Evaluation Of Vegetation And Land Degradation In Central Asia Under The Background Of Climate Change

Central Asia is located in the interior of the Asian continent,characterized by low and erratic precipitation.As frequency of extremely hydroclimatic events increase,especially recurrent drought,the region has suffered from severe vegetation degradation.As one of the important fulcrums of Silk Road Economic Belt,Central Asia has been connecting cities in western China to Europe and furher to the world via cultural and trade exchanges.Dryland rainfed ecosystem in the region support a substantial proportion of the human population and economy,thus analysis of its response to frequent droughts,assessment of the risk in vegetation degradation and the status of land degradation are of great significance for regional sustainable development and the promotion of China's Belt and Road Initiative.Based on CRU meteorological data set,MODIS GPP(Gross Primary Productivity),NPP(Net Primary Productivity),NDVI(Normalized Difference Vegetation Index),LAI(Leaf Area Index)data set,ESACCI land cover data set,the spatial distribution and spatiotemporal trends of six meteorological factors,vegetation growth,land cover,water use efficiency(WUE)and rain use efficiency(RUE)in Central Asia were analyzed using the contextual Mann-Kendall(CMK)test,Sen'slope,Regime Shift Index(RSI)and Trend Surface Analysis(TSA).Based on WUE and RUE,ecological resilience index was applied to evaluate the degradation risk of different vegetation cover types in Central Asia.The higher the index,the lower the risk of degradation.Based on net primary productivity,land cover data and soil carbon storage values,WUE correction method,regression analysis and non-parametric test were applied,and land degradation,stability and improvement were determined by analyzing the rate of change,frequency distribution,percentage grades,etc.,and comparing the relative differences between baseline and target periods.Combined with land productivity,land cover and soil organic carbon,the comprehensive evaluation of land degradation in Central Asia was carried out.The main findings were as follows:During the period 1901 to 2015,the temperature in Central Asia showed an upward trend,and the precipitation showed no significant increase.The increase of potential evapotranspiration was greater than the increase of precipitation.With the minimum temperature rising significantly,diurnal temperature range and frost days showed a downward trend.Temperature increased significantly throughout Central Asia.Diurnal temperature range and frost days shown a significant downward trend in most areas.An increasing trend of precipitation exhibited over half of the study area,and the potential evapotranspiration tended to increase in most regions.With the exception of Tajikistan and Kyrgyzstan,the increase in potential evapotranspiration was higher than the increase in precipitation throughout the region.The spatial variation of SPEI(Standardized Precipitation Evapotranspiration Index)was as follows:the spatial trend of SPEI01,SPEI06,SPEI12 were similar,and the area of drought decreased with the increase of time scale,while SPEI03 decreased significantly in 76.32%of the study area.SPEI03 showed the largest drought area,indicating that the regional seasonal drought intensified.The annual GPP(2000-2015),NPP(2000-2014)and NDVI(2000-2015)showed a downward trend in the total vegetation area of 77.97%,79.61%and 58.89%,respectively,mainly in the northwestern part,central and northeast of Kazakhstan,eastern Uzbekistan,southern Turkmenistan,and northern Xinjiang and Ili Valley of China.Area of water and bareland in Central Asia decreased significantly during 1992 to 2015.Except for rangeland,the area of the remaining land cover types increased significantly.The area of rainfed cropland and irrigated cropland in Kazakhstan increased significantly from 1992 to 2015.The main source of increased rainfed cropland was rangeland,and the increase in irrigated farmland was mainly from rangeland and bareland.During the five stages(1992-1995,1995-2000,2000-2005,2005-2010,2010-2015),the conversion of rangeland to rainfed cropland was concentrating in in the northern and eastern parts of Kazakhstan,and the conversion of rangeland to irrigated cropland was concentrating in central and southern parts of Kazakhstan.During 2000-2014,based on WUE,96.01%of rainfed cropland,68.02%of steppe and 62.70%of sparse vegetation showed moderate degradation risk,while 54.92%and 42.09%of shrub had high and moderate degradation risk,respectively.Based on RUE,89.11%of rainfed cropland showed high risk of degradation,56.39%and 42.43%of steppe were moderate and high degradation risk,54.63%and 42.52%of shrub had high and moderate risk of degradation,while 71.85%of sparse vegetation was at high degradation risk.The degradation risk assessment based on WUE and RUE were the same in 10.12%of rainfed cropland,46.03%of steppe,41.35%of shrub and 37.64%of sparse vegetation.During and after extreme drought,vegetation with high risk of degradation showed a decrease in WUE and RUE,which had a negative impact on vegetation carbon accumulation.Vegetation with high degradation risk caused by low resistance,its WUE and RUE negatively respond to drought,and as the drought in Central Asia becomes continuous and frequent,these vegetation may degenerate and die due to the continuous reduction of WUE and RUE.The proportion of land degradation,stability and improvement in Central Asia from 2001-2015 were 13.46%,36.14%and 15.39%,respectively.The improvement area was mainly located in the northwest,northeast and southeast of Kazakhstan,northwestern Uzbekistan and central and northern Xinjiang of China.The area of degradation occurred mainly in the western parts and the southern parts of Kazakhstan,southern Turkmenistan,central and southern parts of Uzbekistan,southwestern Tajikistan,eastern Kyrgyzstan,and central Xinjiang of China.Land degradation was affected by climate change,the mechanisms by which vegetation responds to drought,and human activities.