Beech forests responded quickly to Switzerland’s 2018 thirst

Beech forests responded quickly to Switzerland’s 2018 thirst

Beech woods quickly responded to the 2018 thirst

Edge of mixed bee forest near Hemishofen (canton of Schaffhausen) in July 2018. The leaves of many deciduous trees had already turned brown, three months earlier than usual. Credit: Ulrich Wasem

How do Swiss bees 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) plots during and after the dry summer of 2018 .Reduced growth and abundance of leaves with premature coloration and Thin crowns provided clear evidence of loss of vigor in many of the bees studied. However, most of the trees survived the following year, when the weather was wetter and hotter.

Since late spring 2018, almost all parts of Switzerland have had a long time without rain. This was followed by above average summer temperatures, which led to severe drought throughout much of Central Europe. In many places on the Swiss Platform and in Jura, the leaves of deciduous trees grow too quickly. As early as mid-July, many bees displayed small, brown and slightly striped leaves that were still attached to their branches.

Lucky time: sample plots surveyed before thirst

Prior to this thirst, field work had begun for the fifth NFI to be carried out by WSL in collaboration with the Federal Office of the Environment (FOEN). By the end of May, NFI WSL survey teams had already measured 75 sample plots in beech forests in Switzerland that are German and French speaking. As part of the WSL Drum Campaign 2018, the researchers took the opportunity to visit these NFI plots again in late summer 2018 and then in 2019 to find out what the 271 trees were like. file growing there to quench the thirst.

Clear diagnosis based on three symptoms

In late summer 2018 and 2019, the researchers evaluated how the vigor of the bees improved during and after the actual drought, based on the brightness of their crown, the color of the foliage and the diameter growth of a gas. The team of researchers from WSL and ETHZurich recently published the results of the study in the journal Ecological Indicators.

The measurements at the end of summer 2018 showed that bees with brown, colored leaves and an unusually prominent crown were much more frequent than average from long-term observation. It was thought that the main possible causes of declining growth in previous years were strong competition from nearby trees and the local lack of water. The researchers also found that larger trees tended to have worse effects than smaller trees.

Timber growth in 2018 was well below the 2010–17 average across the 75 plots. In addition, bees with premature brown leaves and prominent crown made much worse, in terms of growth, the trees that retained a more important appearance.

2019: a year of recovery

Thanks to favorable weather, the condition of many of the bees studied improved in 2019. Only one tree had a bad leaf color, and the crown’s visibility was broadly in line with that of the bees. long-term. Growth in 2019 was much higher than in 2018, and even higher than the average for 2010–17.

This study, conducted at short notice, reveals the rapid response of beeches to the dry summer of 2018, which manifested itself in premature browning of leaves, above-average crown visibility and, at the end over there, low growth rate in the trees studied. However, in most of the bees studied, these effects from hot weather and low rainfall – at this time, at least – were only temporary.

The importance of long-term forest study

If climate change causes further changes in summer rainfall and temperatures, this is likely to affect the health, activity and growth of forests. In addition, forests that produce less timber than usual as a result of extreme drought also capture less carbon.

This study was only possible because there is a permanent network of more than 6,000 NFI sample plots in Switzerland’s forests for more than 35 years. This not only avoided timely and careful analysis for suitable sites but also provided robust, long-term measurements for comparing the data from 2018 to 2019. Such networks could permanent measurements to be useful in the future for the study of environmental or ecological interactions and measuring the impact of disturbances.

Dry summer of 2018: One tenth of central European forests lost leaves prematurely

Further information:
Brigitte Rohner et al. Signs of tree vigor revealed a rapid response of beech forests to the 2018 thirst, Ecological Characteristics (2020). DOI: 10.1016 / j.ecolind.2020.106903

Presented by the Swiss Federal Institute for Forest, Snow and Landscape Research WSL

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