Assessing the Performance of CMIP6 Models in Simulating Droughts across Global Drylands

Both the attribution of historical change and future projections of droughts rely heavily on climate modeling. However,reasonable drought simulations have remained a challenge, and the related performances of the current state-of-the-art Coupled Model Intercomparison Project phase 6(CMIP6) models remain unknown. Here, both the strengths and weaknesses of CMIP6 models in simulating droughts and corresponding hydrothermal conditions in drylands are assessed.While the general patterns of simulated meteorological elements in drylands resemble the observations, the annual precipitation is overestimated by ~33%(with a model spread of 2.3%–77.2%), along with an underestimation of potential evapotranspiration(PET) by ~32%(17.5%–47.2%). The water deficit condition, measured by the difference between precipitation and PET, is 50%(29.1%–71.7%) weaker than observations. The CMIP6 models show weaknesses in capturing the climate mean drought characteristics in drylands, particularly with the occurrence and duration largely underestimated in the hyperarid Afro-Asian areas. Nonetheless, the drought-associated meteorological anomalies, including reduced precipitation, warmer temperatures, higher evaporative demand, and increased water deficit conditions, are reasonably reproduced. The simulated magnitude of precipitation(water deficit) associated with dryland droughts is overestimated by 28%(24%) compared to observations. The observed increasing trends in drought fractional area,occurrence, and corresponding meteorological anomalies during 1980–2014 are reasonably reproduced. Still, the increase in drought characteristics, associated precipitation and water deficit are obviously underestimated after the late 1990s,especially for mild and moderate droughts, indicative of a weaker response of dryland drought changes to global warming in CMIP6 models. Our results suggest that it is imperative to employ bias correction approaches in drought-related studies over drylands by using CMIP6 outputs.

Prevalence of vegetation browning in China’s drylands under climate change

Vegetation greening has long been acknowledged,but recent studies have pointed out that vegetation greening is possibly stalled or even reversed.However,detailed analyses about greening reversal or increased browning of vegetation remain scarce.In this study,we utilized the normalized difference vegetation index(NDVI)as an indicator of vegetation to investigate the trends of vegetation greening and browning(monotonic,interruption,and reversal)through the breaks for the additive season and trend(BFAST)method across China’s drylands from 1982 to 2022.It also reveals the impacts of ecological restoration programs(ERPs)and climate change on these vegetation trends.We find that the vegetation displays an obvious pattern of east-greening and west-browning in China’s drylands.Greening trends mainly exhibits monotonic greening(29.8%)and greening with setback(36.8%),whereas browning shows a greening to browning reversal(19.2%).The increase rate of greening to browning reversal is 0.0342/yr,which is apparently greater than that of greening with setback,0.0078/yr.This research highlights that,under the background of widespread vegetation greening,vegetation browning is pro-gressively increasing due to the effects of climate change.Furthermore,the ERPs have significantly increased vegetation coverage,with the increase rate in 2000-2022 being twice as much as that of 1982-1999 in reveg-etation regions.Vegetation browning in southwestern Qingzang Plateau is primarily driven by adverse climatic factors and anthropogenic disturbances,which offset the efforts of ERPs.

A New Merged Product Reveals Precipitation Features over Drylands in China

Due to the considerable uncertainties inherent in the datasets describing the spatiotemporal distributions of precipitation in the drylands of China,this study presents a new merged monthly precipitation product with a spatial resolution of approximately 0.2°×0.2°during 1980–2019.The newly developed precipitation product was validated at different temporal scales(e.g.,monthly,seasonally,and annually).The results show that the new product consistently aligns with the spatiotemporal distributions reported by the Chinese Meteorological Administration Land Data Assimilation System(CLDAS)product and Multi-Source Weighted Ensemble Precipitation(MSWEP).The merged product exhibits exceptional quality in describing the drylands of China,with a bias of–2.19 mm month^(–1)relative to MSWEP.In addition,the annual trend of the merged product(0.09 mm month^(–1)yr^(−1))also closely aligns with that of the MSWEP(0.11 mm month^(–1)yr^(−1))during 1980–2019.The increasing trend indicates that the water cycle and wetting process intensified in the drylands of China during this period.In particular,there was an increase in wetting during the period from 2001–2019.Generally,the merged product exhibits potential value for improving our understanding of the climate and water cycle in the drylands of China.

