European forests struggle to recover from 2018 drought

A new study from the University of Basel has revealed that the drought of 2018 led to long-term consequences across central European forests. The researchers found that even species that are considered to be resistant to drought, such as beech, pine, and silver fir, sustained major damage. 

As the hottest year in recorded history, 2018 shattered the previous record set in 2003 by 1.2 degrees Celsius. Compared to the average temperature from 1961 to 1990, the summer of 2018 was hotter by 3.3 degrees.

For the current investigation, the team extensively analyzed tree canopies at a research site in Basel called the Swiss Canopy Crane II.

The goal of the study was to gain a better understanding of how and when trees are affected by water deprivation. This insight will ultimately help in developing targeted management measures to offset the impacts of climate change.

Trees lose a lot of water through their surfaces that is impossible to replace through the roots when the soil becomes dry. This water shortage is visible in negative suction tension in the wood’s vascular tissue. 

While trees are capable of reducing their water intake during droughts, a lack of water in the soil ultimately causes cell dehydration and death.

Physiological measurements of trees at the Basel site showed that water shortages and negative suction tension had occurred earlier than usual. 

The water shortages were the worst ever measured throughout Germany, Austria, and Switzerland, according to the researchers. Over the course of the summer, severe stress symptoms such as aging and premature shedding appeared in many tree species important to forestry. 

In 2019, the consequences of the summer heatwave became even more clear when many trees failed to form new shoots. The trees that had managed to survive were increasingly vulnerable to bark beetle infestation or fungus. 

Trees with partially dead canopies were found to be much less capable of recovering from damage.

“Spruce was most heavily affected. But it was a surprise for us that beech, silver fir and pine were also damaged to this extent,” said study lead author Professor Ansgar Kahmen. 

Beech was once widely regarded as ‘the tree of the future.” However, the resilience of beech trees became a subject of debate after the 2003 heatwave.

The latest climate projections show that precipitation in Europe will decline by up to a fifth later this century, while drought and heat become more frequent. This means that it is essential to redesign forests to build up their tolerance to extreme conditions. 

“Mixed woodland is often propagated, and it certainly has many ecological and economic advantages. But whether mixed woodland is also more drought-resistant has not yet been clearly proven,” said Professor Kahmen. “We still need to study which tree species are good in which combinations, including from a forestry perspective. That will take a long time.”

“The damage is obvious. More difficult is precisely quantifying it and drawing the right conclusions for the future.”

Conventional methods must be replaced with more advanced approaches to speed up the process, such as using Earth-observing satellite data. “A system like this already exists in some regions in the US, but central Europe still lacks one.”

The study is published in the journal Basic and Applied Ecology.

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By Chrissy Sexton, Earth.com Staff Writer