Our interest in leaf temperature is because we know that when leaves reach a certain temperature, their photosynthetic machinery breaks down. We have known about these critical temperature thresholds for more than 150 years. Our new study in Nature is an effort to establish how close tropical forest canopies are to these limits.
One of the most remarkable aspects of this study was the methods we were able to use to determine canopy leaf temperatures. It is remarkable that we can observe the temperature of the world tropical forests from an instrument on the International Space Station orbiting 400 km above Earth’s surface and traveling nearly 29,000 km per hour. It is equally remarkable to imagine the painstaking efforts made by my colleagues to measure the temperatures of individual leaves in the canopy by hand. We need both the ground- and satellite-based observations to understand the temperatures of tropical forest canopies.
For leaf temperatures, it is not the averages that are important. It’s the extremes. And this study shows that there are times and places where tropical forest leaves are surpassing their critical temperature thresholds at least once per season. Moreover, we show that when we experimentally warm air temperatures, we often observe leaf temperatures that are higher than that of air temperature. These higher air temperatures exceed the limits of cooling that the leaves can achieve and so the leaves accumulate excess heat. It's a non-linear effect between increasing air and increasing leaf temperatures.
The results do not indicate that reaching a tipping point for tropical forests is fait accompli. We still hold in our power the ability to conserve these places that are so critically important for carbon, water and biodiversity.
Doughty, C.E., Keany, J.M., Wiebe, B.C. et al. Tropical forests are approaching critical temperature thresholds. Nature (2023). https://doi.org/10.1038/s41586-023-06391-z