The past few years have made it abundantly clear how much heat and drought pose a threat to local forests. The long-lasting drought from 2018 to 2021 caused the death of trees in an area of 5,000 square kilometers, estimates the German Aerospace Center (DLR), which corresponds to twice the area of Saarland. Forest experts therefore agree that the forest must be converted to cope with climate change.
However, the scope for this could be smaller than previously assumed, Researchers now warn in the specialist magazine Nature Ecology & Evolution. Scientists led by Johannes Wessely from the University of Vienna and Rupert Seidl from the Technical University of Munich have examined the future distribution areas of 69 tree species native to Europe. Accordingly, climate change will reduce the number of suitable tree species for the 21st century by at least a third compared to today – even if the climate goals are largely met. If climate protection is weak, the number of suitable tree species per square kilometer could even be reduced by half.
Global warming is comparable to a bottleneck, the authors write: Some tree species that can still cope under current conditions will fail by the end of the century – in southern Germany, including the Norway maple. As warming progresses, it will become possible to plant other tree species, such as the chestnut. However, it is sometimes still too cold in winter for these plants, which are used to higher temperatures. In order to make the forest fit for climate change as quickly as possible, trees are needed that can thrive both today and in the future, which narrows the choice.
Pests such as the bark beetle or newly introduced species have not yet been taken into account
The researchers have therefore identified the species that will find good conditions throughout the 21st century. In Northern Europe, only four tree species are possible in a medium climate scenario, such as spruce. However, this tree, which is important for forestry, would disappear from many other areas, for example from large parts of the foothills of the Alps. Overall, suitable areas for coniferous forests are likely to shrink, while areas for deciduous forests expand. Oak, for example, seems to be reasonably robust. In the Alps and low mountain ranges, the slightly lower temperatures could cushion the effects of global warming somewhat.
However, the future suitability of the tree species in the study is only determined by the climatic conditions, points out Henrik Hartmann from the Julius Kühn Institute for Cultivated Plants. Recent years have shown that climatic stress can also promote pests and diseases. “The bark beetle is perhaps the most obvious example, but sooty bark disease in maple trees, locust beetles in oak trees and complex diseases in beech trees also prove this,” says Hartmann, who was not involved in the study. In addition, global warming is encouraging the introduction of new species such as ash shoot dieback or the oak web bug, “with often devastating consequences for existing forest communities.” Hartmann therefore classifies the researchers’ results as “rather conservative”.
“More species in the population increase resistance to climate change,” says Christoph Leuschner from the Department of Plant Ecology and Ecosystem Research at the University of Göttingen. However, foresters would usually understand mixed forests as two-species mixtures, such as Douglas fir as a conifer mixed with beech. “Species-rich mixed forests, such as those that occur naturally in Central Germany – with five to eight species – are or are more likely to be present, are hardly planned anywhere,” criticizes Leuschner. He calls for species that are not economically attractive today, such as the hornbeam or the winter lime tree, to be included. According to Leuschner, these stress-tolerant deciduous tree species are missing today, “because our wood industry is completely focused on softwood. This is where a real forest transition should begin and the material use of wood should be switched to hardwood.”
A mix of forests is necessary, but at the same time one should not rely on this alone, warn the researchers from Vienna and Munich. In order to preserve European forests, it is essential to also reduce greenhouse gas emissions.
The study does not take into account the possibility of importing tree species that are not yet found in Europe. These could potentially help replace the endangered native trees. But not all experts are keen on this option. “Trees from all over the world are currently being planted everywhere in Germany because of their supposedly favorable climatic suitability, while at the same time the soil, micro- and local climate as well as the landscape water balance are being damaged,” says Pierre Ibisch, professor of nature conservation at the Eberswalde University for Sustainable Development. Such management errors could lead to not only ecological but also economic risks in the short term.
With material from the Science Media Center