and clonal fraction. All investigated trees at the saline Ebinur Lake site belonged to one single clone. Our results demonstrate that the genetic pattern of this widespread species is not easily predictable even over small distances as it is a result of a complex interplay of stand history and dispersal of propagules (pollen, seeds, and vegetative diaspores) by wind and water. In conservation and restoration schemes, P. euphratica stands with a high genetic diversity and stands that grow at short distances to the water table and are regularly subjected to flooding (which favors generative over clonal reproduction) should be prioritized.

Estimation of above-ground biomass is vital for understanding ecological processes. Since direct measurement of above-ground biomass is destructive, time consuming and labor intensive, canopy cover can be considered as a predictor if a significant correlation between the two variables exists. In this study, relationship between canopy cover and above-ground biomass was investigated by a general linear regression model. To do so, canopy cover and above-ground biomass were measured at 5 sub-life forms (defined as life forms grouped in the same height classes) using 380 quadrats, which is systematic-randomly laid out along a 10-km transect, during four sampling periods (May, June, August, and September) in an arid rangeland of Marjan, Iran. To reveal whether obtained canopy cover and above-ground biomass of different sampling periods can be lumped together or not, we applied a general linear model (GLM). In this model, above-ground biomass was considered as a dependent or response variable, canopy cover as an independent covariate or predictor factor and sub-life forms as well as sampling periods as fixed factors. Moreover, we compared the estimated above-ground biomass derived from remotely sensed images of Landsat-8 using NDVI (normalized difference vegetation index), after finding the best regression line between predictor (measured canopy cover in the field) and response variable (above-ground biomass) to test the robustness of the induced model. Results show that above-ground biomass (response variable) of all vegetative forms and periods can be accurately predicted by canopy cover (predictor), although sub-life forms and sampling periods significantly affect the results. The best regression fit was found for short forbs in September and shrubs in May, June and August with R² values of 0.96, 0.93 and 0.91, respectively, whilst the least significant was found for short grasses in June, tall grasses in August and tall forbs in June with R² values of 0.71, 0.73 and 0.75, respectively. Even though the estimated above-ground biomass by NDVI is also convincing (R²=0.57), the canopy cover is a more reliable predictor of above-ground biomass due to the higher R² values (from 0.75 to 0.96). We conclude that canopy cover can be regarded as a reliable predictor of above-ground biomass if sub-life forms and sampling periods (during growing season) are taken into account. Since, (1) plant canopy cover is not distinguishable by remotely sensed images at the sub-life form level, especially in sparse vegetation of arid and semi-arid regions, and (2) remotely sensed-based prediction of above-ground biomass shows a less significant relationship (R²=0.57) than that of canopy cover (R² ranging from 0.75 to 0.96), which suggests estimating of plant biomass by canopy cover instead of cut and weighting method is highly recommended. Furthermore, this fast, nondestructive and robust method that does not endanger rare species, gives a trustworthy prediction of above-ground biomass in arid rangelands.

Net primary productivity (NPP), as an important variable and ecological indicator in grassland ecosystems, can reflect environmental change and the carbon budget level. The Ili River Valley is a wetland nestled in the hinterland of the Eurasian continent, which responds sensitively to the global climate change. Understanding carbon budget and their responses to climate change in the ecosystem of Ili River Valley has a significant effect on the adaptability of future climate change and sustainable development. In this study, we calculated the NPP and analyzed its spatio-temporal pattern of the Ili River Valley during the period 2000-2014 using the normalized difference vegetation index (NDVI) and an improved Carnegie-Ames-Stanford (CASA) model. Results indicate that validation showed a good performance of CASA over the study region, with an overall coefficient of determination (R²) of 0.65 and root mean square error (RMSE) of 20.86 g C/(m²?a). Temporally, annual NPP of the Ili River Valley was 599.19 g C/(m²?a) and showed a decreasing trend from 2000 to 2014, with an annual decrease rate of -3.51 g C/(m²?a). However, the spatial variation was not consistent, in which 55.69% of the areas showed a decreasing tendency, 12.60% of the areas remained relatively stable and 31.71% appeared an increasing tendency. In addition, the decreasing trends in NPP were not continuous throughout the 15-year period, which was likely being caused by a shift in climate conditions. Precipitation was found to be the dominant climatic factor that controlled the inter-annual variability in NPP. Furthermore, the correlations between NPP and climate factors differed along the vertical zonal. In the medium-high altitudes of the Ili River Valley, the NPP was positively correlated to precipitation and negatively correlated to temperature and net radiation. In the low-altitude valley and high-altitude mountain areas, the NPP showed a negative correlation with precipitation and a weakly positive correlation with temperature and net radiation. The results suggested that the vegetation of the Ili River Valley degraded in recent years, and there was a more complex mechanism of local hydrothermal redistribution that controlled the growth of vegetation in this valley ecosystem.

