Carrying Capacity For Vegetation Across Typical Sandy Regions In Northern China

Revegetation and afforestation in sandy region are important to curb desertification and prevent the wind-blown sand hazards.However,in recent years,vegetation,constructed by ecological restoration and recovery,in these areas occurred vegetation degradation or death.It is due to the mismatching between the scale of ecological construction and the carrying capacity for vegetation of local environment.However,it is unclear how large the carrying capacity for vegetation(i.e.,the vegetation quantity of the local ecosystem can maintain stably under the environmental limit conditions)is and what differences is in the different sandy regions in northern China.To address these problems,six sandy regions(Horqin,Hulun Buir,Otindag,Mu Us,Southern Tengger,and Southern Gurbantunggut,respectively)were investigated in this study.All analyses were based on the geographical units that were divided and selected the subareas of each region with less human disturbances.Multivariate statistical analyses and empirical dynamic modeling,including convergent cross-mapping(CCM)and multivariate empirical dynamic modelling(Multivariate EDM),were used to detect the spatio-temporal distributions of vegetation(represented by leaf area index,LAI)and their relationships to environmental factors(such as temperature,precipitation,actual evapotranspiration,and potential evapotranspiration)at multi-year,interannual and seasonal scales from 2000 to 2014.Then,we obtained the vegetation responses to the variabilities of environmental factors,and found the most sensitive periods of vegetation growth.Based on these analyses,the threshold of max leaf area index(CCVmax)and the threshold of safe leaf area index(CCVsafe)were proposed to characterize the different aspects of carrying capacity for vegetation(i.e.,the maximum vegetation growth potential and the safe range of vegetation growth to maintain the ecosystem stably under the possible environmental stresses,respectively).The theoretical concepts and calculated principles of these two thresholds were elaborately derived,so that the distributions of carrying capacity for vegetation can be assessed for different typical plant communities and six regions in northern China.The quantitative relation models between the two thresholds and environmental factors were established in each region,to predict the dynamics of carrying capacity for vegetation from 2015 to 2024.The results show:(1)The spatial distribution of vegetation was most closely related to precipitation and actual evapotranspiration(with larger correlation coefficient r)than other factors.Seasonal cycle of vegetation was most strongly driven by temperature(cross-mapping skill ?CCM>0.75 and extent of influence a>0.77),while during the early growing season,the precipitation and actual evapotranspiration had more drivring effects(?CCM>0.3)on vegetation than other factors(?CCM<0.25).The short-term fluctuations of vegetation were related to actual evapotranspiration and precipitation(0.05 ? r ? 0.3),but the annual variations of vegetation was mainly affected by annual precipitation and annual evapotranspiration.The sensitivity of vegetation to variabilities of environmental factors changed over different seasons:the positive responses occurred in the early growing season and the negative in the late growing season.The vegetation of the eastern sandy regions(e.g.,Horqin,Hulunbuir,and Otindag)was more sensitive to the variabilities of environmental factors than that of the western sandy regions(e.g.,Mu Us and Southern Tengger).The most sensitive period of vegetation to variabilities of environmental factors in each region were in the early growing season(about from April 10 to June 20).(2)From 2000 to 2014,the CCVmax of the six regions ranged from 0.36 to 1.03 m2/m2,while the CCVsafe ranged from 0.29 to 0.70 m2/m2.These two thresholds decreased from the east to west.Based on the degrees of aridity index,the CCVmax in dry sub-humid(Horqin,eastern Hulunbuir,and southeastern Otindag),semi-arid(western Hulunbuir,middle Otindag,Mu Us,and Southern Tengger)and arid(western Otindag and Southern Gurbantunggut)regions are 0.7?1.0,0.45?0.7 and 0.36?0.45 m2/m2,respectively;while the CCVsafe had no obvious relationships with aridity index.In the next decade,under the current climatic variability and anthropogenic disturbances,the two thresholds of the six regions varied inconsistently,but most of them showed an upward trend(about 15%increasement).However,the carrying capacity for vegetation in Eastern Horqin would decrease(by about 15%),and in the Southern Tengger it had no obvious changes.(3)For the vegetation communities with landscape of savannas(mainly constituted by Ulmus pumila,Salix gordejevii,Stipa grandis in Horqin,eastern Hulunbuir,and southeastern Otindag),typical grassland(Stipa grandis,Artemisia halodendron in western Hulunbuir,middle Otindag,and eastern Mu Us),desert grassland(Artemisia ordosica,Caragana microphyla,Achnaentherum splndes in western Mu Us and Southern Tengger)and desert(Artemisia ordosica,Reaumuria soongarica,Haloxylon ammodendron in Southern Gurbantunggut),the CCVmax were 0.60?0.92,0.48?0.77,0.46?0.86 and 0.39?0.56 m2/m2,respectively,and CCVsafe were 0.42?0.61,0.39?0.51,0.36?0.58,and 0.33?0.39 m2/m2,respectively.Results suggest that the ecosystems in different sandy regions,structures,and components had different responses related to the variability of environmental factors and the various carrying capacity for vegetation.It would be better to implement subregional and seasonal ecosystems management to revegetation and restoration in sandy region of northern China,especially the eastern sandy regions(Horqin,Hulun Buir,and Southeastern Otindag)need critical protection.This study provide a theoretical basis and data support for the mechanism studies of plant sand fixation and differentiated ecosystem management for sustainable development,and contribute to the ecological restoration and reconstruction and the realization of the target of land degradation neutrality(2030 Agenda of Sustainable Development Goal 15.3 by UN)in desertification areas in northern China.However,it remains unclear that the impacts of socio-economic factors and climate change on the carrying capacity for vegetation in various sandy areas.It is in need of the comprehensively vegetation configuration on geographic units or watershed to apply the carrying capacity for vegetation to ecological restoration and ecosystem process management.These aspects should be further studied in the future.