Temporal And Spatial Variation Of Grassland Net Primary Productivity And Its Response To Drought In Inner Mongolia, As Well As Pattern Analysis Of Precipitation Use Efficiency In Northern Grasslands

In order to explore the change of net primary productivity(NPP)and its relationship with drought in the growing season of Inner Mongolia grassland and the distribution pattern of rain utilization efficiency(RUE)in northern grassland,based on NPP simulated by BEPS model,standardized precipitation evapotranspiration index(SPEI)and grassland RUE,the temporal and spatial variation characteristics of NPP and SPEI in Inner Mongolia grassland from 2001 to 2015 and the distribution pattern of RUE in northern grassland from 2016 to 2018 were analyzed.(1)During 2001-2015,NPP increased gradually from southwest to northeast in Inner Mongolia grassland growing season,with obvious spatial heterogeneity,which was consistent with the distribution of annual precipitation.On the time scale,the average growth rate of NPP was 2.20 g C·m^-2·a^-1in the growing season of Inner Mongolia grassland from 2001 to 2015;the average growth rate of NPP in meadow steppe was 3.73g C·m^-2·a^-1,the fastest;the average growth rate of NPP in typical steppe was 1.69g C·m^-2·a^-1,the second;the average growth rate of NPP in desert steppe was 0.30g C·m^-2·a^-1,the slowest.On the spatial scale,the annual average trend of grassland NPP in the whole region showed a distribution pattern of"southwest region decreased,central and northeast region increased".Among them,meadow steppe was the highest,with an average annual NPP of 387.90 g C·m^-2·a^-1;followed by typical steppe,with an average annual NPP of 291.26 g C·m^-2·a^-1;desert steppe was the lowest,with an average annual NPP of 133.70 g C·m^-2·a^-1.(2)During the growing season of grassland in Inner Mongolia from 2001 to 2015,SPEI at different scales showed a similar trend in drought indicator,but the ability of SPEI at different time scales to capture drought in growing season was different.On the whole,the correlation between NPP and SPEI decreased with the increase of SPEI time scale.The grassland net primary productivity in Inner Mongolia had a better response to short-term surface water anomalies and seasonal drought changes,but a relatively poor response to medium and long-term drought changes,especially annual drought changes.Among them,NPP in meadow steppe had the best correlation with SPEI-1(R²=0.58,P<0.001),NPP in typical steppe had the best correlation with SPEI-3(R²=0.76,P<0.001),and NPP in desert steppe had the best correlation with SPEI-3.Determination coefficient R²=0.85(P<0.001).Growing season drought interannual variability of Inner Mongolia grassland has obvious spatial heterogeneity,largely in xilingol league area in central and central chifeng city as the boundary,is becoming more and more humid in northeast line,more and more drought in southwest boundary,the meadow steppe on the whole is more and more moist,typical steppe on the whole is more and more drought,desert steppe on the whole is more and more drought.The rate of drought was faster than that of typical steppe.In terms of correlation between NPP and SPEI,it was almost opposite to the spatial distribution pattern of annual mean precipitation in Inner Mongolia.Most of the correlations were positive,and the correlation coefficients were mostly between 0.6-1.0.In terms of the correlation between NPP and SPEI of various grasslands,from strong to weak:desert steppe>typical steppe>meadow steppe.(3)From 2016 to 2018,the NPP of grassland in northern China was 0-550g C·m^-2·a^-1,and the average NPP was 292.44 g C·m^-2·a^-1.The correlation between NPP and precipitation was the highest in Inner Mongolia,r=0.740(P<0.001);Xinjiang took the second place,r=0.295(P<0.05);Qinghai was the lowest,r=0.103(P<0.05).From the perspective of the distribution pattern of RUE of the whole northern grassland,the RUE distribution of the grassland in Xinjiang is fragmented,showing a distribution characteristic of"high in the north and low in the south".The grassland RUE in the other areas showed a distribution characteristic of"increasing from southwest to northeast".The average RUE of the whole northern grassland was 0.80 g C·m^-2·a^-1·mm^-1.In some areas,the higher the annual mean temperature,the higher the RUE of grassland in Xinjiang,and the correlation coefficient between RUE and annual mean temperature r=0.433(P<0.01).In the southeastern part of Gansu Province,Ningxia Hui Autonomous Region,northern part of Shaanxi Province and Inner Mongolia,RUE generally showed a distribution characteristic that the higher the annual mean temperature was,the lower the RUE was,and the correlation coefficient between RUE and annual mean temperature was r=-0.621(P<0.001).However,under the influence of low annual average temperature,the RUE value of grassland in Qinghai Province and the northwest of Gansu Province was relatively low,and the RUE value was positively correlated with the annual average temperature.The correlation coefficient between RUE and annual mean temperature was0.781(P<0.001),which was stronger than that of grassland in Xinjiang Province.