The Risk Assessment & Research Bureau (BuRO) of the NVWA has conducted a risk assessment for eight exotic tree species that may be planted as part of climate-smart forest management. These species include: Tulip tree (Liriodendron tulipifera), Turkish hazel (Corylus colurna), Downy oak (Quercus pubescens), European hornbeam (Torminalis glaberrima), Italian alder (Alnus cordata), Oriental beech (Fagus orientalis), Maritime pine (Pinus pinaster), and Black walnut (Juglans nigra). Due to its invasive character, it is advised that forest and land managers do not plant Italian alder in the Netherlands, and that Tulip tree, Downy oak, Oriental beech, and Maritime pine should only be planted in limited numbers.
This risk assessment was carried out by the Probos Foundation in collaboration with Wageningen Environmental Research, FLORON, and NVWA.
Reason
Due to climate change, forest managers are looking for tree species that are better adapted to the future climate. Some of the tree species that may be planted in Dutch forests are exotic. For these species, there is a risk that they may have harmful effects on native biodiversity or other societal values. Reliable information about the (potential) invasiveness was not available. Therefore, BuRO conducted a risk assessment on the eight mentioned tree species.
Results
The risk assessment shows that there is a high chance that Italian alder, Maritime pine, and Oriental beech will behave invasively. Black walnut, Turkish hazel, Tulip tree, European hornbeam, and Downy oak have a moderate risk score.
Italian alder grows quickly and can easily colonize open areas. This tree can hybridize with native black alder. However, natural hybridization of the two alder species is rare. The hybrids seem limited in their ability to backcross. The tree is a nitrogen fixer that increases nitrogen in the soil. On nutrient-poor soils, this can lead to the disappearance of plants that require nitrogen-poor conditions. The pollen of Italian alder, like that of other alder species, is allergenic.
Maritime pine can easily colonize open plains and can convert open dune areas into forest. Therefore, it is advised not to plant this species in or near open dune areas or other areas where openness is desired.
Oriental beech can easily hybridize with native common beech. The hybrids are fertile and can backcross with one of the parent species. Therefore, it is advised not to plant this species near common beech, particularly for indigenous populations of common beech, as these are a source of unique genetic material adapted to local conditions and often contain traits not found elsewhere.
Even tree species with a moderate risk score can have negative effects. Downy oak can hybridize with summer oak and winter oak, and the hybrids from these crosses are fertile. The Tulip tree can quickly colonize open and/or disturbed areas. Due to its rapid growth and high seed production, competition with other trees may arise. Black walnut, European hornbeam, and Turkish hazel are not very competitive.
The risk assessment of exotic climate-resilient tree species can be found on the NVWA website.