THERMOS:Thermal Safety Margins of Earth's Tropical Forests

Earth's tropical forests provide an array of ecosystem services, housing over 50% of global biodiversity, taking up 8-13% of annual anthropogenic CO2 emissions, recycling rainfall at continental scales and directly providing livelihoods to millions of people. The biological and ecological processes that sustain these services (e.g. photosynthesis and transpiration) are strongly climate-sensitive, such that the future large-scale functioning of tropical forests depends on keeping their climate space within safe operating limits. Currently we do not know what the safe operating temperature limits for tropical forests are nor how close they are to upper limits of temperature function. There are three main reasons for this:

1) different plant processes are subject to different temperature thresholds - e.g. there are optimal temperatures for photosynthesis and also temperatures at which the photosynthetic apparatus begin to break down, but large data gaps prevent us from understanding how these limits vary across tropical forests and species

2) even for species where we do know the temperature thresholds for key physiological functions (e.g. breakdown of photosynthesis machinery), we usually do not have the leaf temperature records that allow us to gauge how close tropical trees are to these thresholds. The distinction between leaf and air temperature is key here - leaf temperatures are the physiologically meaningful measure of temperature and can be substantially different to air temperatures

3) we do not know what leaf-level metrics of temperature tolerance mean for the performance of the whole plant in terms of growth and mortality. It is unclear whether leaf traits can predict risk of heat-induced mortality. Temperature can affect plant performance directly (e.g. by reducing photosynthetic rate) but also indirectly by increasing the vapour pressure difference between the air and leaves (leaf-to-air vapour pressure deficit). Higher VPD increases plant water losses...