Tropical Forest Photosynthesis Response to High Temperatures
Figure shows (a) the response of photosynthesis rates to temperature in observations, and using multiple versions of two vegetation models, to test the effect of different processes on the temperature sensitivity of photosynthesis. (b) Separation the direct temperature effects from indirect effects driven by humidity shows that plant hydraulics and photosynthesis acclimation have different sensitivities to temperature and humidity.
Zarakas et al. 2024
Photosynthesis in tropical forests appears to go down at high temperatures, but is it driven directly by hot temperatures or indirectly by changes in humidity? The authors disentangled the effects of hot temperatures and dry air using creative experiments with a computer model to separate out the role of different factors and things we don’t know about tropical trees. They find that multiple assumptions about trees in computer models match the observed response of real trees about equally well, but that the different assumptions give diverging predictions under future climate conditions.
This study tells us that understanding why and how tropical forests are responding to hot temperatures now is critical for predicting how they respond in the future. It also tells us that we need more observational data on hot conditions at multiple different levels of humidity to understand these responses to hot temperatures now. This study provides an example for how to use models to better disentangle many assumptions to really test our understanding of why and how.
Hot air acts to both slow photosynthesis and dry out leaf tissues. With global warming, the relationship between humidity and temperature may change, so it is important to know how much humidity versus temperature governs the net effect on plants. Recent observations show that plants can acclimate to direct effects of higher temperatures, but also that the drying of leaves and other plant tissues in hot temperatures have strong effects on photosynthesis. In natural ecosystems, it can be difficult to separate these two effects, but experiments such as the Biosphere 2 greenhouse, which is both hot and humid, and large enough to grow tropical forest trees, can be used to separate these effects. We ran two vegetation models under both natural and experimental conditions, to separate these effects, and showed that two processes of acclimation and plant tissue drying offset each other's effects on the temperature-photosynthesis relationship.