Optimized NPK fertilizer recommendations based on topsoil available nutrient criteria for wheat in drylands of China

The optimized management of crop fertilization is very important for improving crop yield and reducing the consumption of chemical fertilizers.Critical nutrient values can be used for evaluating the nutritional status of a crop,and they reflect the nutrient concentrations above which the plant is sufficiently supplied for achieving the maximum potential yield.Based on on-farm surveys of 504 farmers and 60 field experimental sites in the drylands of China,we proposed a recommended fertilization method to determine nitrogen(N),phosphorus(P),and potassium(K)fertilizer input rates for wheat production,and then validated the method by a field experiment at 66 different sites in northern China.The results showed that wheat grain yield varied from 1.1 to 9.2 t ha^(-1),averaging 4.6 t ha^(-1),and it had a quadratic relationship with the topsoil(0-20 cm)nitrate N and soil available P contents at harvest.However,yield was not correlated with the inputs of N,P,and K fertilizers.Based on the relationship(exponential decay model)between 95–105%of the relative yield and topsoil nitrate N,available P,and available K contents at wheat harvest from 60 field experiments,the topsoil critical nutrient values were determined as 34.6,15.6,and 150 mg kg^(-1)for soil nitrate N,available P,and available K,respectively.Then,based on five groups of relative yield(>125%,115–125%,105–115%,95–105%,and<95%)and the model,the five groups of topsoil critical nutrient levels and fertilization coefficients(Fc)were determined.Finally,we proposed a new method for calculating the recommended fertilizer input rate as:Fr=Gy×Nr×Fc,where Fr is the recommended fertilizer(N/P/K)input rate;Gy is the potential grain yield;Nr is the N(N_(rN)),P(N_(rP)),and K(N_(rK))nutrient requirements for wheat to produce 1,000 kg of grain;and Fc is a coefficient for N(N_c)/P(P_c)/K(K_c)fertilizer.A 2-year validated experiment confirmed that the new method reduced N fertilizer input by 17.5%(38.5 kg N ha^(-1))and P fertilizer input by 43.5%(57.5 kg P_(2)O_(5) ha^(-1))in northern China and did not reduce the wheat yield.This outcome can significantly increase the farmers’benefits(by 7.58%,or 139 US$ha^(-1)).Therefore,this new recommended fertilization method can be used as a tool to guide N,P,and K fertilizer application rates for dryland wheat production.

Characteristics and drivers of the soil multifunctionality under different land use and land cover types in the drylands of China

The drylands of China cover approximately 6.6×106 km2 and are home to approximately 5.8×10^(8)people,providing important ecosystem services for human survival and development.However,dryland ecosystems are extremely fragile and sensitive to external environmental changes.Land use and land cover(LULC)changes significantly impact soil structure and function,thus affecting the soil multifunctionality(SMF).However,the effect of LULC changes on the SMF in the drylands of China has rarely been reported.In this study,we investigated the characteristics of the SMF changes based on soil data in the 1980s from the National Tibetan Plateau Data Center.We explored the drivers of the SMF changes under different LULC types(including forest,grassland,shrubland,and desert)and used structural equation modeling to explore the main driver of the SMF changes.The results showed that the SMF under the four LULC types decreased in the following descending order:forest,grassland,shrubland,and desert.The main driver of the SMF changes under different LULC types was mean annual temperature(MAT).In addition to MAT,pH in forest,soil moisture(SM)and soil biodiversity index in grassland,SM in shrubland,and aridity index in desert are crucial factors for the SMF changes.Therefore,the SMF in the drylands of China is regulated mainly by MAT and pH,and comprehensive assessments of the SMF in drylands need to be performed regarding LULC changes.The results are beneficial for evaluating the SMF among different LULC types and predicting the SMF under global climate change.

Long-term light grazing does not change soil organic carbon stability and stock in biocrust layer in the hilly regions of drylands

