Dynamics Of Vegetation Net Primary Productivity And Its Response To Drought In The Mongolian Plateau

With the accelerating process of global warming,the distribution of water resources has been changed,which led to the frequent occurrences of drought,flood and other natural disasters worldwide.As an important component of the global carbon cycle,the evolution of vegetation net primary productivity(NPP)is closely related to climate change.Geographically,the Mongolian Plateau is located between 37° N and 53° N,which is a typical arid and semi-arid region.From the perspective of vegetation diversity,the Mongolian Plateau is still relatively rich in vegetation resources and plays a crucial role in the carbon cycle and ecological construction of East Asia and even the world.However,at present,the negative impact of drought and water shortage on vegetation productivity is an important factor restricting ecological construction and economic development in the Mongolian Plateau.Therefore,it is of great significance for the sustainable development of the Mongolian Plateau to strengthen the comprehensive ability of drought monitoring and study the spatial-temporal variation of NPP at regional scale,and to explore the response relationship between NPP and drought.In this research,the standardized precipitation evapotranspiration index(SPEI)was used as drought index,and CASA model was used to simulate the net primary productivity data of vegetation in the Mongolian Plateau.Based on the results,the spatio-temporal variation characteristics of drought and vegetation NPP,drought frequency and drought duration in the Mongolian Plateau from 1982 to 2014 were analyzed,and the response of NPP to drought in the Mongolian Plateau was evaluated.The main conclusions are as follows:(1)The vegetation NPP simulated by the CASA model was verified by the MOD17A3 NPP data,and the results showed that the CASA model was suitable for simulating the vegetation NPP of the Mongolian Plateau.NPP in the growing season of the Mongolian Plateau and the NPP in spring,summer and autumn all showed an increasing trend,among which the NPP in the growing season increased the most,and from the perspective of different regions,the NPP in Inner Mongolia showed the largest increase trend.Spatially,the slope of NPP increased significantly in the growing season and spring of the Mongolian Plateau,accounting for a large part,while the NPP in summer and autumn showed an insignificant increasing trend in most areas.The decrease of NPP in Mongolia was greater than that in Inner Mongolia.In the growing season,spring,summer,and autumn of the Mongolian Plateau,the vegetation NPP on different altitude gradients showed an overall increasing trend,but the NPP performance at different elevations was different.The vegetation growth in low altitude areas was better,and the variation range of vegetation NPP was also larger.(2)SPEI in the growing season,spring,summer and autumn of the Mongolian Plateau showed a downward trend as a whole,and drought aggravated.The decrease of SPEI in Mongolia was greater than that in Inner Mongolia,and drought was the most serious in summer.Spatially,the degree of aridity increased in most areas of the Mongolian Plateau during the growing season,spring,summer and autumn,and the increase of drought in summer accounted for the largest proportion of the area,while in autumn,drought eased only in western Mongolia and western Inner Mongolia.In the past 33 years,light drought occurred more frequently in the growing season of the Mongolian Plateau,spring drought occurred more frequently in the western part of Inner Mongolia and autumn drought occurred more frequently in the central and eastern part,and drought duration less than 3 months occurred the most frequently in the Mongolian Plateau.(3)In the Mongolian Plateau,the vegetation NPP and SPEI in different seasons were more positively correlated,and the NPP and SPEI of each vegetation type were positively correlated in the growing season and summer.Drought in the growing season of the Mongolian Plateau had a lag effect on vegetation NPP,and showed no significant response in the early stage of the growing season.NPP of different vegetation responded strongly to the SPEI of the current month and the previous month.Through the analysis of the established drought loss rate curve,it can be found that the vegetation NPP was more sensitive to regional drought,and with the increase of drought degree,the loss rate of vegetation NPP also fluctuated and increased in various vegetation types of the summer.The NPP loss rate of grassland types fluctuated relatively high,the loss rate of NPP in forest areas was in the range of 0-60%,and the loss rate of NPP in shrub,sandy land and cropland was between grassland types and forest areas.