Potential thermal safety margin for plant photosynthesis derived from local temperature variability

The search for globally applicable features of the terrestrial responses to climate change, amenable to numerical representation in Earth system models (ESMs), underpins our ability to accurately predict the ecological consequences of different potential future climate change trajectories. We hypothesize an example of potential near universality by introducing the concept of a unified thermal safety margin for plant leaf photosynthesis, supported by data where available. Using measured optimum temperature for photosynthesis across several locations across the globe, we first show that the difference between the optimum temperature for photosynthesis and local growth temperature (ToptA – Tgrowth) is proportionally related to growth temperature variability. Hence, for these locations, the probability of growing-season temperature crossing the optimum temperature for photosynthesis is a near-invariant value. Furthermore, this probability is small. If the optimal temperature is crossed, this corresponds to a severe reduction in productivity due to the strong asymmetry of the photosynthetic response to temperature, which vegetation tries to avoid. We then extend the analysis at a global-scale using a temperature reanalysis dataset that gives local statistics of mean growing-season temperature and its interannual variability and combine this with the data-led temperature-dependent equation for ToptA. Our results show that across the globe, dividing (ToptA – Tgrowth) by the standard deviation of yearly variability in Tgrowth is also highly invariant, suggestive of a universal temperature safety margin, TSMn. Thus, if vegetation does take a globally applicable level of risk, then our finding implies that the point-calibrated equation for ToptA is valid everywhere and hence available for inclusion in the parameterisation of the land surface components of ESMs. Our findings provide a useful mathematical interpretation of how plant photosynthesis may respond to different levels of temperature variability across the globe, should there be a common level of risk to avoid crossing the optimum level of photosynthesis.