Sensitivity of Tropical Forest to Heat Stress -Trop-Heat

Tropical forests are biodiversity hotspots and important biological conservation regions. They deliver key ecosystem services such as carbon sequestration and storage, and water for electricity generation via hydropower (a large source of electricity in many tropical countries) and freshwater provision, serving the needs of millions of people and fast-growing populations in these regions.

However, tropical regions have experienced the largest recent increases in heat extremes over the globe, with ongoing warming predicted to exceed the bounds of historic climate variability in the next two decades. This climate change has potentially large but poorly understood consequences for tropical forests.

Recent findings suggest that these critical forests appear at substantial risk, in terms of their vulnerability and exposure to warming and its extremes. For example, extreme temperatures in lowland forest reduces tree growth and carbon storage. Furthermore, in the tropical Andes, recent warming has been associated with increased mortality of species in the warm extreme of their thermal ranges, triggering a compositional change towards warm-adapted species across all elevations. The mechanisms underpinning reduced tree growth and species compositional changes remain largely unknown. To predict species composition changes and their implications for forest function and ecosystem services, a mechanistically-informed understanding of the physiological strategies employed by thermally resilient and susceptible species is needed.

At our unique warming experiments along elevation gradients in the tropics in the Colombian Andes and in Rwanda in the Albertine Ridge we obtain a range of responses to the warming treatment: some species have died, some have shown reduced growth, while others have increased their growth. Importantly, and contrary to some expectations, plant physiological responses to average site temperatures cannot predict growth patterns. Rather, preliminary evidence suggests that tree growth and survival in the North Andean region and in our experiments in Colombia and Rwanda, is related to species abilities to deal with heat stress. Multiple mechanisms may be involved in determining the ability of species to cope with heat stress, but their relative roles in different settings is unknown. In Rwanda, preliminary data suggest that the most successful species thermoregulate, cooling their leaves via high rates of evapotranspiration to cope with extreme temperature, while species that have shown reduced growth with warming reach very high leaf temperatures (ie they cannot thermoregulate). In contrast, in Colombia, the most successful species are those that emit isoprene to ameliorate heat stress suggesting enhanced thermotolerance may be a key mechanism. Overall, our results demonstrate an urgent need to understand how different tropical tree species cope with extreme rather than average temperatures.

Using our experiments in Colombia and Rwanda, this project will deliver new mechanistic understanding of heat stress physiology for tropical forests and possible links to plant growth responses to warming which will inform how we understand and predict composition changes along elevation and climate gradients. We will use a holistic combination of measurements not done before in any ecosystem- thermoregulation, thermal tolerance thresholds, in situ isoprene emissions, and their thermal plasticity- to evaluate tree heat stress strategies. We will combine our experimental data with mechanistic modelling to generalise our results to other ecosystems and with data from Andean trees to determine the extent to which the new understanding of species-level heat stress strategies can explain compositional changes in Andean forest tree species. Our project will support better prediction of future biodiversity shifts and forest function, tropical forest restoration and conservation.