During the extremely dry summer of 2018, beech leaves turned color in many places as early as July.

Research by the Swiss Federal Institute for Forest, Snow and Landscape Research WSL now shows that up to 10% of these trees have died — a multiple of the natural mortality rate. The summer of 2022 is likely to have put further strain on beeches at dry sites.

The summer of 2022 felt like déjà vu: similar to 2018, the leaves of various trees began to change color as early as the end of July, and in the Mendrisiotto entire forests turned brown across wide areas in August. Then as now, the question arises as to how the trees with early leaf drop fare in the years that follow.

The Swiss Federal Institute for Forest, Snow and Landscape Research WSL launched an ad-hoc research program in 2018 to investigate the consequences of the dry summer. As part of this program, 1,000 beeches with premature leaf drop were repeatedly observed; these trees were located in the regions of Baselland, Schaffhausen, and Knonauer Amt/Bremgarten in the cantons of Zurich and Aargau.

The research team estimated annually until 2021 the proportion of dead branches and leaf loss in the tree canopies of around 830 beech trees with early leaf drop in summer 2018 and 139 beech trees with normal leaf drop in autumn. In addition, the team counted pests and signs of disease and determined further site factors such as soil conditions and long-term water deficits during the summer months. The researchers are now presenting the results in two specialist articles as well as in the forestry journal Wald und Holz.

Dry sites unfavourable for beech trees

“It became apparent that tree damage and a dry climate are related,” explains project coordinator Esther Frei. “Trees at sites with low precipitation and on soils that can store little water were more severely affected.” Of the beech trees with premature leaf drop, 10 percent in the Schaffhausen region died completely within three years, 7 percent in the Baselland region, and 4 percent in the Knonaueramt/Bremgarten region. A further fifth of the 1,000 monitored beech trees were felled prematurely for safety reasons.  Of the 139 trees with normal autumn colouring, only two trees have died, which corresponds approximately to the natural attrition rate.

The fact that drought caused greater damage to weakened trees at dry sites is also confirmed by tree-ring data analysed by Stefan Klesse from the Dendrosciences research group. The wood cores had been extracted in 2020 from beech trees with damaged crowns in the Ajoie region of the canton of Jura. The tree rings showed that trees with severe crown damage had already grown less well in previous years. The drought thus took its greatest toll on already weaker individuals. In the Ajoie, too, the connection with soil conditions was evident: trees growing on shallow or stony soils with poor water retention capacity showed markedly more pronounced damage than those growing on deep soils.

A sign of weakness

What is significant for foresters: Trees with early leaf drop, particularly on dry sites, did not recover in the following years, which were also quite low in rainfall. On the contrary, the proportion of dead branches increased with each passing year. “The shedding of leaves there cannot therefore be interpreted as a protective mechanism of the tree to better survive the dry period, but rather as a sign of weakness,” explains Frei. In contrast, on moister soils such as in the Knonauer Amt, many of the beeches with early leaf drop did recover.

In the summer of 2022, autumn colours were once again visible in many places. On shallow soils on south-facing slopes in northern Switzerland and in the southern Ticino, the trees were most severely affected this year. “On such drought-prone sites, beech will have a difficult future,” says Esther Frei. Foresters will sooner or later have to abandon beech on such sites.