Livestock grazing is the most extensive land use in global drylands and one of the most extensive stressors of biological soil crusts(biocrusts).Despite widespread concern about the importance of biocrusts for global carbon(C)cycling,little is known about whether and how long-term grazing alters soil organic carbon(SOC)stability and stock in the biocrust layer.To assess the responses of SOC stability and stock in the biocrust layer to grazing,from June to September 2020,we carried out a large scale field survey in the restored grasslands under long-term grazing with different grazing intensities(represented by the number of goat dung per square meter)and in the grasslands strictly excluded from grazing in four regions(Dingbian County,Shenmu City,Guyuan City and Ansai District)along precipitation gradient in the hilly Loess Plateau,China.In total,51 representative grassland sites were identified as the study sampling sites in this study,including 11 sites in Guyuan City,16 sites in Dingbian County,15 sites in Shenmu City and 9 sites in Ansai District.Combined with extensive laboratory analysis and statistical analysis,at each sampling site,we obtained data on biocrust attributes(cover,community structure,biomass and thickness),soil physical-chemical properties(soil porosity and soil carbon-to-nitrogen ratio(C/N ratio)),and environmental factors(mean annual precipitation,mean annual temperature,altitude,plant cover,litter cover,soil particle-size distribution(the ratio of soil clay and silt content to sand content)),SOC stability index(SI)and SOC stock(SOCS)in the biocrust layer,to conduct this study.Our results revealed that grazing did not change total biocrust cover but markedly altered biocrust community structure by reducing plant cover,with a considerable increase in the relative cover of cyanobacteria(23.1%)while a decrease in the relative cover of mosses(42.2%).Soil porosity and soil C/N ratio in the biocrust layer under grazing decreased significantly by 4.1%–7.2%and 7.2%–13.3%,respectively,compared with those under grazing exclusion.The shifted biocrust community structure ultimately resulted in an average reduction of 15.5%in SOCS in the biocrust layer under grazing.However,compared with higher grazing(intensity of more than 10.00 goat dung/m2),light grazing(intensity of 0.00–10.00 goat dung/m2 or approximately 1.20–2.60 goat/(hm2•a))had no adverse effect on SOCS.SOC stability in the biocrust layer remained unchanged under long-term grazing due to the offset between the positive effect of the decreased soil porosity and the negative effect of the decreased soil C/N ratio on the SOC resistance to decomposition.Mean annual precipitation and soil particle-size distribution also regulated SOC stability indirectly by influencing soil porosity through plant cover and biocrust community structure.These findings suggest that proper grazing might not increase the CO_(2) release potential or adversely affect SOCS in the biocrust layer.This research provides some guidance for proper grazing management in the sustainable utilization of grassland resources and C sequestration in biocrusts in the hilly regions of drylands.

Sensitivity of Potential Evapotranspiration Estimation to the Thornthwaite and Penman–Monteith Methods in the Study of Global Drylands

Drylands are among those regions most sensitive to climate and environmental changes and human-induced perturbations.The most widely accepted definition of the term dryland is a ratio,called the Surface Wetness Index(SWI),of annual precipitation to potential evapotranspiration(PET)being below 0.65.PET is commonly estimated using the Thornthwaite(PET Th)and Penman–Monteith equations(PET PM).The present study compared spatiotemporal characteristics of global drylands based on the SWI with PET Th and PET PM.Results showed vast differences between PET Th and PET PM;however,the SWI derived from the two kinds of PET showed broadly similar characteristics in the interdecadal variability of global and continental drylands,except in North America,with high correlation coefficients ranging from 0.58 to 0.89.It was found that,during 1901–2014,global hyper-arid and semi-arid regions expanded,arid and dry sub-humid regions contracted,and drylands underwent interdecadal fluctuation.This was because precipitation variations made major contributions,whereas PET changes contributed to a much lesser degree.However,distinct differences in the interdecadal variability of semi-arid and dry sub-humid regions were found.This indicated that the influence of PET changes was comparable to that of precipitation variations in the global dry–wet transition zone.Additionally,the contribution of PET changes to the variations in global and continental drylands gradually enhanced with global warming,and the Thornthwaite method was found to be increasingly less applicable under climate change.

Spatial variability and driving factors of soil multifunctionality in drylands of China

Drylands are highly vulnerable to climate change and human activities.The drylands of China account for approximately 10.8%of global drylands,and China is the country most severely affected by aridity in Asia.Therefore,studying the spatial variation characteristics in soil multifunctionality(SMF)and investigating the driving factors are critical for elucidating and managing the functions of dryland ecosystems in China.Based on the environmental factors(mean annual precipitation(MAP),mean annual temperature(MAT),solar radiation(Srad),soil acidity(pH),enhanced vegetation index(EVI),and cation exchange capacity(CEC))and aridity from the“dataset of soil properties for land surface modeling over China”,we used non-linear regression,ordinary least square(OLS)regression,structural equation model(SEM),and other analytical methods to investigate the relationships of SMF with environmental factors across different aridity levels in China.SMF in different dryland regions varied significantly and showed a patchy distribution,with SMF index values ranging from–1.21 to 2.42.Regions with SMF index values from–0.20 to 0.51 accounting for 63.0%of dryland area in China.OLS regression results revealed that environmental factors like MAP,MAT,Srad,pH,EVI,and CEC were significantly related to SMF(P<0.05).MAP and MAT were correlated to SMF at the whole aridity level(P<0.05).SEM results showed that the driving factors of SMF differed depending on the aridity level.Soil pH was the strongest driving factor of SMF when the aridity was less than 0.80(P<0.001).Both soil CEC and EVI had a positive effect on SMF when aridity was greater than 0.80(P<0.01),with soil CEC being the strongest driving factor.The importance ranking revealed that the relative importance contribution of soil pH to SMF was greatest when aridity was less than 0.80(66.9%).When aridity was set to greater than 0.80,the relative importance contributions of CEC and EVI to SMF increased(45.1%and 31.9%,respectively).Our findings indicated that SMF had high spatial heterogeneity in drylands of China.The aridity threshold controlled the impact of environmental factors on SMF.

