The planting of non-native tree species in our forests is controversial. In some cases, however, such “foreigners” can even become refuges for native animals – for example when native tree species are decimated by drought or disease. This is proven by the example of the North American red ash in the lowland forests of the Elbe. After the native ash trees there have almost all died, they now offer tree-living beetles a new home, as biologists have found out.
As a result of the drought periods in recent years, there has been a pronounced dieback of trees in Germany, leaving clearly visible gaps in forests and parks. All climate projections indicate that such events will occur more frequently in the future. In response to the tree dieback, forest science is planning to convert commercial forests into more robust mixed forests. This also includes the increased planting of heat and drought-resistant tree species from other countries. These plants, introduced by human activity at non-native sites, are also known as neophytes.
Study of beetle biodiversity in different ash species
But this strategy involves risks and is controversial: If plants or animals are introduced into regions where they are not native, this can disrupt the biodiversity and function of the ecosystems there. “The type and extent of such disturbances cannot be predicted with the current level of knowledge,” explain Andreas Floren from the University of Würzburg and his colleagues. The scientists therefore investigated how beetles react to planting with non-native tree species that can withstand periods of drought. To do this, the scientists took a closer look at the biodiversity of insects in two special tree species in the alluvial forests on the Elbe. In addition to the native ash Fraxinus excelsior, the red ash Fraxinus pennsylvanica, which comes from North America, also grows there. The exotic plants were planted there at the beginning of the 20th century because of their high flood tolerance.
The study of beetle biodiversity was carried out using insecticide fogging, also known as “fogging”. With this method, individual tree crowns are misted with a substance that is poisonous to insects. As a result, beetles and other insects fall into trapping tarpaulins on the ground and can then be systematically identified. The insecticide decomposes within a few hours without leaving any residue, so the disturbance to the ecosystem remains low and is limited to individual trees.
Red ash offers a chance for beetle refugees
The researchers found that non-native trees do indeed offer opportunities in times of climate change – they can become a refuge for insects if their traditional host trees die off in large numbers. This is shown by comparisons of the diversity before the extreme droughts of 2018/19 with the situation afterwards. “In 2016 and 2017 we analyzed the biodiversity of the beetles in the crowns of the two tree species,” says Floren. It turned out that the greatest beetle diversity of all the tree species examined was found on the native ash trees, which at that time were still growing in large numbers in the forests. The beetle populations differed significantly from the less species-rich beetle communities of the red ash.
But in 2020, when the team repeated the analyses, the situation was completely different: By then, more than 80 percent of the native ash trees had died, which comes close to a “knock-out” of this tree species. The heat and drought of the previous two years took their toll. The ash trees from North America, on the other hand, remained unharmed, which the beetles promptly took advantage of: many species had now switched to the red ash tree. “This indicates a functional restructuring of the ecosystem,” explains Floren. In particular, fungus-eating beetles left their native trees and now increasingly lived on the North American ash trees. However, wood-living and wood-eating beetles now colonized the original ash trees in large numbers.
American red ash could replace the native ash
The migration of the many beetle species to the red ash ensured the survival of the small creepy-crawlies, but worsened the situation of the native ash even further: several red-listed species that were originally classified as endangered now appeared so frequently that they developed into pests . These beetles burrow into the bark of native ash trees and destroy their bark layer, causing the trees to die.
Nevertheless, Floren and his colleagues draw a positive balance: “All in all, our data suggest that Fraxinus pennsylvanica could become a kind of lifeline for the native fauna if the native ash tree disappears. In this case, the red ash offers the second best habitat”. Further investigations into the diversity and function of the treetop fauna, including neophytes, are now necessary in order to be prepared for possible consequences of climate change. Because what happened in the lowland forests could also be the fate of other forests.
Source: University of Würzburg; Specialist article: Sustainability, doi: 10.3390/su14031914