Temporal And Spatial Variation Of Vegetation NPP And Its Influencing Factors In Wujiang River Basin

Under the background of global climate change and the intensification of human activities,the ecological problems of watershed have become the focus of attention of scientists all over the world.Net Primary Productivity（NPP）is an important index to measure the quality and carbon sequestration capacity of the regional ecosystem,which is of great significance for the future ecological planning and construction of the basin and carbon balance.As the largest tributary of the upper reaches of the Yangtze River,the health of the ecosystem in the Wujiang River basin is related to the sustainability of the future development of the province（city）.Therefore,the study on NPP of vegetation in Wujiang River Basin is helpful to clarify the future development direction of the region,and can also provide a theoretical reference for the study on how vegetation in the basin responds to global changes.Based on meteorological data,vegetation index and type data,this study obtained the 2000-2021 vegetation NPP data set of Wujiang River Basin with the help of CASA model,and conducted accuracy verification.On the premise of ensuring reliability,the temporal and spatial variation characteristics of vegetation NPP in Wujiang River Basin in the past 22years are analyzed firstly.Secondly,the response relationship between vegetation NPP and various influencing factors is deeply explored from the perspectives of climate,terrain,land use and population density.On this basis,the geographical probe and residual analysis method are used.The force of various influencing factors is analyzed quantitatively.The main conclusions are as follows:（1）The spatial distribution of NPP in Wujiang River Basin was significantly different.The spatial distribution of mean values on the annual scale is consistent with that on the seasonal scale.The high value areas are in Wuling Mountain area in the north and Longli area in the south of the basin,while the low value areas are mainly distributed in the urban areas.Among them,on the interannual scale,the multi-annual mean value of the high value area can reach more than 1000g C·m^-2·a^-1,while the mean value of the low value area such as Guiyang,Zunyi city and water area is not more than 400g C·m^-2·a^-1.On the seasonal scale,the high value area can reach more than 300g C·m^-2·a^-1in summer,but not more than 100g C·m^-2·a^-1in winter,and 200-300g C·m^-2·a^-1in the rest of spring and autumn.（2）The vegetation NPP in Wujiang River Basin mainly increased in time.At the annual scale,the change rate of NPP was 3.8g C·m^-2·a^-1（P<0.01）,the annual mean range was 525-668g C·m^-2·a^-1（total range was45.8-58.2Tg C）,and the multi-annual mean was 593.7g C·m^-2·a^-1（total range was 51.7Tg C）.On a seasonal scale,the rate of change in spring was the highest(1.25g C·m^-2·a^-1;P<0.05),followed by autumn and winter,summer was the least,and the change was not significant（P>0.05）.The mean NPP values of different vegetation types increased at the annual scale,but decreased at the seasonal scale in the summer of evergreen broad-leaved forest,evergreen coniferous forest and shrub.（3）The areas with a high degree of urbanization are also the main areas of NPP reduction in Wujiang River Basin.In terms of spatial change,both at interannual scale and seasonal scale,about 80%（54%in summer）of the regions showed an increasing trend,while the decrease trend was most significant in urban areas.The stability of urban areas is also the lowest（CV>0.2）,and Guiyang and Zunyi are the most obvious,while the other regions are mainly medium-high stability.Attention should be paid to vegetation protection and restoration in the process of urbanization.（4）Different influencing factors have different mechanisms and degrees of influence on NPP.In terms of climatic factors,the correlation between climatic factors and vegetation NPP has regional differences.The upper,middle and lower reaches of Wujiang River Basin show different response rules when responding to climate change.There is a"hump shape"relationship between altitude,slope and vegetation NPP.When the altitude is 1250-1500m and 750-1000m,the mean value and total amount of NPP in the corresponding range are the largest.When the slope grade is in the range of 25-30°and 5-10°,the mean and total amount of NPP in the corresponding range are the largest.The main land use types in Wujiang River Basin are forest land and cultivated land,which account for about 98%of the total land use.The mutual transfer of forest land and cultivated land is also the main reason for the NPP loss in the study area.There is an inverse correlation between population density and vegetation NPP.With the gradual increase of population density range,the mean value of vegetation NPP in the corresponding range also decreases.（5）Human factors were the main driving force of NPP growth in Wujiang River Basin.The results of single factor detection by geographic detector show that population density,whether as a single factor or interacting with other factors,has a strong explanatory power on the spatial distribution of vegetation NPP in Wujiang River Basin.Secondly,all interaction types are double-factor enhancement and nonlinear enhancement.Among the results of eco-detector,no significant difference only occurred between temperature,solar radiation and land use.The results of residual analysis showed that human activities were the main driving force for NPP growth in the Wujiang River Basin,and the relative contribution rate of human activities in the NPP increase area was about65.4%,while the relative contribution rate in the NPP decrease area was only about 39%,mainly distributed in urban areas.