The Spatio-temporal Dynamics And Driving Factors Of Grassland Net Primary Productivity In The Farming-pastoral Ecotone Of Inner Mongolia

The farming-pastoral ecotone of Inner Mongolia is the main part of the farmingpastoral ecotone in northern China.It is located in the transition area of forest,grassland and desert ecosystems,and has diverse and drastic changes in natural environmental conditions in its fragile natural ecosystems.On the other hand,frequent and intensive human activities play an important role in natural ecosystems,especially grassland ecosystems.The issues of degradation and resources reduction of grassland in the region are noticeable.It has become major problems that need to be solved urgently in the process of building a northern ecological security barrier in China.Net primary productivity(NPP)of vegetation is a comprehensive reflection of ecosystem states and ecosystem services.In recent years,the study of the spatial and temporal dynamics of grassland vegetation NPP has received great attention and has become an important basis for monitoring changes of regional grassland ecosystems and revealing the driving mechanisms from natural factors and anthropogenic activities.In this thesis,based on the data of land use,MODIS NDVI,and climate factors in the farming-pastoral ecotone of Inner Mongolia,by analyzing the land use changes between 2000 and 2015,this study clarified the spatial distribution of persistent grassland in the study area since 2000(i.e.,the area that always remained as grassland between 2000 and 2015);An improved CASA model was used to estimate the persistent grassland NPP,and a combination of Theil-Sen median trend analysis and Mann-Kendall significance test was used to portray the spatial and temporal dynamics of NPP during 2001-2017.The spatio-temporal heterogeneity of NPP was further explored by grouping 47 counties in the study area using hierarchical clustering.Spatial and temporal dynamics of annual precipitation and annual mean temperature in the study area with persistent grassland during 2001-2017 using Theil-Sen median and Mann-Kendall methods.;The correlation analysis method was used to reveal the spatial distribution of the correlation between NPP and annual precipitation and annual mean temperature,and to explain the similarities and differences of the effects of hydrothermal changes on NPP at the county scale by grouping statistics.Finally,the XGBoost algorithm was used to clarify the contribution of different hydrothermal factors to NPP.Finally,the XGBoost algorithm was used to clarify the contribution of different hydrothermal factors to NPP.The main findings are as follows.1.During 2000-2015,grassland and arable land were the main types in the farming-pastoral ecotone of Inner Mongolia,and the interconversion between them was the main manifestation of land use change,with grassland showing a decreasingincreasing-decreasing trend,and the spatial distribution of persistent grassland was not uniform.2.During 2001-2017,70.49% of the persistent grassland NPP increased,and14.07% of the area increased significantly,indicating that overall condition of persistent grassland is good over the study period.3.The annual precipitation in the eastern and northeastern parts of the study area is relatively abundant,while the annual precipitation in the central and western parts of the study area was low.During 2001-2017,the annual precipitation of persistent grassland generally increased,and the annual mean temperature increased and decreased.Since 2001,the climate in which most of the persistent grasslands were located tends to be warm and humid,and the climate of persistent grasslands in counties within Xing’an League tends to be cold and humid.4.During 2001-2017,most of the persistent grassland NPP showed positive correlation with annual precipitation,with a significant positive correlation area of49.13%;Nearly 60% of the persistent grassland NPP was negatively correlated with the annual average temperature,with a significant negative correlation accounting for30.52%.In addition,NPP was significantly positively correlated with mean annual temperature over 8.04% of the area.Multi-year mean temperature and mean annual precipitation are the main factors driving spatial variation in NPP at the regional scale,and at the county scale,significant enhancements to NPP from warm and humid climate change occur only in a few counties;the significant increase in NPP of the renewed grassland in some counties was weakly correlated with climatic factors,presumably with non-natural factors such as ecological conservation and construction playing a positive role.In summary,the state of the persistent grassland in the farming-pastoral ecotone of Inner Mongolia improves gradually during 2001-2017,driven by precipitation increase and human ecological restoration activities.However,there was significant regional heterogeneity in NPP variability of persistent grassland.In a lot of counties,warming offsets the benefits derived from precipitation increasing.The above findings enrich the knowledge of the mechanisms of NPP changes in grassland vegetation in the farming-pastoral ecotone,and also h have certain guiding value for the study of the region’s in-depth ecological civilization construction.