Heterogeneous elevation-dependence of grassland greenness and greening on the Tibetan plateau

The change in vegetation activity along elevational gradients holds important value in maintaining biodiversity and function of ecosystems. The Tibetan Plateau (TP), a highly climate-sensitive region, has undergone pronounced warming in recent decades, with its complex terrain and diverse climatic conditions driving variability in vegetation activity along elevational gradients. In this study, we utilized 250 m resolution leaf area index (LAI) records to investigate the elevation dependence of grassland greenness on the TP. Our results indicate that grassland LAI across the entire TP decreased with increasing elevation at a rate of 1.16 m² m^-2 km^-1. However, marked spatial differences were observed: LAI decreased with elevation mainly in the energy-limited regions, i.e., eastern TP (accounting for 68.4 % of TP grassland), but increased with elevation predominantly in the water-limited areas, i.e., western TP (accounting for 31.6 % of TP grassland). From 2000 to 2021, rising temperatures have driven an overall increase in grassland LAI across the TP. However, the increasing rate declined with elevation at a rate of 0.01 m² m^-2 y^-1 km^-1, resulting in a 25.93 % increase in the elevation dependence of greenness and thereby intensifying greenness heterogeneity along elevation. Spatially, 42.4 % of grassland areas exhibited more homogeneous grassland greenness along elevation, while 57.6 % showed increasing heterogeneity, reflecting a fragmented distribution across the TP. These patterns were attributed to the spatial variability in the elevational variations of grassland sensitivities to climate change. These findings enhance our understanding of vegetation dynamics and responses to global change in mountainous ecosystems.