Fig. 1. Where beech trees will still be able to grow by the end of the century depends very much on the course of climate change and future site conditions. Photo: Andreas Rigling (ETHZ)
Around 20 years ago, a scientific debate began on the future of the European beech(Fagus sylvatica) in central Europe. It was triggered by an article published by Rennenberg et al. (2004) under the [translated] title “European beech (Fagus sylvatica L.) – a forest tree without future in the south of central Europe?”. The authors postulated that beech could lose vitality and competitiveness under the more extreme and variable conditions predicted for the future by climate models. Ammer et al. (2005) disagreed with these theses and saw no reason to question the prevailing views on the central role of beech in central European forest structures. They assumed that most assertions on the suitability of beech would continue to apply even in view of the predicted climate changes.
Fig. 2. Publications listed in the Web of Science (Core Collection) under the keywords Climate change AND (Fagus sylvatica OR European beech) as well as Climate change AND Forest* for the period from 1990 to 2022.
The debate clearly shows the uncertainties that existed at the time and the different perspectives on the effects of climate change on beech trees. A look at the development of research on climate change and the beech makes clear how limited the state of knowledge was in 2004 (fig. 2).
Today, climate change has progressed considerably, and many new scientific findings are available. It is time to re-evaluate the discussion carried out in the past against the current background, and to venture a new prognosis for the future of beech in central Europe.
Climate change and extreme events: new realities
The extremely dry years of 2018, 2019, 2020, and 2022 clearly showed the vulnerability of the beech (Fig. 3). In these years, there was premature leaf fall and crown damage in many places in summer. In 2018, in particular, severe damage occurred in Germany, Switzerland and Austria. Surprisingly, even beech trees on good sites where they usually grow vigorously were affected. Satellite data also show that in 2018, 2019, and especially in 2022, there was unprecedented premature browning of forests in Europe over extensive areas, going beyond the range of beech. Such effects were not observed in previous extreme summers, such as 2003, or only in certain very small areas.
Fig. 3. More and more often, European forests show signs of premature leaf shedding, extensive browning and crown damage as early as the summer months. Photos: Andreas Rigling (ETHZ)
Sites with a low capacity to store water were particularly affected, but even in deep soil with a good water supply, beech trees showed increased vulnerability. The combined effects of consecutive dry years and secondary pathogens such as the red pustular fungus Neonectria coccinea that causes beech bark disease have very probably led to a long-term weakening of the vitality of the beech.
Models and scenarios: the future of the beech in Switzerland
New modelling for Switzerland gives a rather dramatic vision of the future for the beech (fig. 4): Even in the moderate climate scenario (RCP4.5), beech will generally no longer find suitable growing conditions on the Swiss Plateau. The greatest losses are expected at altitudes of up to 1000 m a.s.l., where lower summer precipitation and more frequent extreme events severely compromise the suitability of the habitat. Small gains in area in montane zones can only partially offset the losses (around 30% of the losses). For “business as usual” scenarios (fig. 4d), it is predicted that beech will only be able to survive at altitudes above 1200 m a.s.l., and that the net loss of potential range for Switzerland will be around 8500 km².
Fig. 4. Changes in the potential range of the beech in Switzerland under the influence of climate change. The maps show the losses, gains, and the unchanging proportion of the potential range for beech under the emission scenarios RCP4.5 (limited climate protection) and RCP8.5 (“business as usual”) for the periods 2045-2074 (top) and 2070–2099 (bottom). The bar charts show the changes in the area suitable for beech, according to altitude.
If CO2 emissions are reduced in line with the goals of the Paris Agreement, the beech could continue to thrive in at least parts of its current range in Switzerland, even if not in all locations. However, it should be noted that global CO2 emissions between 2005 and 2020 largely corresponded to the assumptions made in the “business as usual” scenario. This raises doubts as to whether the climate goals can actually be achieved.
The uncertainties in the debate 20 years ago are also reflected in today’s models. The current climate projections confirm the temperature prognoses made by Rennenberg et al. (2004), whereas the precipitation forecasts remain more uncertain, although not implausible.
Physiological adaptation and sensitivity to drought
The drought tolerance of the beech is closely related to its biomass distribution and the hydraulic properties of its xylem. Under optimum conditions, the beech invests more resources in its above-ground biomass, while prolonged drought increases root formation (fig. 5).
