Early autumn colours are a bad omen

In the extremely dry summer of 2018, beech leaves discoloured in many places as early as July. Now, studies by the Swiss Federal Institute for Forest, Snow and Landscape Research WSL show that up to 10% of these trees died - a multiple of the natural mortality rate. The summer of 2022 is likely to have further affected beech trees in dry locations.
Forest in autumn colours

The summer of 2022 seemed like a déjà vu: Similar to 2018, the leaves of various trees were already changing colour from the end of July, and in the Mendrisiotto, entire forests were brown in August. Today, as then, the question is how the trees with early leaf fall will fare in the following years.

In 2018, the Swiss Federal Institute for Forest, Snow and Landscape Research WSL launched an ad-hoc research programme on the consequences of the dry summer. As part of this, 1000 beech trees with early leaf fall were repeatedly observed in the regions of Baselland, Schaffhausen and Knonauer Amt/Bremgarten in the cantons of Zurich and Aargau.

The research team estimated the proportion of dead branches and leaf loss in the treetops of around 830 beech trees with early leaf fall in summer 2018 and 139 beech trees with normal leaf fall in autumn each year until 2021. It also counted pests and signs of disease and determined other site factors such as soil quality and long-term water deficits in the summer months. Now the researchers present the results in two scientific articles and in the forestry journal Wald und Holz.

Dry sites bad for beech trees

"It turned out that tree damage and a dry climate are linked," explains project coordinator Esther Frei. "Trees in locations with low precipitation and on soils that can store little water were more severely affected." Of the beech trees with early leaf fall, 10 percent died completely within three years in the Schaffhausen region, 7 percent in the Baselland region and 4 percent in the Knonaueramt/Bremgarten region. Another fifth of the 1,000 monitored beech trees were felled prematurely for safety reasons.  Of the 139 trees with normal autumn discolouration, only two trees died, which is about the natural death rate.

That the drought caused more damage to weak trees on dry sites is also confirmed by tree ring data evaluated by Stefan Klesse from the Dendrosciences research group. The wood cores had been taken in 2020 from beech trees with damaged crowns in the Ajoie in the canton of Jura. The tree rings showed that trees with severe crown damage had already grown less well in previous years. The drought therefore mainly affected weaker individuals. In the Ajoie, too, the connection with soil composition was found: trees on shallow or stony soils, which are poor at storing water, suffered much more damage than those growing on deep soils.

A sign of weakness

What is significant for foresters is that the trees with early leaf fall did not recover in the following years, especially on dry sites, which were also quite poor in rainfall. On the contrary, the proportion of dead branches increased with each year. "The dropping of the leaves there is therefore not to be interpreted as a protective mechanism of the tree to better survive the dry season, but as a sign of weakness," explains Frei. In contrast, on wetter soils like in the Knonauer Amt, many of the beech trees with early leaf fall recovered.

The early autumn colours were again visible in many places in the summer of 2022. On shallow soils on southern slopes in the north of Switzerland and in southern Ticino, the trees were most affected this year. "On such drought-prone sites, beech will have a hard time in the future," says Esther Frei. Sooner or later, foresters will have to do without beech there.