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 due to greater atmospheric demand for water but also results in reduced stomatal conductance and carbon assimilation rates. Recent studies have suggested that increasing tree mortality patterns observed in some temperate and tropical zones may be driven by increasing VPD. However, no study to date has sought to isolate the role of direct temperature effects vs. indirect VPD effects in inducing heat stress-driven mortality.

THERMOS will address each of these current bottlenecks to deliver unprecedented large-scale insights into the thermal risk of tropical forests. To do this, a diverse set of complementary methodologies will be used including: 1) extensive field data collection in tropical forests in four continents to determine the high temperature thresholds of key plant processes, 2) drone-based thermal imaging to determine maximum leaf temperatures reached in different sites, 3) new extreme heating greenhouse experiments to test the ability of leaf thermal traits to predict mortality and to evaluate the importance of direct vs. indirect VPD effects in driving mortality, 4) remote sensing to determine how thermally 'safe' forests are across the Tropics and 5) analysis of forest dynamics records to evaluate the role of increasing temperature and VPD in driving increased mortality across tropical forests.