Tropical trees and climate change: new insights into heat and drought stress
How do tropical trees cope in a warming world? Olivier Manzi demonstrates in a new thesis that that leaf temperatures can reach extreme levels, challenging previous models of heat stress in trees. The thesis also highlights how species-specific traits influence resilience to heat and drought, offering important conclusions for forest restoration and climate-resilient management.
The title was ”Heat and drought responses of tropical trees in a warming world”, could you tell us a bit more about the work you did?
My thesis investigated the physiological responses of tropical trees to heat and drought stress, focusing on leaf temperature regulation, heat tolerance, and water use. The research was conducted in tropical tree plantations at three sites in Rwanda along an elevation and climate gradient. One key finding was that leaf temperatures (Tleaf) can exceed air temperatures (Tair) by up to 20°C and canopy temperatures (Tcan) by significant margins. This suggests that relying on Tair or Tcan underestimates leaf heat stress and overestimates thermal safety margins. Traits like leaf size and stomatal conductance (gs) were found to control interspecific variation in Tleaf.
I also explored heat tolerance mechanisms, revealing that species with larger leaves or lower gs had greater photosynthetic heat tolerance but narrower thermal safety margins due to insufficient thermal acclimation. Adjustments in membrane lipid composition provided some acclimation but could not fully offset warming effects.
Lastly, I studied tree water use and responses to drought. Larger trees, likely supported by deeper roots, maintained better water status and growth under drought, while smaller trees were more vulnerable. These results highlight the critical role of species-specific traits in modulating resilience to heat and drought stress in tropical forests.
The insights from this work contribute to improving vegetation models and informing strategies for forest restoration and climate-resilient forest management.
… were there any surprises?
Yes! I was particularly struck by the extreme leaf temperatures (Tleaf) observed in sunlit leaves, sometimes exceeding 50°C, and 20°C warmer than air temperatures. This revealed that relying on Tair can significantly underestimate the heat stress experienced by individual leaves. Another unexpected finding was the limited thermal acclimation of tropical tree species, with heat tolerance increasing by only 0.31°C per 1°C rise in growth temperature. Despite their evolutionary adaptation to warm environments, tropical trees seem to have narrow thermal safety margins (TSMs), making them highly vulnerable to climate warming.
Your research is part of a successful collaborations between Sweden and Rwanda, what has been the biggest difference between the two countries?
Sweden is more developed in many areas compared to Rwanda, offering advanced scientific infrastructure, state-of-the-art technologies, and access to extensive long-term ecological datasets. Rwanda, on the other hand, is still developing, with growing facilities but more limited resources and technology. The collaboration between the two countries has been transformative, merging Rwanda’s rich biodiversity and deep ecological knowledge with Sweden’s advanced methodologies. This synergy has produced globally relevant insights into how tropical trees respond to climate change.
Sweden has beautiful forests, although they are not as diverse as Rwanda’s. The Swedish weather, however, was quite a challenge for me! The people in Sweden are kind but tend to be more reserved compared to the warm, open nature of Rwandans. I particularly enjoyed embracing the fika culture and celebrating Midsummer, both of which were unique and delightful experiences.
And finally, what’s next for you now?
In March 2025, I will begin a new role as a postdoctoral research fellow at the University of Leeds in the UK. My work will focus on African forest remeasurement in the Congo Basin, studying carbon dynamics across 225 permanent plots located in four countries: the Democratic Republic of Congo, Republic of Congo, Cameroon, and Gabon.
