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Drought & beech: medium-term effects of the 2018 summer drought on beech north of the Alps

 

The extreme 2018 summer drought induced premature leaf senescence in beech as early as July in various parts of the Swiss Plateau and Jura. In this project, we investigate possible causes of this phenomenon, in particular soil properties and the damage due to previous drought periods. Furthermore, the tree vitality will be monitored during several years from 2018 on.

 

In Switzerland, as in many other regions of Europe, the summer of 2018 was marked by a heatwave accompanied by several weeks of drought. For example, some areas of the Swiss Plateau received only about two-thirds of the usual amount of rain. The severe drought led to premature leaf senescence, i.e., discoloration of beech foliage in various regions of Switzerland, followed by defoliation beginning in July and August (Fig. 1). This raised the question of whether the early defoliation is a defense strategy that provides beech trees with a physiological advantage, or whether it is a clear sign of weakness. In addition, we are interested in whether and how the beech trees recover from the drought in the following years.

As part of the WSL Drought 2018 initiative, the project "1000 beech trees" was launched to investigate whether the damage pattern of beech trees with early leaf senescence in 2018 differ in subsequent years from those trees showing a normal timing of leaf senescence, i.e., around October. In total, 964 beech trees were marked and drought damage was assessed in the regions of Baselland, Schaffhausen, Bremgarten (canton of Aargau) and western canton of Zurich The same trees were re-examined in the summers of 2019 and 2020. Several WSL research groups are collaborating on the project (Disturbance Ecology, Ecophysiology, Swiss Forest Protection, Forest Entomology, Soil Functions and Soil Protection).

The follow-up project "Drought and beech" was launched in spring 2020 and includes two modules: "1000 beech trees" and "Ajoie".

 

Module 1: 1000 beech trees

In this first module, the time series with 2018 and 2019 data on dead or recovering beech trees from the "1000 beech trees" project is extended by two more years. Furthermore, the project is complemented by tree-ring analysis in order to examine the medium-term effects of drought and the causes of different types of damage. The field surveys will show if and how well the trees can recover and how many will die.

The following research questions will be examined: How many beech trees with premature leaf senescence will survive in 2019 and 2020? What damage did the trees suffer in the first four years? Does this damage depend on the characteristics of the soil, or is it caused by pests or pathogens? How does premature leaf senescence affect the diversity of arthropod species living in the trees (insects and arachnids)?

 

Methods

In August 2018, 964 beech trees were marked and assessed for visible damage in the regions of Baselland, Schaffhausen, Bremgarten, and the western canton of Zurich (Figure 2).

 

The vitality (crown condition, total mortality) of all these trees was assessed again in 2019 and 2020.

In addition, side effects of biotic origin such as bark exudation, fungal infections and insect infestations on the stems (Figure 3) that might be late effects of the 2018 summer drought were recorded on all trees.

In July 2019 and 2020, leaf samples (Figure 4) were collected from a subsample trees for chemical analyzes and to insect herbivore damage. A soil study was also conducted on all sites.

From September to November 2020, cores were sampled from all beech trees in order to reconstruct the growth history of each tree using tree ring analysis and to check whether trees seriously affected by the drought of 2018 had not already been weakened by previous droughts.

 

Preliminary results (2018-2019)

While more than half of the beech trees that had lost their leaves prematurely during the summer of 2018 showed crown mortality of at least 10%, this damage was only observed on 20% of the control trees.

A second survey in July 2019 confirmed the increase in crown mortality since 2018 as well as the increase in damage caused by pathogens. For example, bark exudation was observed in May 2019 in one fifth of the beech trees showing premature leaf senescence in 2018, whereas this defense strategy against pathogens and pests was observed in only 6% of the control trees.

These preliminary results suggest that premature leaf senescence may be a sign of weakness.

 

Module 2: Ajoie

The extreme 2018 drought caused significant and widespread damage to the beech forests of the Ajoie region (canton of Jura; figure 6). In this region, damage levels vary on a relatively small scale, sometimes even within a stand.

Therefore, this second module focuses on the predisposing factors that may explain the observed small-scale differences in drought-induced damage of beech stands. As predisposing factors, the local climate, soil properties and genetic factors will be examined.

In addition, long-term growth patterns in tree-rings will be analyzed to investigate whether past stresses (e.g., droughts or pests) have already weakened the beech trees.

 

Methods

In collaboration with the Office for the Environment of the canton of Jura, nine beech stands with different proportions of severely damaged beech trees were selected in early 2020, i.e. three stands each with a proportion of > 75%, 50% and 10% of damaged beech trees (figure 7).

In collaboration with the Institute of Applied Plant Physiology (IAP) and Swiss Forest Protection (WSL), 21 beech trees were selected and marked within each stand in spring 2020, i.e. 189 trees in total.

 

 

During summer 2020, two cores were collected from each tree (Figure 8) in order to reconstruct their growth history. With this approach, we expect to see that the trees most severely affected by the 2018 drought are those that had previously been weakened by droughts, resulting in narrower rings. The vitality of the beech trees was also evaluated by applying the same parameters as in the project “1000 beech trees” (module 1).

In fall 2020, soil pits were dug in all nine stands. Soil depth and other parameters were measured. Based on these data, a soil water balance model will be calculated for each stand to show possible differences in water balance between stands.

In addition, samples of severely and slightly damaged beech leaves were collected from three of these stands during the summer of 2020. These leaf samples were genetically analyzed by IAP to provide evidence of a possible genetic predisposition to drought susceptibility.

 

Drought and beech: How do beech trees in Switzerland react to extreme drought​?

Follow the project team during field work in summer 2020.

 

FURTHER INFORMATION