by Swiss Federal Institute for Forest, Snow and Landscape Research WSL
Non-native forest tree species can reduce native species diversity if they are planted in uniform stands. In contrast, the effects of introduced species on soil properties are small. This was found by an international review study with the participation of the Swiss Federal Institute for Forest, Snow and Landscape Research WSL.
Curse or blessing? Opinions are divided on non-native tree species. In addition to native species, many foresters also plant non-native species that can withstand the increasing summer drought. In various parts of Europe, the latter are already important suppliers of timber. However, conservationists fear ecological damage, for example if native species are displaced or tree pathogens and insect pests are introduced.
Now a team of European researchers, led by Thomas Wohlgemuth of WSL, has looked at the state of knowledge on the ecological consequences of alien tree species in Europe. They analyzed the results of 103 studies on seven such species. All of these studies had investigated how stands dominated by non-native tree species affected biodiversity or soil condition under the trees compared to stands of native tree species. The organisms studied included plants, mosses, microorganisms and insects from the forest floor to the treetops.
Of the seven alien species studied, only the Douglas fir is currently planted in larger numbers in the Swiss forests. While foresters used to value its fast, straight growth and its versatile wood, today they appreciate its higher drought tolerance compared to spruce. Other species are problematic because they can spread uncontrollably. The North American Robinia, for example, is invasive and can displace native species. It was already introduced in Europe 400 years ago and used in Switzerland, among other things, to stabilize soils.