A bibliometric analysis of carbon exchange in global drylands

Drylands refer to regions with an aridity index lower than 0.65,and billions of people depend on services provided by the critically important ecosystems in these areas.How ecosystem carbon exchange in global drylands(CED)occurs and how climate change affects CED are critical to the global carbon cycle.Here,we performed a comprehensive bibliometric study on the fields of annual publications,marked journals,marked institutions,marked countries,popular keywords,and their temporal evolution to understand the temporal trends of CED research over the past 30 a(1991-2020).We found that the annual scientific publications on CED research increased significantly at an average growth rate of 7.93%.Agricultural Water Management ranked first among all journals and had the most citations.The ten most productive institutions were centered on drylands in America,China,and Australia that had the largest number and most citations of publications on CED research."Climate change"and climate-related(such as"drought","precipitation","temperature",and"rainfall")research were found to be the most popular study areas.Keywords were classified into five clusters,indicating the five main research focuses on CED studies:hydrological cycle,effects of climate change,carbon and water balance,productivity,and carbon-nitrogen-phosphorous coupling cycles.The temporal evolution of keywords further showed that the areas of focus on CED studies were transformed from classical pedology and agricultural research to applied ecology and then to global change ecological research over the past 30 a.In future CED studies,basic themes(such as"water","yield",and"salinity")and motor themes(such as"climate change","sustainability",and"remote sensing")will be the focus of research on CED.In particular,multiple integrated methods to understand climate change and ecosystem sustainability are potential new research trends and hotspots.

Abkhandari (Aquifer Management): a Green Path to the Sustainable Development of Marginal Drylands

Recurrent droughts and occasional floods are the facts of life in drylands. The presence of innumerable societies in deserts is the living proof that even the extremely dry environments are livable; the secret is how to adapt to the situation. Floodwater management, the most important art of sedentarized desert dwellers, is the secret of living with deserts. Floodwater irrigation alone, or in combination with the artificial recharge of groundwater （ARG）, has sustained the Persians for millennia. The scientists at the Kowsar Floodwater Spreading and Aquifer Management Research, Training and Extension Station in the Gareh Bygone Plain have been working since January 1983 to revive this ancient art and upgrade it to the level of science. A summary of important findings is provided as follows： 1. Debris cones and coarse alluvial fans are the best places for the ARG as they provide the potential aquifer for groundwater storage. The recharged water may be used for irrigation on the lower lying, fine alluvium; 2. Flood is not a proverbial curse but a blessing in disguise, and the turbid floodwater is the best resource for the sustainable development of drylands; 3. Translocation of fine clay minerals eventually makes the vadose zone impermeable. Planting deep-rooted, drought-resistent trees and shrubs, and introducing sowbugs （Hemilepistus shirazi Schuttz） are reliable methods to increase infiltration rate and the saturated hydraulic conductivity of the ARG systems; 4. The moving sand stabilization is best achieved by spreading turbid floodwater onto them. Establishing of living windbreaks and planting of fodder trees and shrubs turn a sand menace into a verdant pasture; 5. The 10 year average of native forage yield in the ARG systems has been 445 kg·ha^-1·year^-1 as opposed to 92 kg·ha^-1.year^-1 for the control. At 4×4 m^2 spacing, Atriplex lentiformis （Torr.） Wats. can annually yield 1500 kg ha^-1 of dry matter and support 3 heads of sheep; 6. The stem- and fuel wood yield of 18 year old Eucalyptus camaldulensis Dehnh. in the ARG system have been 4,684 and 781 kg·ha^-1.year^-1, respectively. The mean annual carbon sequestration of this tree has been 2.975 tons per ha; 7.Up to 80% of the diverted floodwaters reach the unconfined aquifer. As evaporation practically wastes large volumes of water in surface reservoirs, storing water underground is logical in deserts; 8. The high evaporation rate, the large sediment load, the environmental hazards, the undesirable social costs, the long time needed for the different phases of study, and the very large price tag make dam building the most hydro-illogical choice in deserts where the ARG is practicable; 9. Each 5.5 ha of the irrigated farm provides income for a family of 7.64 member and 0.38 employment opportunities for a farm hand. On average, 4-ha of an ARG system provides one full time job for irrigation farmers. The benefit： cost ratio for this project is 22; 10. The number of wells in the area affected by the ARG activities has increased lo-fold to 130 wells, the irrigated area has increased 8-fold to 1,193 ha, and 345 job opportunities have been created in 4 villages that surround the Station.

