Non-native tree species (NNT) whose natural range is outside Europe have long been part of the cultural development of the Alpine Space forest landscape. To date, NNT have been promoted in forests, for example, to increase timber production and diversify wood products, or to replace some native tree species in regions where they face increased stress as a result of rising temperatures and more frequent droughts.
The large-scale cultivation of some NNT that have the potential to become invasive, however, is a growing ecological concern worldwide. Species are typically termed “invasive” when they spread from the sites of their initial introduction and rapidly colonise new areas, with possible negative consequences for the environment, economy, or human health. In the Alpine Space, some NNT are considered invasive because they have spread from managed forest sites into neighbouring areas, where they can threaten native species communities. NNT which spread and establish in protected areas, are of particular concern for conservationists and forest managers because they may lead to the competitive exclusion (displacement) of native species populations. Conservationists are also concerned that large-scale cultivation of NNT pose a threat to native forest-dwelling plant and animal species. This concern is based on the assumption that these “new” tree species are not closely related to any native tree species and have not co-evolved with the native flora and fauna. Thus, native insects for example, do not or hardly feed on NNT, and in turn diversity of insect-dependent species, such as birds, or small mammals may also decline.
Negative environmental impacts
In conservation biology, a distinction is made between four different types of possible negative environmental impacts associated with the use of NNT in forests:
NNT can have a negative impact on biodiversity if they have a strong tendency to establish dominant stands due to their competitive behaviour, causing native animal and plant species to lose their native habitats. Although this effect cannot be directly described as competition for resources dominant stands of NNT can also reduce local biodiversity by providing less suitable habitats for native forest dwelling species, such as lichen, insects, or birds.
For example, ash maple (Acer negundo L.) was introduced to Central Europe from North America at the end of the 17th century. Although this tree species has never attained forestry importance due to its low wood quality, it is widespread today in many riparian forest areas, especially in southern and eastern Europe, where it regenerates naturally and can establish dominant stands. It has a high potential for spreading, especially in disturbed floodplain forest habitats with a high light availability. Due to its high competitive strength in the young growth stage it poses a risk to native floodplain forest species.
An example of an indirect consequence of dominant NNT stands on forest biodiversity is the interaction of the north American Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) with native forest ants. Due to the unsuitability of the Douglas fir wooly adelgids as a honeydew resource for forest ants and the lack of other honeydew-producing aphid species on Douglas fir, dominant and large-scale populations of this NNT may have a negative local impact on forest ant populations.
However, there are also many examples that show that NNT are colonized by diverse native species (Gossner 2016).
2. Alteration of the function of the ecosystems
NNT can negatively affect ecosystem functioning and species communities by changing nutrient cycling, for example, by fixing atmospheric nitrogen (eutrophication), by poorly degradable litter (soil acidification), by plant-inhibiting litter-ingredients or secretions (allelopathy), which can severely impair living conditions for native species.
For example, black locust (Robinia pseudoacacia L.), native to North America, can alter the soil on which it grows by accumulating nitrogen in the soil, as do all leguminous plants. In addition, it spreads rapidly via root suckers. These two characteristics make black locust a threat to specific protected areas. In particular, nutrient-poor sites such as dry grasslands or inland dunes are rare and valuable ecosystems. Their preservation helps to maintain species that are specialized in this habitat. Yet, the more nitrogen is accumulated in the soil, the more nitrophylic species migrate, displacing typical native species.
3. Genetic pollution of native tree species through hybridization
NNT with the potential to hybridize can dilute the native genotype of native tree species of the same genus such that there are no longer any “pure” native species and there is a risk that rare endemic tree species will become extinct.
For example, in Italy, the Siberian elm (Ulmus pumila L.), was planted extensively to replace the native elm, (Ulmus minor L.) as many trees were decimated by the Dutch elm disease during the 20th century. Because the two tree species are closely related, Siberian elm has hybridized with native elm, which threatens the genetic diversity of the native elms.
4. Introduction or transmission of pests and pathogens to native species
Another ecological risk associated with the introduction of NNT is the possibility of unintentional introduction of new species or organisms (e.g. fungi, insects, viruses) with unknown ecological and economic consequences. Such pests can harm the NNT itself, but also endanger native species, for example, compete with native insects for food-resources, and/or they can also switch to native tree species and damage them.
For example, some cone wasp species (Megastigmus spp.) which have been co-introduced with non-native firs (Abies spp.) have displaced native cone wasps on the native silver fir (Abies alba Mill.). The western conifer seed bug (Leptoglossus occidentalis Heidemann), probably first introduced to Italy in 1999 with seeds or building materials from North America, has spread throughout Europe in recent years. It damages not only the seeds of the introduced Douglas-fir, but also those of native pines (Pinus spp.), and can thus limit the natural regeneration of native species and reduce their fitness by spreading pathogens.
Other possible negative effects
In addition to the impacts on biodiversity, NNT may negatively affect other ecosystem services, for example when they reduce forestry productivity or increase fire or erosion risks, or they may have negative impacts on human health, e.g., when causing allergies in humans. In addition, invasive tree species can incur high costs if their populations must be removed at great expense, for example in protected areas, or if habitats must be restored after removal. However, not every NNT is problematic. In fact, only a small proportion of NNT are able to establish themselves and have an effect that is considered harmful.