Increasingly Important Role of Atmospheric Aridity on Tibetan Alpine Grasslands

Pronounced warming occurring on the Tibetan Plateau is expected to stimulate alpine grassland growth but could also increase atmospheric aridity that limits photosynthesis. But there lacks a systematic assessment of the impact of atmospheric aridity on alpine grassland productivity. Here we combine satellite observations, flux-tower-based productivity, and model simulations to quantify the effect of atmospheric aridity on grassland productivity and its temporal change between 1982 and 2011. We found a negative impact of atmospheric vapor pressure deficit on grassland productivity. This negative effect becomes increasingly intensified in terms of the impact severity and extent, suggesting an increasingly important role of atmospheric aridity on productivity. We further demonstrated that this negative effect is mitigated but cannot be overcompensated by the positive effect of rising CO2. Given that vapor pressure deficit is projected to further increase by similar to 10-38% in the future, Tibetan alpine grasslands will face an increasing stress of atmospheric drought. Plain Language Summary Significant climatic warming occurred in most parts of the globe and could lead to increased atmospheric drought stress world widely. However, the trends of atmospheric drought condition gained inadequate attentions and their effects on vegetation growth are not clear. Here we combined observational and model evidences to point out a significant increasing trend of atmospheric aridity (vapor pressure deficit [VPD]) across Tibetan alpine grasslands during the past decades. Moreover, an increasingly detrimental role of VPD was found for grassland productivity, with respect to both extent and intensity of the impact. Under the future gas emission scenarios, VPD will further increase and ecosystems might face amplified drought stress from atmosphere, in addition to commonly recognized soil aridity.