Total Carbon Stock and Potential Carbon Sequestration Economic Value of Mukogodo Forest-Landscape Ecosystem in Drylands of Northern Kenya

Carbon sequestration is one of the important ecosystem services provided by forested landscapes. Dry forests have high potential for carbon storage. However, their potential to store and sequester carbon is poorly understood in Kenya. Moreover, past attempts to estimate carbon stock have ignored drylands ecosystem heterogeneity. This study assessed the potential of Mukogodo dryland forest-landscape in offsetting carbon dioxide through carbon sequestration and storage. Four carbon pools (above and below ground biomass, soil, dead wood and litter) were analyzed. A total of 51 (400 m2) sample plots were established using stratified-random sampling technique to estimate biomass across six vegetation classes in three landscape types (forest reserve, ranches and conservancies) using nested-plot design. Above ground biomass was determined using generalized multispecies model with diameter at breast height, height and wood density as variables. Below ground, soil, litter and dead wood biomass;carbon stocks and carbon dioxide equivalents (CO2eq) were estimated using secondary information. The CO2eq was multiplied by current prices of carbon trade to compute carbon sequestration value. Mean ± SE of biomass and carbon was determined across vegetation and landscape types and mean differences tested by one-way Analysis of Variance. Mean biomass and carbon was about 79.15 ± 40.22 TB ha-1 and 37.25 ± 18.89 TC ha-1 respectively. Cumulative carbon stock was estimated at 682.08 TC ha-1;forest reserve (251.57 TC ha-1) had significantly high levels of carbon stocks compared to ranches (209.78 TC ha-1) and conservancies (220.73 TC ha-1, P = 0.000). Further, closed forest significantly contributed to the overall biomass and carbon stock (58%). The carbon sequestration potential was about 19.9MTCO2eq with most conservative worth of KES 39.9B (US$40M) per annum. The high carbon stock in the landscape shows the potential of dryland ecosystems as carbon sink for climate change mitigation. However, for communities to benefit from bio-carbon funds in future, sustainable landscape management and restorative measures should be practiced to enhance carbon storage and provision of other ecosystem services.

Estimating the optimal vegetation coverage for the dominant tree and shrub species over China's northwest drylands

Anthropogenic revegetation is an effective way to control soil erosion and restore degraded ecosystems in China's northwest drylands(NWD).However,excessive vegetation cover expansion has long been known to increase evapotranspiration,leading to reduced local water availability,which can in turn threaten the health and services of restored ecosystems.Determining the optimal vegetation coverage(OVC)is critical for balancing the trade-off between plant growth and water consumption in water-stressed areas,yet quantitative assessments over the entire NWD are still lacking.In this study,a modified Biome BioGeochemical Cycles(Biome-BGC)model was used to simulate the long-term(1961–2020)dynamics of actual evapotranspiration(ET_(a)),net primary productivity(NPP),and leaf area index(LAI)for the dominant non-native tree(R.pseudoacacia and P.sylvestris)and shrub(C.korshinkii and H.rhamnoides)species at 246 meteorological sites over NWD.The modified model incorporated the Richards equation to simulate transient unsaturated water flow in a multilayer soil module,and both soil and eco-physiological parameters required by the model were validated using field-observed ETadata for each species.Spatial distributions of OVC(given by the mean maximum LAI,LAI_(max))for the dominant species were determined within three hydrogeomorphic sub-areas(i.e.,the loess hilly-gully sub-area,the windy and sandy sub-area,and the desert sub-area).The modified Biome-BGC model performed well in terms of simulating ET_(a) dynamics for the four plant species.Spatial distributions of mean ET_a,NPP,and LAI_(max)generally exhibited patterns similar to mean annual precipitation(MAP).In the loess hilly-gully sub-area(MAP:210 to 710 mm),the OVC respectively ranged from 1.7 to 2.9 and 0.8 to 2.9 for R.pseudoacacia and H.rhamnoides.In the windy and sandy sub-area(MAP:135 to 500 mm),the OVC ranged from 0.3 to 3.3,0.5 to 2.6 and 0.6 to 2.1for P.sylvestris,C.korshinkii and H.rhamnoides,respectively.In the desert sub-area(MAP:90 to 500 mm),the OVC ranged from 0.4 to 1.7 for H.rhamnoides.Positive differences between observed and simulated plant coverage were found over 51%of the forest-and shrub-covered area,especially in the loess hilly-gully sub-area,suggesting possible widespread overplanting in those areas.This study provides critical revegetation thresholds for dominant tree and shrub species to guide future revegetation activities.Further revegetation in areas with overplanting should be undertaken with caution,and restored ecosystems that exceed the OVC should be managed(e.g.,thinning)to maintain a sustainable ecohydrological environment in the drylands.

