Climate and land use influences on changing spatiotemporal patterns of mountain vegetation cover in southwest China

Understanding the spatiotemporal patterns of vegetative cover in relation to climate and land uses is essential for effective management of ecology and the environment. In this study, spatial and temporal changes of the normalized difference vegetation index (NDVI) and potential influencing factors were analyzed in different elevations and land uses across southwest China. Results showed: (1) there was a critical elevation of 3400 m, with different NDVI responses to climate and human interventions above and below 3400 m. Below 3400 m, mean NDVI in each land use area and the whole region did not change with elevation due to compensative effects of decreasing cultivated land and increasing woodland and grassland towards high elevations. Above 3400 m, cultivation effectively ceases. NDVI decreased with elevation as alpine plant species shifted from woody trees to alpine grass, primarily related to declining temperature towards high altitudes. (2) NDVI responses to climate change and human activities are also different above and below 3400 m. NDVI below 3400 m increased significantly after 1980s, primarily as a result of reforestation on hillslopes and improved agricultural productivity. Above 3400 m, under climate warming since the 1980s, NDVI did not increase significantly in 1990s and even decreased in 2000s as the consecutive rise of temperature is higher towards higher altitudes in the 2000s. (3) The area-weighted NDVIs illustrated that from 1980s to 2000s, the increased mean NDVI in the whole region arose from contributions of 20.93, 60.66 and 18.41% changes in NDVIs in cultivated land, woodland and grassland, respectively. In 2000s, the proportion of the woody trees contribution to NDVI increased due to reforestation in the low elevation area (<3400 m), but decreased due to shift of the woody trees to alpine grass under the consecutive climate warming in the high elevation area (>3400 m). The decease of NDVI in the high elevations did not alter increasing trend of NDVI across the whole region during 1982-2015. However, in future, the greening could diminish or even cease as climate warming continues and effects of artificially managed ecological restoration reduce.