Young trees can withstand high temperatures if there is enough water available in the ground. If the ground is too dry, however, they can’t properly cool themselves off and become vulnerable to overheating and leaf damage, a joint WSL-EPFL study shows.

• For 5 years, researchers from WSL and EPFL studied how young beech and oak trees respond to heat, drought, or both.
• The trees grew even when exposed to significantly elevated temperatures, as long as they had sufficient water. If not, they experienced overheating and sunburn.
• With climate change, the likelihood of such conditions increases.

Can our forests adapt to a hotter and drier future climate? Temperatures are predicted to rise up to 5°C compared to pre-industrial times. Forest management needs to adapt to these conditions, which requires better understanding of how heat and drought affect trees. A new study by a team of the Swiss Federal Institute for Forest, Snow and Landscape Research WSL and EPFL shows that the warming itself is not the biggest issue for trees.

On the WSL grounds in Birmensdorf (ZH), young beech and downy oak trees have been growing inside climate chambers for over 5 years. In these miniature greenhouses ((Modoek – link)), the researchers subjected the trees to varying degrees of soil moisture and air temperature. While some trees experienced drought, others were growing at a temperature elevated by 5°C, to simulate future conditions, or had to endure both heat and drought.

The researchers used beech and downy oak because they are important to Central Europe’s economy and forest ecosystems. “Downy oak mainly grows in Mediterranean areas but is also found in Switzerland. We wanted to compare its water-use strategy with that of the beech, a tree more sensitive to droughts, especially in the sapling stage,” says Alyssa Therese Kullberg, a plant ecophysiologist at the EPFL-WSL’s Plant Ecology Research Laboratory who has conducted the research.

Sunburn and scorching

Both heat and drought are a source of stress for trees. To prevent overheating at high temperatures, leaves cool themselves down by releasing water through their pores, in a process akin to sweating in humans and known as transpiration. If soil water is scarce, however, the pores close, transpiration stops and the leaf faces permanent damage called scorching. In this situation, trees face the difficult decision to either continue transpiring to cool their leaves, or to close their pores to conserve water.

In the experiment, the researchers looked at the trees’ thermoregulation by measuring leaf temperatures and at colour changes due to damage. In direct sunlight, leaf surfaces can reach temperatures of 40–45°C. Even in this intense heat, their cooling mechanism appears to work without issue. “We might see a little sunburn on the leaves with minor damage to the photosynthesis system, but that damage is usually temporary,” says Kullberg.

But when subjected to the same temperatures in combination with dry soil, there is not enough water for transpiration and the leaves’ pores close to preserve water. The leaves become dehydrated and they overheat. In beech trees, this leads to scorching which is visible as brown patches of damaged leaf tissue. Oaks, however, are different: “We didn’t see much scorching on oak leaves, even though their thermal safety margins were exceeded with pretty much the same frequency as those of beech leaves,” says Kullberg. “In other words, both species reached critical temperatures just as often, but the oak seemed better able to tolerate those conditions. That came as a surprise – and it can’t be fully explained by the mechanisms we measured in our study.”
The study is the first to document with experimental data that when tree leaves go beyond their thermal safety margin – meaning the temperature reaches critical levels – this is associated with permanent damage in the form of scorching, particularly under drought conditions. “That’s why we are really excited about the finding,” says Kullberg.

Drier air “sucks up” more water

The trees in the experiment will continue to grow under experimental conditions for two more years. With more data, the researchers will be able to investigate further whether trees that have already experienced water scarcity will be more resistant to heat and drought because they adapt to difficult conditions. With climate change, the co-occurrence of extreme weather events such as heatwaves and droughts is becoming increasingly frequent. Even if precipitation remains stable, higher temperatures will increase the “pull” of evaporation in the future, because warmer air takes up more moisture. This will dry up soils more quickly than today, causing problems for trees, especially beeches. In the summer droughts in 2018 and 2022 in Switzerland, for example, leaf scorching has occurred in many beech trees on dry soils.