Response Relationship Between Meteorological And Agricultural Drought And Terrestrial Vegetation Carbon Sinks In Northwest China

Global warming has exacerbated the problem of drought,caused vegetation degradation and resulted in a decline in vegetation productivity and carbon sink function.Northwest China is one of the most frequently occurring and severely affected regions by drought.In recent years,due to the uneven distribution of regional precipitation and seasonal or intermittent water shortages in vegetation,droughts have been frequent and the carbon sink function of vegetation has been significantly degraded.Therefore,there is an urgent need to understand and assess the characteristics of drought and terrestrial vegetation carbon sink changes and their response relationships in Northwest China,to improve the ability of human society to adapt to climate change and provide an important scientific basis for achieving regional carbon sink management comprehensively and accurately.However,the current analysis of drought and terrestrial vegetation carbon sinks in Northwest China is not sufficiently detailed,and does not consider the diversity of climatic conditions and land cover types,nor does it determine the quantitative response relationship between drought and vegetation carbon sinks.In this paper,the Standardized Precipitation Evapotranspiration Index（SPEI）and Vegetation Condition Index（VCI）were calculated based on multi-source remote sensing and meteorological data,which were used to characterize meteorological drought and agricultural drought,respectively.The Theil-Sen and Mann-Kendall（TS-MK）method was used to analyze the spatial and temporal patterns of meteorological dryness and agricultural drought in Northwest China from 2000 to 2019.Furthermore,the modified Carnegie-Ames-Stanford-Approach model（CASA）and soil heterotrophic respiration model was used to estimate vegetation Net Ecosystem Productivity（NEP）in Northwest China,while the Solar-induced Chlorophyll Fluorescence（SIF）was introduced as an indicator of vegetation carbon sink,and the TS-MK method was used to analyze the spatial and temporal variation of vegetation carbon sink in Northwest China.Based on the above analysis results,the quantitative response of meteorological drought and agricultural drought to vegetation carbon sinks in Northwest China was assessed by Wavelet Coherence analysis,Spearman correlation and the Akaike Information Criterion（AIC）.The main conclusions are as follows:（1）During 2000-2019,the overall trend of SPEI and VCI in Northwest China showed increasing trends of 0.028 yr^-1 and 0.002 yr^-1,respectively,while the frequency,duration and proportion of mild drought,moderate drought and severe drought were decreasing,indicating that the degree of meteorological drought and agricultural drought in Northwest China was slowing down and the climate was shifting toward wetness.However,SPEI and VCI under different climatic zones and land cover types showed different patterns.SPEI was highly sensitive to meteorological drought in semi-arid and semi-humid regions,while VCI was highly sensitive to agricultural drought in arid and semi-arid regions;grassland was more sensitive to meteorological drought and agricultural drought,while the forest was not easily affected by meteorological drought and agricultural drought.（2）During 2000-2019,vegetation NEP and SIF in Northwest China showed increasing trends of 7.09 g C m^-2yr^-1 and 0.001 W m^-2μm^-1sr^-1yr^-1,respectively,mainly distributed spatially in northern Shaanxi,eastern and southern Gansu,and southern Ningxia（p<0.05）.However,the trend changes of NEP and SIF under different climatic zones and land cover types were different,and the trend changes of NEP under different climatic zones were from large to small:semi-arid region>humid region>semi-humid region>arid region,while the trend changes of SIF were from large to small:humid region>semi-humid region>arid region>semi-arid region;the trend changes of NEP under different land cover types were from large to small:cropland>forest>grassland,while the trend change of NEP for different land cover types was from large to small:forest>cropland>grassland,while the trend change of SIF was from large to small:forest>cropland>grassland.Furthermore,NEP and SIF of forest,cropland and grassland showed different trend changes along the aridity gradient,indicating that there were differences in carbon sink capacities of different climate zones and land cover types.The strongest carbon sink capacity was in humid regions and forests,and the weakest was in arid regions and grasslands.（3）The response patterns of vegetation SIF-SPEI and SIF-VCI in Northwest China were similar to those of vegetation NEP-SPEI and NEP-VCI,which were positively correlated under different climate zones and different land cover types.The vegetation NEP,SIF and SPEI in Northwest China showed a significant positive correlation during 2000-2019（coherence greater than 0.8）.However,there was a certain lag effect between 2003 and 2007.The NEP-VCI and SIF-VCI showed a cyclical correlation of 8-20 months.SPEI and VCI were significantly positively correlated with vegetation NEP and SIF in most areas of Northwest China（p<0.05）,mainly in Shaanxi,eastern and southern Gansu,eastern Qinghai,and southern Ningxia.Overall,the correlations of vegetation NEP and SIF with SPEI and VCI showed nonlinear changes along the drought gradient,reflecting multiple trend differences,especially in grasslands.The correlations of grassland NEP and SIF with VCI were the highest,indicating that the carbon sink capacity of grassland was the most sensitive to changes on agricultural drought.