Study on Land Use Changes and Spatiotemporal Distribution of Carbon Storage in the Middle Yarlung Zangbo Basin Based on the InVEST Model

To reveal the characteristics of land use change and the spatio-temporal distribution of carbon storage in the middle reaches of the Yarlung Zangbo River, the seven counties of Shannan City in the middle reaches of the Yarlung Zangbo River, including Konggar County and Zhanang County, were taken as the study area. Taking a typical afforestation area within this region as the study site, Land use data from 1980 to 2020 were employed, carbon density data from analogous regions were referenced, and the carbon storage module of the InVEST model was applied to simulate land use/cover change processes under different historical periods and scenarios. The spatiotemporal evolution of carbon storage was analyzed to reveal its changing patterns. The main findings are as follows: (1) From 1980 to 2020, land use types underwent significant transformations. Overall, the areas of unused land, builtup land, water bodies, shrubland, and cropland showed a dynamic increase, while glaciers, grassland, and forestland exhibited decreasing trends. (2) During 1980-1990, grassland decreased most substantially, whereas cropland expanded at the fastest rate; during 1990-2000, forestland increased most markedly, while water area shrank most rapidly; during 2000-2010, grassland reduction was the largest and glacier retreat was the fastest; during 2010-2020, builtup land experienced the greatest increase in both area and growth rate. (3) From 1980 to 2020, the carbon storage in the study area showed an overall fluctuating downward trend, with a net reduction of approximately 165.86× 10⁴ t, representing a decrease of around 6.39%. The variation process can be divided into three stages: an initial increase (1980-1995), a subsequent decline (1995-2010), and a final slight increase (2010-2020). Among these stages, the cumulative reduction exceeded 205×10⁴ t during the rapid decline in the middle stage, which dominated the overall downward trend. (4) Among all land use types in the study area, grasslands account for the largest proportion of total carbon storage, ranging from approximately 75.42% to 85.19%, yet this proportion showed a fluctuating downward trend over the past three decades. (5) From 1980 to 2010, the spatial distribution of carbon storage in the study area exhibited consistent characteristics. Regions with high carbon storage were primarily distributed in Zhanang County, Konggar County, the southern area of Naidong District, Qonggyai County, and most areas of Sangri County, which is highly consistent with the spatial distribution of grasslands. (6) Land use changes exerted differential impacts on terrestrial ecosystem carbon sequestration capacity. The conversion of forestland (with higher carbon density) to land types with lower carbon density was the primary factor driving the dynamic decline in carbon storage. In summary, the carbon storage of the ecosystem in the middle reaches of the Yarlung Zangbo River exhibits a fluctuating increasing trend overall. Given the dynamic structure of di-fferent land use types, future efforts should focus on controlling builtup land expansion, strengthening the protection of grassland and forestland, optimizing land use layout, and enhancing regional carbon storage.