Divergent driving mechanisms of community temporal stability in China's drylands

Climate change and anthropogenic activities are reshaping dryland ecosystems globally at an unprecedented pace,jeopardizing their stability.The stability of these ecosystems is crucial for maintaining ecological balance and supporting local communities.Yet,the mechanisms governing their stability are poorly understood,largely due to the scarcity of comprehensive field data.Here we show the patterns of community temporal stability and its determinants across an aridity spectrum by integrating a transect survey across China's drylands with remote sensing.Our results revealed a U-shaped relationship between community temporal stability and aridity,with a pivotal shift occurring around an aridity level of 0.88.In less arid areas(aridity level below 0.88),enhanced precipitation and biodiversity were associated with increased community productivity and stability.Conversely,in more arid zones(aridity level above 0.88),elevated soil organic carbon and biodiversity were linked to greater fluctuations in community productivity and reduced stability.Our study identifies a critical aridity threshold that precipitates significant changes in community stability in China's drylands,underscoring the importance of distinct mechanisms driving ecosystem stability in varying aridity contexts.These insights are pivotal for developing informed ecosystem management and policy strategies tailored to the unique challenges of dryland conservation.

Discussion of the“warming and wetting”trend and its future variation in the drylands of Northwest China under global warming

Since Shi et al.proposed that the climate in the drylands of Northwest China experienced a significant transition from a“warming and drying”trend to a“warming and wetting”trend in the 1980s,researchers have conducted numerous studies on the variations in precipitation and humidity in the region and even in arid Central Asia.In particular,the process of the“warming and wetting”trend by using obtained measurement data received much attention.However,there remain uncertainties about whether the“warming and wetting”trend has paused and what its future variations may be.In this study,we examined the spatiotemporal variations in temperature,precipitation,the aridity index(AI),vegetation,and runoff during 1950-2019.The results showed that the climate in the drylands of Northwest China and the northern Tibetan Plateau is persistently warming and wetting since the 1980s,with an acceleration since the 1990s.The precipitation/humidity variations in North China,which are mainly influenced by summer monsoon,are generally opposite to those in the drylands of Northwest China.This reverse change is mainly controlled by an anomalous anticyclone over Mongolia,which leads to an anomalous easterly wind,reduced water vapor output,and increased precipitation in the drylands of Northwest China.While it also causes an anomalous descending motion,increased water vapor divergence,and decreased precipitation in North China.Precipitation is the primary controlling factor of humidity,which ultimately forms the spatiotemporal pattern of the“westerlies-dominated climatic regime”of antiphase precipitation/humidity variations between the drylands of Northwest China and monsoonal region of North China.The primary reasons behind the debate of the“warming and wetting”trend in Northwest China were due to the use of different time series lengths,regional ranges,and humidity indices in previous analyses.Since the EC-Earth3 has a good performance for simulating precipitation and humidity in Northwest and North China.By using its simulated results,we found a wetting trend in the drylands of Northwest China under low emission scenarios,but the climate will gradually transition to a“warming and drying”trend as emissions increase.This study suggests that moderate warming can be beneficial for improving the ecological environment in the drylands of Northwest China,while precipitation and humidity in monsoon-dominated North China will persistently increase under scenarios of increased emissions.

Long-term changes in the effect of drought stress on ecosystems across global drylands

Droughts are one of the climate extremes that occur frequently in drylands,constraining the terrestrial carbon uptake and water cycle processes.Studies about the long-term changes in the effect of drought stress on dryland ecosystems under global warming are still insufficient so far.Here we comprehensively investigate long-term changes in droughts and the effect of drought stresses on ecosystems across global drylands for 1982–2015,and further reveal changes in the configurations of the key meteorological and ecological factors of droughts,by using observation and reanalysis datasets.Climatically,the spatial patterns of intensity,occurrence,duration and severity of all drought categories across global drylands are highly consistent with each other.Associated with severe droughts,drylands witness insufficient precipitation(-12.4 mm mon^(-1)area-averaged over global drylands),warmer temperature(0.6℃),water deficit(-19.0 mm mon^(-1)),higher atmospheric aridity(0.06 k Pa),lower soil moisture(-0.03 m^(3)m^(-3)),suppressed vegetation growth and declined carbon uptake(-0.13 g C m^(-2)d^(-1)).The affected area,occurrence,duration and severity for mild to severe droughts in drylands and the anomalies of meteorological and ecological factors for all drought categories have increased significantly during 1982-2015.Specifically,when droughts occur,the precipitation deficit is weakened at a rate of 3.7–9.6 mm mon^(-1)(30 yr)^(-1),the higher temperature(atmospheric aridity)is further intensified at 0.71–0.87-1(30 yr)^(-1)(39–47 Pa(30 yr)^(-1)),the water(soil moisture)deficit is relieved at 1.7–5.1 mm mon^(-1)(30yr)^(-1)(4.1×10^(-3)–7.4×10^(-3)m^(3)m^(-3)(30 yr)^(-1)),and the suppressed vegetation growth is generally alleviated at 0.98×10^(-2)–1.2×10^(-2)(30 yr)^(-1).Since the 2000s,the increasing trend in the suppressed vegetation growth during droughts tends to stagnate over about 50%of the dryland area.By checking the probability density function,the configurations of the key meteorological and ecological factors of droughts show significant differences between the periods before and after 2000.The probability of drought stress caused by precipitation and soil moisture deficit has decreased by 20%and 10%,respectively,while that caused by higher temperature and atmospheric aridity has been doubled,and increased by 26%and 15%,respectively.Thus,the impact of drought stress on dryland ecosystems caused by atmospheric aridity are increasing under global warming.

