Impact Of Climate Change And Grazing Disturbance On Vegetation Structure In The Upper Heihe River Basin

Under the background of global change,as a sensitive area where ecological processes and hydrological processes are coupled,water resources utilization and integrated management in arid watersheds are facing severe challenges.Interactions between natural processes and frequent human activities have caused changes in vegetation pattern.Accurate detection and quantification of the driving mechanisms and impacts of vegetation changes have become the focus of current research.As a sensitive and fragile agricultural-pastoral ecotone,the upper Heihe River Basin have a great impact on the downstream.A series of datasets,such as GLASS Leaf Area Index(LAI)data,meteorological data,grazing data,and vegetation type map,are used to identify the temporal and spatial LAI change caused by climate change and grazing disturbance.Then,detecting the contribution rate of each driving factor quantitatively.Finally,we use CART method to simulate the spatial distribution of vegetation LAI in different future scenarios.The results show:(1)The average growing season of grasses in the UHRB was 127d,which increased significantly in the past 30 years(p<0.05).Between 1983 and 2010,the contribution rate of the first modal variance of the whole basin's arid index(AI)reached 77%.The sensitive area has shifted from the southwest region to the central north region.About 29%from 1983 to 2000,about 41%of LAI major pattern changes from 2001 to 2010 can be explained by AI.During 1983-2010,the UHRB has become increasingly warm and wet,and the regional LAI became better gradually.The strongest connections between climate and LAI were found in the southwest part of the basin prior to 2000,but they shifted towards the north central area afterwards.Climate change(increased AI)is the main reason for the improvement of LAI in the whole basin,while human disturbance,such as ecological restoration(afforestation)policy and over grazing,leads to changes in local(middle and northern)LAI.At the basin level,the positive effects of regional climate warming and precipitation increase as well as local ecological restoration efforts overwhelmed the negative effects of overgrazing.In some specific climate zones,human activities can magnify or reduce the risk of drought to vegetation in the context of global warming.(2)Overgrazing was a dominant negative driver for LAI reduction in alpine grasslands and shrubs,especially for the periods of 1985-1991 and 1997-2004.With the implementation of grazing exclusion and grazing rotation policy in grassland,the grazing management contributed to the improvement of LAI,partially offsetting the negative effects due to increasing livestock.The grazing days decreasing from west to east,and the shrubs in the eastern part of the watershed has maximum grazing density.Altitude,vegetation type,and regional grazing management may jointly lead to regional differences in grazing patterns.Overgrazing is a big challenge to the restoration of the eastern shrub grassland ecosystem.(3)During 2001-2010,the LAI change in the UHRB was affected by grazing disturbance and climate change.Those two driving forces coexisted with positive and negative effects.Climate dominated the change of LAI.The positive impact of climate in the western part of the basin is increasing.The increasing negative grazing affects masked the positive effect of a part of climate warming.Grazing intensifies the negative effects on LAI in the eastern vegetation caused by bad climatic conditions.The contribution of climate to vegetation may be underestimated in the absence of long-term grazing changes.Therefore,it's necessary to strengthen grazing management in the study area to ease the current grazing pressure.(4)When the depth is 8,the number of nodes is 128 and the minimum mean square error is 0.218,our CART vegetation model has the accuracy rate 78.2%.After adding human interference parameters,the accuracy rate of the model increased 12%.The future scenario simulation results show that:on the watershed scale,warming climate conditions will increase vegetation LAI(0.015),but the negative effect of grazing disturbance on vegetation will weaken some of the positive effects of climate(-0.021).