High and efficient use of limited rainwater resources is of crucial importance for the crop production in arid and semi-arid areas.To investigate the effects of different soil and crop management practices (i.e., mulching mode treatments: flat cultivation with non-mulching, flat cultivation with straw mulching, plastic-covered ridge with bare furrow and plastic-covered ridge with straw-covered furrow; and planting density treatments: low planting density of 45,000 plants/hm², medium planting density of 67,500 plants/hm² and high planting density of 90,000 plants/hm²) on rainfall partitioning by dryland maize canopy, especially the resulted net rainfall input beneath the maize canopy, we measured the gross rainfall, throughfall and stemflow at different growth stages of dryland maize in 2015 and 2016 on the Loess Plateau of China. The canopy interception loss was estimated by the water balance method. Soil water storage, leaf area index, grain yield (as well as it components) and water use efficiency of dryland maize were measured or calculated. Results showed that the cumulative throughfall, cumulative stemflow and cumulativecanopy interception loss during the whole growing season accounted for 42.3%-77.5%, 15.1%-36.3% and 7.4%-21.4% of the total gross rainfall under different treatments, respectively. Soil mulching could promote the growth and development of dryland maize and enhance the capability of stemflow production and canopy interception loss, thereby increasing the relative stemflow and relative canopy interception loss and reducing the relative throughfall. The relative stemflow and relative canopy interception loss generally increased with increasing planting density, while the relative throughfall decreased with increasing planting density. During the two experimental years, mulching mode had no significant influence on net rainfall due to the compensation between throughfall and stemflow, whereas planting density significantly affected net rainfall. The highest grain yield and water use efficiency of dryland maize were obtained under the combination of medium planting density of 67,500 plants/hm² and mulching mode of plastic-covered ridge with straw-covered furrow. Soil mulching can reduce soil evaporation and retain more soil water for dryland maize without reducing the net rainfall input beneath the maize canopy, which may alleviate the contradiction between high soil water consumption and insufficient rainfall input of the soil. In conclusion, the application of medium planting density (67,500 plants/hm²) under plastic-covered ridge with bare furrow is recommended for increasing dryland maize production on the Loess Plateau of China.

Reduced tillage provides ecological and economic benefits to arable land on the Loess Plateau of China, where soil erosion has long been a serious problem and soil water availability is largely restricted. However, high abundances of weeds in reduced tillage systems cause significant yield losses. In this study, we explored the effects of no-tillage and stubble retention on the number and density of weeds and weed seeds in a 12-year maize-winter wheat-common vetch rotation on the Loess Plateau. Four treatments including conventional tillage, no-tillage, conventional tillage+stubble retention and no-tillage+stubble retention were designed and applied. We found that no-tillage increased the number of weed species and weed density in most of the crops, while stubble retention decreased weed density in maize and tended to suppress weeds in both no-tillage treatments (no-tillage and no-tillage+stubble retention). No-tillage led to an increase in the number of weed species in the weed seedbank and tended to increase seed density during the spring growth of winter wheat, but it decreased seed density during post-vetch fallow. Stubble retention tended to reduce seed density during the spring growth of winter wheat and post-vetch fallow. We concluded that no-tillage can promote weeds in the experimental crop rotation, while stubble retention suppresses weeds in untilled fields. The combined effects of stubble retention and no-tillage on weed suppression varied among the three crops. Based on these results, we recommend stubble retention in untilled legume-crop rotations on the Loess Plateau to improve the control of weeds.