Heat stress: the climate is putting European forests under sustained pressure

In a forest in Hölstein near Basel researchers study the effects of climate chan

In a forest in Hölstein near Basel researchers study the effects of climate change on the most important and sensitive part of the trees - the canopy. A total of 450 trees between 50 and 120 years old grow on the 1.6 hectare research area. (Photo: University of Basel)

No year since weather records began was as hot and dry as 2018. A first comprehensive analysis of the consequences of this drought and heat event shows that central European forests sustained long-term damage. Even tree species considered drought-resistant, such as beech, pine and silver fir, suffered. The international study was directed by the University of Basel, which is conducting a forest experiment unique in Europe.

Until now, 2003 has been the driest and hottest year since regular weather records began. That record has now been broken. A comparison of climate data from Germany, Austria and Switzerland shows that 2018 was significantly warmer. The average temperature during the vegetation period was 1.2°C above the 2003 value and as high as 3.3°C above the average of the years from 1961 to 1990.

Part of the analysis, which has now been published, includes measurements taken at the Swiss Canopy Crane II research site in Basel, where extensive physiological investigations were carried out in tree canopies. The goal of these investigations is to better understand how and when trees are affected by a lack of water in order to counter the consequences of climate change through targeted management measures.

When trees die of thirst

Trees lose a lot of water through their surfaces. If the soil also dries out, the tree cannot replace this water, which is shown by the negative suction tension in the wood’s vascular tissue. It’s true that trees can reduce their water consumption, but if the soil water reservoir is used up, it’s ultimately only a matter of time until cell dehydration causes the death of a tree.

Physiological measurements at the Basel research site have shown the researchers that the negative suction tension and water shortage in trees occurred earlier than usual. In particular, this shortage was more severe throughout all of Germany, Austria and Switzerland than ever measured before. Over the course of the summer, severe drought-related stress symptoms therefore appeared in many tree species important to forestry. Leaves wilted, aged and were shed prematurely.

Spruce, pine and beech most heavily affected

The true extent of the summer heatwave became evident in 2019: many trees no longer formed new shoots - they were partially or wholly dead. Others had survived the stress of the drought and heat of the previous year, but were increasingly vulnerable to bark beetle infestation or fungus. Trees with partially dead canopies, which reduced the ability to recover from the damage, were particularly affected.

“Spruce was most heavily affected. But it was a surprise for us that beech, silver fir and pine were also damaged to this extent,” says lead researcher Professor Ansgar Kahmen. Beech in particular had until then been classified as the “tree of the future”, although its supposed drought resistance has been subject to contentious discussion since the 2003 heatwave.

Future scenarios to combat heat and drought

According to the latest projections, precipitation in Europe will decline by up to a fifth by 2085, and drought and heat events will become more frequent. Redesigning forests is therefore essential. “Mixed woodland is often propagated,” explains plant ecologist Kahmen, “and it certainly has many ecological and economic advantages. But whether mixed woodland is also more drought-resistant has not yet been clearly proven. We still need to study which tree species are good in which combinations, including from a forestry perspective. That will take a long time.”

Another finding of the study is that it is only possible to record the impacts of extreme climate events on European forests to a limited extent using conventional methods, and thus new analytical approaches are needed. “The damage is obvious. More difficult is precisely quantifying it and drawing the right conclusions for the future,” says Kahmen. Earth observation data from satellites could help track tree mortality on a smaller scale. Spatial patterns that contain important ecological and forestry-related information can be derived from such data: which tree species were heavily impacted, when and at which locations, and which survived without damage? “A system like this already exists in some regions in the US, but central Europe still lacks one.”

Original source

Schuldt, Bernhard & Buras, Allan & Arend, Matthias & Vitasse, Yann & Beierkuhnlein, Carl & Damm, Alexander & Gharun, Mana & Grams, Thorsten & Hauck, Markus & Hajek, Peter & Hartmann, Henrik & Hilbrunner, Erika & Hoch, Günter & Holloway-Phillips, Meisha & Körner, Christian & Larysch, Elena & Luebbe, Torben & Nelson, Daniel & Rammig, Anja & Kahmen, Ansgar.

A first assessment of the impact of the extreme 2018 summer drought on Central European forests.
Basic and Applied Ecology (April 2020); doi: 10.1016/j.baae.2020.04.003


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In einem Wald in Hölstein bei Basel betreibt das Departement Umweltwissenschaften der Universität Basel ein für Europa einmaliges Waldexperiment. Auf der 1,6 Hektar grossen Untersuchungsfläche wachsen insgesamt 450 Bäume, die zwischen 50 und 120 Jahre alt sind, darunter Rotbuchen, Stieleichen, Hagebuchen, Fichten, Waldföhren, Elsbeeren und andere. Ein 50 Meter hoher Kran ermöglicht es, die Auswirkungen des Klimawandels in den wichtigsten und sensibelsten Teilen der Bäume - den Baumkronen - zu erforschen. Weiter soll ab 2021 die Hälfte der Untersuchungsfläche mit einem Regendach überdeckt werden, um Trockenheit zu simulieren. Die Forschenden möchten klären, welche einheimischen Baumarten besonders empfindlich auf Trockenheit ansprechen und ob ausgewachsene Bäume in der Lage sind, ihren Stoffwechsel an Klimaveränderungen anzupassen. Zudem geht es um die Frage, wie sich der Wassermangel auf biogeochemische Kreisläufe im Wald auswirkt und ob unsere Wälder auch unter zukünftigen Klimabedingungen grosse Mengen an Kohlenstoff speichern können. Das Waldexperiment startete 2018 und hat eine Laufzeit von 20 Jahren.

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