Spatio-temporal distribution characteristics and driving factors of forest land in the Da-Xiao Liangshan mountains based on topographic gradient.

The Da-Xiao Liangshan Mountains serve as a crucial ecological barrier in the upper reaches of the Yangtze River in China. This study uses high-precision land use and land cover data from 1980 to 2020, applying kernel density analysis, topographic gradient classification, the PLUS model, landscape pattern indices, and standard deviational ellipse theory to analyze the spatio-temporal changes, topographic gradient differentiation, fragmentation, and driving factors of forest land in the study area. The results reveal the following: (1) From 1980 to 2020, forest land was one of the dominant land use types in the study area, covering more than 57.65% of the total area. The forest land area initially decreased and then increased, with a net gain of 120.41 km(2) over the 40-year period. (2) The spatial distribution of forest land exhibited a pattern of sparse forest cover in the central and eastern regions and dense cover in the southeastern, northeastern, and western regions. During the study period, the degree of fragmentation intensified, and the centroid of forest land shifted 0.84 km to the northeast between 1980 and 2020. (3) Forest land distribution demonstrated significant vertical zonation, being highly concentrated on sunny slopes at altitudes between 1,622 m and 3,976 m, with slopes ranging from 8.77° to 44.16° and relief degree of land surface between 199 m and 969 m. Forest land area generally decreased across different topographic gradients from 1980 to 2020, with the opposite trend observed between 2000 and 2020. (4) Changes in forest land area were strongly influenced by both natural factors (such as altitude, NDVI, and annual precipitation) and economic factors (such as per capita GDP, distance to railways, and population density), with elevation being the primary driving factor, contributing 0.11. The study indicates that forest land in the Da-Xiao Liangshan Mountains exhibits significant spatio-temporal and topographic gradient differentiation. Tailored forest conservation strategies should be implemented to promote the sustainable development of forest land resources.