Heat and Drought Threaten Beech and Downy Oak Trees

A warmer future sounds ominous for forests, and it is likely to be. Yet a new experiment indicates that, for many trees, high temperatures alone are not the greatest threat. The real risk comes when heat is combined with dry ground.

Researchers at the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) and EPFL led the study, drawing on an unusual long-term experiment in Birmensdorf, near Zurich.

For over five years, young beech and downy oak trees have been cultivated in climate chambers that allow scientists to raise temperatures, reduce water supplies, or impose both conditions simultaneously.

The premise was straightforward: recreate the summers central Europe may experience if warming reaches around +5°C, then observe the consequences for leaves.

The findings are somewhat unexpected. Leaves can withstand remarkably high temperatures when water is plentiful, whereas identical heat can inflict permanent harm when it occurs during drought.

A miniature forest experiment

At the WSL site, the researchers constructed open-top chambers, essentially small outdoor “model ecosystems”. Within them, saplings develop under precisely managed combinations of air temperature and soil moisture.

One set of trees received normal water levels and experienced normal temperatures. Another grew in air roughly 5°C warmer than outdoor conditions, representing a future climate.

A third set experienced drought, while the most demanding treatment exposed trees to both hotter air and dry soil at once.

The research examined two species of major importance in central Europe. Beech is common and commercially valuable, but it is recognised as vulnerable to drought, particularly at a young age.

Downy oak differs in this respect. Although more typical of Mediterranean-style climates, it also grows in Switzerland and is consequently considered a potential “replacement” tree for warmer environments.

“We wanted to compare its water-use strategy with that of the beech, a tree more sensitive to droughts, especially in the sapling stage,” said Alyssa Therese Kullberg, a plant ecophysiologist at EPFL and WSL who led the work.

How trees control leaf heat

Leaves can become hotter far more quickly than many people assume. Under direct sunshine, their surfaces can readily reach 40 to 45°C, even where the surrounding air is cooler.

Trees generally protect themselves through transpiration, a process similar to plant sweating. Water vapour is released through minute leaf pores, cooling the surface as that water evaporates.

However, this mechanism depends on sufficient water being present in the soil. During drought, trees must make a difficult choice.

They may leave their pores open to cool their leaves, but this can deplete their water reserves and damage the entire plant. Alternatively, they can close the pores to retain water, but cooling then ceases and the leaves become extremely hot.

During the experiment, scientists monitored leaf temperatures and checked for visible injury, including colour changes. Where trees had adequate water, even intense heat was not as disastrous as might be expected.

“We might see a little sunburn on the leaves with minor damage to the photosynthesis system, but that damage is usually temporary,” Kullberg said.

Heat by itself therefore often produced only temporary stress, even when leaf temperatures reached uncomfortable levels. Serious problems began when drought was added.

Drought makes heat hazardous for trees

When soil is dry, leaves cannot continue evaporating water without endangering the tree. Their pores therefore close, transpiration slows or stops, and leaf temperatures climb still higher.

This is when “scorching” occurs: brown areas of dead or irreversibly damaged tissue.

The scientists found that lasting damage, particularly in drought conditions, was associated with exceeding a leaf’s “thermal safety margin”, a critical temperature threshold.

Using controlled experimental data, the team also demonstrated that surpassing thermal safety margins may lead to permanent scorching, especially where water is limited.

“That’s why we are really excited about the finding,” Kullberg said.

Downy oak performs better than beech in drought

Beech showed clear signs of harm. In beech saplings, the combination of heat and drought often produced scorched leaves, with brown patches extending across the leaf surface.

Downy oak responded differently, which is where the research becomes particularly notable.

“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,” Kullberg said.

“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.”

Put simply, oak and beech reached “too hot” temperatures at comparable rates, but beech leaves suffered a far greater cost. Oak leaves appeared able to cope better.

The researchers cannot yet fully account for the difference, an important qualification. Nevertheless, the finding matters because it distinguishes between two concepts that are often treated as one: reaching a critical temperature and actually being damaged by it.

The overlooked effect of dry air

Drought is not solely a matter of receiving “less rain”. Even where rainfall does not change, warming itself can make soils functionally drier.

Warmer air can contain more moisture. As temperatures increase, the atmosphere demands more water, extracting additional moisture from both soils and leaves through evaporation.

As a result, forests may experience water stress earlier, without substantial shifts in rainfall. This is one reason drought risk is projected to increase under climate change.

In Switzerland, for instance, the hot, dry summers of 2018 and 2022 resulted in widespread scorching of beech leaves. This was particularly apparent in locations where soils were already dry. The experiment helps account for this pattern.

Beech is able to keep cool while water remains available. Once the soil dries out, however, it must switch off its leaf-cooling system, making heat far more dangerous.

Monitoring trees over time

The trees in the climate chambers will remain under these controlled conditions for a further two years. This will allow the researchers to investigate a follow-up issue relevant to real forests: can trees “learn” from stress?

Put another way, if a tree has already faced water shortages, does it adapt in a manner that helps it endure later periods of heat and drought?

Additional years of data may enable the team to establish whether earlier stress makes trees more resilient, or instead simply leaves them more vulnerable.

For forest managers, the message already appears clear. Warming poses a problem, but water is the fuse.

If both heatwaves and drought become more frequent, tree survival will be determined less by how hot the air becomes and more by whether the soil holds enough water to prevent leaves from overheating.

Differences between species, including beech and downy oak, may determine the appearance of future forests, even in areas where beech has been dominant for centuries.