Trees under heat stress: Researchers unravel the reaction of trees to extreme temperatures

Climate change is leading to an increasing exposure of plants to high temperatures. WSL researchers have examined how trees adapt to extreme temperatures under experimental conditions. In all species studied, they found that CO₂ uptake, which allows plants to produce sugar from sunlight during photosynthesis, decreases above 30°C, while water loss through transpiration continues to increase. A novel experimental facility called X_iBox (see inset) has enabled the scientists to maintain water vapor pressure deficit (VPD) at a constant level while gradually increasing temperature.

Limitations in the biochemistry of trees

It is noteworthy that CO₂ uptake is reduced even when there is actually enough CO₂ in the air and in the leaves. «We were able to rule out that the decrease in CO₂ absorption at high temperatures is due to reduced CO₂ availability. This suggests a limitation in the biochemistry of trees at around 30°C,» says study leader Marco Lehmann. This limitation appears to be due to an alteration of the enzymatic processes of photosynthesis, according to the researcher. At the same time, the plant loses proportionally a lot of water compared to the amount of CO₂ absorbed. «In summary, we can say that heat leads to a very inefficient photosynthesis,» explains Lehmann. Inefficient photosynthesis over a long period of time can severely impair the growth, development and adaptability of trees (or plants) and ultimately affect the entire forest ecosystem.

The scientists examined the following tree species: beech (Fagus sylvatica), spruce (Picea abies), sessile oak (Quercus petraea) and small-leaf lime (Tilia cordata). «All the trees reacted in the same way. There were no significant differences between species», adds Lehmann.

Understanding the impact of climate change

Using the X_iBox, scientists were able to expose plants to different environmental conditions under controlled conditions and monitor their behavior through gas exchange and isotope measurements. Isotopes are naturally occurring atom variants of the same element with different masses. Carbon, for example, contains both light and heavy variants of atoms. The ratio between light and heavy isotopes changes when CO₂ is absorbed by the plant. «Studying the effects of temperature and humidity on tree-atmosphere gas exchange independently poses a significant challenge, owing to the intricate between these two climatic factors,» comments Lehmann.

«The experiments were technically very complex. In the X_iBox, we can carry out experiments and simulate environmental conditions that are hardly feasible in this form under natural conditions», he continues. According to him, such simulations are important for better understanding and predicting the ability of plants and their functions to adapt to new climatic conditions and the resulting rise in temperatures.