Beech forests responded quickly to 2018 drought

How do Swiss beeches respond to extreme drought? To answer this question, researchers at the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) studied 75 National Forest Inventory (NFI) sample plots during and after the dry summer of 2018. Reduced growth and an abundance of prematurely discoloured leaves and thinned crowns provided clear evidence of a loss of vitality in many of the beeches examined. However, most of the trees recovered the following year, when the weather was wetter and less hot.

From late spring 2018, almost all parts of Switzerland experienced a prolonged period without rainfall. This was followed by above-average summer temperatures, which led to extreme drought across much of Central Europe. In many places on the Swiss Plateau and in the Jura, the leaves of deciduous trees aged prematurely. As early as mid-July, many beeches displayed small, brown and slightly shrivelled leaves that remained attached to their branches.

Lucky timing: sample plots surveyed prior to drought

Prior to this drought, field work had begun for the fifth NFI being carried out by WSL in cooperation with the Federal Office for the Environment (FOEN). By the end of May, WSL's NFI survey teams had already taken measurements at 75 sample plots in beech forests in German-speaking and French-speaking Switzerland. As part of the WSL Drought Initiative 2018, the researchers took the opportunity to visit these NFI plots again in late summer 2018 and then in 2019 to find out how the 271 beech trees growing there had responded to the drought.

Clear diagnosis based on three symptoms

In late summer 2018 and 2019, the researchers assessed how the vitality of the beeches developed during and after the extreme drought, based on their crown transparency, leaf discolouration and stem diameter growth. The team of researchers from WSL and ETH Zurich recently published the study findings in the journal Ecological Indicators.

The measurements in late summer 2018 revealed that beeches with brown, discoloured leaves and unusually transparent crowns were much more frequent than the average from long-term observations. The main possible causes were deemed to be reduced growth in previous years, strong competition from neighbouring trees and the local precipitation deficit. The researchers also found that larger trees tended to be more severely affected than smaller ones.

Wood growth in 2018 was significantly lower than the average for 2010–17 across all 75 plots. Moreover, beeches with both prematurely brown leaves and transparent crowns did significantly worse, in terms of growth, than trees that retained a more vital appearance.