In 2018, the leaves of many beech trees in some regions of Switzerland changed colour in July instead of October (Figure 1). This could only have been a response to the very hot and dry weather. At the time, it was not known exactly how the increasingly dry conditions were affecting beeches and why the tree’s foliage did not change colour at all in some forests yet changed very markedly in others. To answer these questions, researchers from WSL combined pre-existing measurements from beech forests with measurements taken at the maximum of the 2018 drought at two additional locations with strongly discoloured beeches (Figure 2).
In all, nine sites across Switzerland were investigated, located at altitudes of 550 to 850 metres above sea level, where beech is the dominant tree species in many forests. The soils at the nine study sites differ in terms of the depth of the root zone and the amount of water they can store and make available to trees.
Beeches' response to lack of water in soil
The water availability was measured at several depths in each soil, down to 2 metres where possible (Figure 3). The researchers found that drought stress in beeches is not solely determined by the weather and current precipitation. The water retention present in the soil and whether the roots are deep enough to tap into the soil’s water-bearing layer are also important factors.
Beeches growing in soils that have limited water storage capacity and are thus very prone to desiccation showed increasing signs of stress in the summer of 2018. The first manifestation of this water stress is that the tree restricts evaporation through the leaves, the trunk stops growing and the leaves change colour or even fall prematurely. Despite these water-saving measures, if the drought continues the water transport system in the tree's trunk and branches becomes empty, and eventually the most exposed parts of the crown start to desiccate (Figure 4).
"Our study at very dry beech locations broke new ground," says WSL forest ecologist Lorenz Walthert, lead author of the article published in the journal Science of The Total Environment: "Until now, symptoms of extreme stress in mature beeches have only rarely been observed as they have only occurred widely in exceptionally dry years such as 2003 and 2018. It is also important that water availability is being recorded in the entire root zone where possible, and being measured using suitable sensors." By adopting this approach, the researchers were able for the first time to quantify the relationship between water availability and stress over a very wide range of conditions, extending to extreme drought.
Beeches at risk in future climate
The results suggest that over the coming decades, as the climate gets warmer, the beech will gradually be replaced by more drought-tolerant tree species at drier sites, but should continue to thrive in soils with very good water storage capacity. "Future beech distribution across Switzerland and Europe can be predicted using models. Thanks to our findings, these models will be able to make more accurate predictions in the future," explains Walthert. However, foresters can make their own assessments of the future risk to beeches, without using model predictions. So-called site maps for many forests are available from the forest services. These can be used to roughly estimate the soil's water storage capacity and the drought risk. Although these maps are gradually becoming outdated as a result of climate change, they can still be helpful.
For foresters, information about future beech distribution is useful for planning and forest tending. Depending on the forest site, it allows them to specifically promote tree species that are more drought tolerant than beech, with the aim of encouraging climate-adapted forests that will still be able to perform their diverse functions in the future.