Fig. 5. On sites with a good water (and nutrient) supply, trees invest more in above-ground biomass (leaves, trunk, branches) to optimise light capture. If water availability in the soil decreases, they invest more in root biomass to counteract the water shortage and, at the same time, reduce their above-ground biomass. If the water supply is very low, the root biomass can increase by up to 10% (graph according to Poorter et al., 2012). This adaptability and other anatomical adaptations ensure a constant hydraulic safety margin of approx. 2MPa (Choat et al., 2012).
Despite this adaptability, beech trees are still at risk in extreme drought conditions as soon as certain safety margins are exceeded. This is of particular importance for sites that are normally well supplied with water: Due to the trees’ pronounced crowns and relatively low root biomass, the water requirement can no longer be met during extreme periods of drought because of the large surface area available for transpiration. This effect could be intensified in the case of beech by another seasonal mechanism: very strong leaf development when there is a good water supply in spring may lead to a greater imbalance between the water requirement and water availability in dry summers. This increases the vulnerability of the beech to drought. Observations show that particularly vigorous specimens in deep soils are particularly endangered by these dynamics.
These points contradict the argumentation of Ammer et al. (2005) cited at the beginning of this article, that most assertions on the suitability of beech would continue to apply even under the predicted climate changes.
The rising frequency and intensity of extreme events, especially hot summers and extreme drought, mean that even sites with hitherto good to very good growing conditions may no longer be suitable for the beech. This underlines the need for careful planning of future stocks. The ability of beech trees with an intact water balance to recover does however give rise to the hope that at least some individual extreme years can still be compensated for in the future.
Silvicultural measures: strategies for the future
Climate-adapted silviculture with diverse, drought-adapted tree species and an increase in structural diversity through different age classes can slow down the climate change-related displacement of the beech. In view of the rapid climatic changes, however, even tree species that are still well adapted to the climatic conditions to be expected by the middle of the century on their sites are at risk of losing their suitability in the distant future.
Moderate thinning measures that take account of local site conditions can reduce water consumption. Excessive intervention can, however, be counter-productive.
Despite our growing knowledge of climate change and its effects on native tree species, we must continue to promote research into climate-adapted tree species and provenances. One example of this are the planting trials being carried out by the Swiss Federal Research Institute WSL: In collaboration with cantonal forestry services, forestry enterprises and tree nurseries, a network of 57 trial sites throughout Switzerland was set up between autumn 2020 and spring 2023, covering a wide range of tree species and provenances (fig. 6). Such projects make an important contribution to the development of sustainable forestry strategies.
Fig. 6. Trial area of the project “Trial planting of sustainable tree species” in Ronco sopra Ascona (TI). Photo: Gottardo Pestalozzi
Today, web-based applications offer valuable support in the selection of climate-adapted tree species. Platforms such as tree-app.ch in Switzerland or klimafitterwald.at in Austria are aimed explicitly at forestry practitioners. They link climate projections to the local management context, thus facilitating evidence-based planning for the future composition of tree species. These developments show the enormous progress that science has made since 2004.
The bottom line
Beech will remain an important tree species in central Europe in the future, too, but no longer in all the places where it is still found today. Large parts of its core area are affected. The intensity of the changes depends heavily on the actual course of climate change. It is particularly important to implement silvicultural measures to make the tree stocks more climate-stable and to introduce other suitable tree species in good time while taking account of the local site conditions.
Over the last 20 years, research has significantly improved our understanding of how the beech reacts to climate change. It transpires that the study from 2004, which was considered to be very pessimistic, is largely confirmed by today’s knowledge. The controversial discussion at that time was, however, also the starting point for intensive research into the climate sensitivity of beech forests. This research now allows for better risk assessment and a more targeted adaptation of silvicultural measures.
New web-based applications help us to make site-appropriate decisions. Nevertheless, further research efforts are needed, in particular, to investigate provenances adapted to the climate and their long-term development.
Literature
Detailed methodological information and literature references can be found in the scientific publication on which this article is based:
- Gessler A., Wilhelm M., Brun P., Zimmermann N., Rigling A. (2024) Zurück in die Zukunft – Ein neuer Blick auf die Perspektiven für die Buche nach 20 Jahren Forschung und weiter fortschreitendem Klimawandel. Allg. Forst- Jagdztg. 193(9-12), 206-224. (PDF)
Translation: Tessa Feller
