Spatiotemporal Variation Characteristics Of Vegetation Net Primary Productivity In The Source Region Of The Yangtze River Based On CASA Model

The source region of the Yangtze River is a typical region of alpine ecosystem in China,which has unique and fragile ecosystem and special hydrological characteristics,and plays an extremely important role in the ecological environment,hydrological cycle and climate change of the cold region.Net primary productivity of ecosystem vegetation refers to the difference between the total amount of organic matter consumed by plants to maintain autotrophic respiration and the total amount of organic matter fixed through photosynthesis.As an important variable linking the water cycle and carbon cycle in the ecosystem,Net primary productivity has important hydrological and ecological significance.It is also an important index to evaluate the regional ecological status.Therefore,the research of the Yangtze river source area spatial and temporal variation characteristics of vegetation net primary productivity,and to explore all kinds of factor influence on the Yangtze river source area vegetation net primary productivity,help to understand the status of the Yangtze river source area ecological environment,the ecological security of the source area of the Yangtze river protection has a certain significance,for scientific evaluation of the ecological system adjusting process has important theoretical and practical significance.Based on MODIS remote sensing data,topographic data,land use data,vegetation type data and meteorological and hydrological data from 2001 to 2018,this paper firstly analyzes the spatial-temporal variation characteristics of meteorological and hydrological factors such as precipitation,air temperature and soil water content in the source region of the Yangtze River.Then,based on the light energy utilization model,Based on CASA model,the net primary productivity of vegetation in the source region of the Yangtze River was estimated and analyzed.Finally,the impacts of climate factors,terrain factors,land use and other factors on the net primary productivity of vegetation in the source region of the Yangtze River were analyzed.The main conclusions of this paper are as follows:(i)The annual precipitation,annual temperature and annual soil water content in the source region of the Yangtze River show an increasing trend on the whole.The annual and seasonal precipitation in the source region of the Yangtze River gradually increases from northwest to southeast,and the annual precipitation tendency rate gradually increases from west to east.The temperature in most areas of the source region of the Yangtze River presents an increasing trend,with the temperature trend rate ranging from-0.4?/10 a to 0.8?/10 a.On the seasonal scale,the temperature in most areas increases gradually,especially in spring.The Yangtze river source area throughout the year and each season,soil water content increase gradually from north to south,space on the Yangtze river source area in much of the year and each season is to increase the soil water content,a small number of areas to reduce soil water content,reduce the fastest region is located in the changjiang river source area source,the Yangtze river source area soil moisture content in the autumn has the largest increase trend and the largest decrease trend.(ii)From the perspective of time scale,the NPP of vegetation in the source region of the Yangtze River showed an upward trend on the whole.The variation of NPP of vegetation in the source region of the Yangtze River was mainly affected by the seasons.The total annual NPP of vegetation in the source region of the Yangtze River was mainly concentrated in summer,accounting for more than 90% of the total annual NPP.From the perspective of spatial scale,the NPP of vegetation in the source region of the Yangtze River presents an overall distribution trend of low in the west and high in the east,and an increasing trend from northwest to southeast.This is due to the high altitude,large temperature difference and sparse vegetation distribution in the northwest region,which leads to the lowest total NPP.Under the influence of altitude and seasonality,the variation degree of vegetation NPP in the northwest region of the source region of the Yangtze River is higher,while the variation degree of vegetation NPP in the southeast region is smaller,and the vegetation NPP in the southeast region shows a slight increase trend,while the vegetation NPP in the northwest region shows a decrease trend.(iii)Precipitation in the source region of the Yangtze River had relatively little effect on NPP,and temperature and soil water were the main controlling factors.The NPP in the source region of the Yangtze River decreases with the increase of altitude,and the annual mean NPP in the source region increases with the increase of slope.