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

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".

The vitality (crown condition, total mortality) of all these trees was assessed again in 2019, 2020 and 2021.

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.

The surveys showed that of the beech trees selected in 2018, 4.4% died over the course of four years (Figure 5 a). Of the beech trees with premature leaf fall, about 10% died in the region of Schaffhausen, about 7% in the region of Baselland, and about 4% in the region of Zurich. Beech trees that lost their foliage at the normal time in 2018 showed significantly lower mortality; 2% in Baselland, while all survived in the region of Zurich in 2021. In addition to dead trees, 21% of the observed beech trees were cut and harvested during the study - mostly for safety reasons. In the second year after the summer drought (2020), the proportion of dead branches in the crown reached a peak of 25% of the crown volume (Figure 5b). Thereafter, the dead branch percentage decreased slightly again as new branches grew back. This observation could be an indication of an - at least temporary - recovery.

The evaluation of the different influencing variables shows that beech trees at sites with a drier climate and those trees with premature leaf fall showed more pronounced crown damage in 2018. In addition, larger trees were more damaged on average. The proportion of beech trees that exhibited bark lesions with fresh slime flow reached a maximum of 22.1% affected stems in 2019. In 2021, bark beetle boreholes were found on 21.5% of beech trees. These two secondary pests also occurred more frequently on trees with early leaf discoloration or premature leaf fall and were often also associated with higher crown mortality.

The research shows that premature leaf discoloration during the drought summer of 2018 was a symptom of weakness in a majority of the beech trees studied. The extreme lack of water triggered a progressive damage process, especially on sites with dry climates and shallow soils, which in some cases led to the death of the tree. Often, the increased crown mortality of beech has been accompanied by insect and disease infestations. The degree to which this secondary infestation further weakens the trees could not be clarified in this study. The drought of the past years was extremely exceptional for the growth of beech trees in the lowlands of Switzerland: After the period 2014 - 2018 already belonged to the driest phases of the last 250 years, the extreme drought of 2018 was followed by further, although not quite as extreme, droughts in the two following years. That beech trees suffered from intensified drought stress as a result is shown by other studies that observed a decline in tree ring widths in response to drought.

The intensity of the climatic drought, i.e. the perennial precipitation deficit in the summer half-year, determined the degree of damage at the regional level: in the north of Switzerland with less precipitation, the damage was greater than in the region of Zurich. The precipitation deficit was superimposed by local and small-scale differences in soil, tree and stand properties. This resulted in small-scale damage patterns with severely damaged beech trees on shallow soils in the neighborhood of hardly damaged trees on deeper soils. The regrowth of crown tissue, particularly the formation of glue trees, in the years following the drought indicates a partial recovery of beech. However, for now, it is not clear whether this is sustainable or temporary. Ultimately, beech resilience may increase over the long term through adaptation.

It remains unknown whether the pre-damaged trees will be even more susceptible to damage in a renewed drought phase, as has been put forward for discussion in an international study. Since an increased occurrence of dry periods and heat waves is to be expected in the course of climate change, a long-term decline or failure of beech trees that cannot recover from repeated droughts must be expected on the dry sites in northern Switzerland.

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.

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. Crown mortality of the beech trees was estimated by the proportion of dead branches and the crown transparency.

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.