Estimating site-specific optimum air temperature and assessing its effect on the photosynthesis of grasslands in mid- to high-latitudes

The effect of air temperature on photosynthesis is important for the terrestrial carbon cycle. The optimum air temperature for photosynthesis is one of the major parameters in data-driven and process-based photosynthesis models that estimate the gross primary production (GPP) of vegetation under a changing climate. To date, most models use the biome-specific optimum air temperature (Topt-b) parameter. Towhat degreewill the site-specific optimumair temperature (Topt-s) affectGPP simulation results remains unclear. In this study, we estimated Topt-s by usingGPP data from11 grassland eddy flux tower sites (GPP(EC)) and satellite vegetation indices (NDVI and EVI). We found that Topt-s parameter values estimated from EVI have good consistency with those from GPPEC at individual sites. Wealso evaluated the effects of site-specific and biome-specific optimum air temperature parameters on grassland photosynthesis. The results showed that the use of Topt-s in the Vegetation Photosynthesis Model improved to various degrees in both daily and annual GPP estimates in those grassland flux tower sites. Our results highlight the necessity and potential for the use of Topt-s in terrestrial GPP models, especially in those situations with large temperature variation (heatwave and cold spill events).