Optimal temperature of vegetation productivity and its linkage with climate and elevation on the Tibetan Plateau.

Vegetation productivity first increases and then decreases with temperature; and temperature corresponding to the maximum productivity is called optimal temperature (T ). In this study, we used satellite derived near-infrared reflectance of vegetation (NIR ) data to map T of vegetation productivity at the spatial resolution of 0.1° on the Tibetan Plateau (TP), one of most sensitive regions in the climate system. The average T of non-forest vegetation on the TP is about 14.7°C, significantly lower than the T value used in current ecosystem models. A remarkable geographical heterogeneity in T is observed over the TP. Higher T values generally appear in the north-eastern TP, while the south-western TP has relatively lower T (<10°C), in line with the difference of climate conditions and topography across different regions. Spatially, T tends to decrease by 0.41°C per 100 m increase in elevation, faster than the elevational elapse rate of growing season temperature, implying a potential CO regulation of T in addition to temperature acclimation. T increases by 0.66°C for each 1°C of rising mean annual temperature as a result of vegetation acclimation to climate change. However, at least at the decadal scale, there is no significant change in T between 2000s and 2010s, suggesting that the T climate acclimation may not keep up with the warming rate. Finally, future (2091-2100) warming could be close to and even surpass T on the TP under different RCP scenarios without considering potential climate acclimation. Our analyses imply that the temperature tipping point when the impact of future warming shifts from positive to negative on the TP is greatly overestimated by current vegetation models. Future research needs to include varying thermal and CO acclimation effects on T across different time scales in vegetation models.