Desertification dynamics in China's drylands under climate changeAuthor links open overlay panel

Much of drylands has been hit by land degradation during the recent decades.But whether and how desertification is related to climate change is poorly understood.Here,using the Normalized Difference Vegetation Index(NDVI)time series and climatic variables,we assessed the desertification dynamics over China's drylands from 1982 to 2016 and explored the climate change impacts,with a particular focus on the influence of the 2015/2016 El Niño event.We found a fluctuant downward trend of barren land area from 1982 to the early 2010s,but followed by a sharp increase afterwards.Decrease in precipitation combined with high temperature are major contributors to the barren land expansion.During the 2015/2016 El Niño,the concurrence of severe drought and heat stress gave rise to the most serious land desertification in the record.Hyper-arid and arid drylands are the predominant contributors to the abrupt barren land area increase during 2015/2016.Our findings,therefore,highlight the climate change impacts on dryland desertification processes.Future dryland expansion and accompanying drought stress may exacerbate the risk of land degradation in these regions.

How can ecosystem engineer plants boost productivity in east Mediterranean drylands

Background Water availability is the key limiting factor for plant productivity in drylands covering ca.40%of Earth’s land surface.For such ecosystems to retain productivity and biodiversity under climatic change,it would be valu-able to identify/promote keystone plant species that(i)have developed strategies to more efficiently utilize moisture resources not easily accessible and(ii)improve moisture conditions for neighboring plants.The very deep-rooted Ziziphus lotus,considered an ecosystem engineer,is one such example.However,it is not known which biotic traits:(a)canopy interception of moisture/rainfall,(b)hydraulic redistribution of deep ground moisture by roots,or non-biotic factors:(c)soil’s volume,and(d)organic matter content,Z.lotus activates/modulates to play such a role.We,thus,selected dryland ecosystems where the plant dominates and measured for potential effects on the less deep-rooted Thymbra capitata.For assessing impacts on ecosystem productivity,we measured the spatial aggregation of ca.3600 T.capitata plants.As a proxy for soil moisture availability and its spatial variability,we conducted a 7-year-long study using thymes’nighttime rehydration.Sampling extended up to 15 m away from Z.lotus.Results The density of T.capitata plants growing up to 5 m around Z.lotus vs.thymes growing 10-15 m away was found significantly increased(2.5-4.5 times),while their stem/leaf moisture was ca.10%higher at predawn compared to nightfall during the dry season.This suggests that ecosystem productivity is driven by a greater soil moisture avail-ability around Z.lotus permitting more thyme daytime transpiration,in contrast to thymes growing further away.The phenomenon appeared only under dry topsoil(during the dry season;becoming stronger during dry years).Morning dew/rainfall interception from the canopy or soil depth/organic matter did not show significant effects,leaving only the hydraulic lift properties of Z.lotus as the most likely driver for soil moisture availability.Conclusions The deep-rooting properties and hydraulic lift potential of Z.lotus may be the key in permitting it to boost ecosystem productivity.Such hydraulic plant traits require more attention as they may prove valuable in com-bating desertification and restoring ecosystems in arid/semiarid regions threatened by climate change.

