Elevation-dependent response of vegetation dynamics to climate change in a cold mountainous region

Mountain regions are experiencing more pronounced climate change than other global land areas. How have vegetation dynamics responded to these changes and what are the implications for hydrology? To answer these questions, we examine the impacts of changes in mean air temperature (Tmean ), precipitation (P) and winter snow cover extent (SCE) in the headwaters of the Yellow River basin (HYRB) on two important vegetation dynamic metrics: (i) the maximum growing-season greenness (represented by the monthly maximum NDVI); and (ii) the beginning of growing season (BGS). Satellite-derived NDVI and SCE, along with observation-based gridded climate data, show that during the past 34 years (1982-2015) the HYRB experienced widespread vegetation greening, while no significant trend in BGS was observed. Spring greenness and phenology were significantly affected by SCE change, highlighting the importance of snow-related process to spring vegetation activity. We observed a clear signal of elevation-dependent warming below 4300 m elevation, which is absent at higher elevations. Changes in NDVI and BGS are elevation-dependent, and trends in Tmean, P, and SCE with elevation play different roles in this dependence. Both observed and estimated watershed annual evapotranspiration series show increasing trends, suggesting that vegetation greening imposes positive effects on evaporative fluxes. Given steady-state and non-stationary hydrological conditions, increasing evapotranspiration should result in runoff reduction, which agrees with catchment-scale runoff observations across the HYRB. These findings represent new knowledge regarding the vegetation response to climate change in alpine environments which has important implications for the hydrology of the region and for other high-water yielding mountainous regions worldwide.