Climate and topography regulate the spatial pattern of soil salinization and its effects on shrub community structure in Northwest China

Soil salinization may affect biodiversity and species composition,leading to changes in the plant community structure.However,few studies have explored the spatial pattern of soil salinization and its effects on shrub community structure at the ecosystem scale.Therefore,we conducted a transect sampling of desert shrublands in Northwest China during the growing season(June–September)in 2021.Soil salinization(both the degree and type),shrub community structure(e.g.,shrub density and height),and biodiversity parameters(e.g.,Simpson diversity,Margalf abundance,Shannon-Wiener diversity,and Pielou evenness indices)were used to assess the effects of soil salinization on shrub community structure.The results showed that the primary degree of soil salinization in the study area was light salinization,with the area proportion of 69.8%.Whereas the main type of soil salinization was characterized as sulfate saline soil,also accounting for 69.8%of the total area.Notably,there was a significant reduction in the degree of soil salinization and a shift in the type of soil salinization from chloride saline soil to sulfate saline soil,with an increase in longitude.Regional mean annual precipitation(MAP),mean annual evapotranspiration(MAE),elevation,and slope significantly contributed to soil salinization and its geochemical differentiation.As soil salinization intensified,shrub community structure displayed increased diversity and evenness,as indicated by the increases in the Simpson diversity,Shannon-Wiener diversity,and Pielou evenness indices.Moreover,the succulent stems and leaves of Chenopodiaceae and Tamaricaceae exhibited clear advantages under these conditions.Furthermore,regional climate and topography,such as MAP,MAE,and elevation,had greater effects on the distribution of shrub plants than soil salinization.These results provide a reference for the origin and pattern of soil salinization in drylands and their effects on the community structure of halophyte shrub species.

Plasticity of photorespiratory carbon concentration mechanism in Sedobassia sedoides(Pall.)Freitag&G.Kadereit under elevated CO_(2)concentration and salinity

Rising atmospheric CO_(2)(carbon dioxide)concentrations and salinization are manifestations of climate change that affect plant growth and productivity.Species with an intermediate C_(3)-C_(4)type of photosynthesis live in a wide range of precipitation,temperature,and soil quality,but are more often found in warm and dry habitats.One of the intermediate C_(3)-C_(4)photosynthetic type is C_(2)photosynthesis with a carbon concentration mechanism(CCM)that reassimilates CO_(2)released via photorespiration.However,the ecological significance under which C_(2)photosynthesis has advantages over C_(3)and C_(4)plants remains largely unexplored.Salt tolerance and functioning of CCM were studied in plants from two populations(P1 and P2)of Sedobassia sedoides(Pall.)Freitag&G.Kadereit Asch.species with C_(2)photosynthesis exposed to 4 d and 10 d salinity(200 mM NaCl)at ambient(785.7 mg/m^(3),aCO_(2)and elevated(1571.4 mg/m^(3),eCO_(2))CO_(2).On the fourth day of salinity,an increase in Na+content,activity catalase,and superoxide dismutase was observed in both populations.P2 plants showed an increase in proline content and a decrease in photosynthetic enzyme content:rubisco,phosphoenolpyruvate carboxylase(PEPC),and glycine decarboxylase(GDC),which indicated a weakening of C_(2)and C_(4)characteristics under salinity.Treatment under 10 d salinity led to an increased Na^(+)content and activity of cyclic electron flow around photosystem I(PSI CEF),a decreased content of K^(+)and GDC in both populations.P1 plants showed greater salt tolerance,which was assessed by the degree of reduction in photosynthetic enzyme content,PSI CEF activity,and changes in relative growth rate(RGR).Differences between populations were evident under the combination of eCO_(2)and salinity.Under long-term salinity and eCO_(2),more salt-tolerant P1 plants had a higher dry biomass(DW),which was positively correlated with PSI CEF activity.In less salt-tolerant P2 plants,DW correlated with transpiration and dark respiration.Thus,S.sedoides showed a high degree of photosynthetic plasticity under the influence of salinity and eCO_(2)through strengthening(P1 plants)and weakening C_(4)characteristics(P2 plants).

Short lifespan and ‘prime period’ of carbon sequestration call for multi-ages in dryland tree plantations

Enhancing forest cover is important for effective climate change mitigation.Studies suggest that drylands are promising areas for expanding forests,but conflicts arise with increased forest area and water consumption.Recent tree mortality in drylands raises concerns about carbon sequestration potential in tree plantations.Using Chinese dryland tree plantations as an example,we compared their growth with natural forests.Our results suggested plantation trees grew 1.6–2.1 times faster in juvenile phases,significantly shortening time to maturity(13.5 vs.30 years)compared to natural forests,potentially stemming from simple plantation age structures.Different from natural forests,74%of trees in plantations faced growth decline,indicating a short“prime period”for carbon sequestration and even a short lifespan.Additionally,a negative relationship between evapotranspiration and tree growth was observed in tree plantations since maturity,leading to high sensitivities of trees to vapor pressure deficit and soil water.However,this was not observed in natural forests.To address this,we suggest afforestation in drylands should consider complex age structures,ensuring a longer prime period for carbon sequestration and life expectancy in tree plantations.