Reading 1.1: What are non-native trees?

The black locust (Robinia pseudoacacia) is a fast-growing tree native to eastern North America. Alongside the northern white cedar (Thuja occidentalis), it was one of the first alien tree species introduced to Europe more than 400 years ago. For this reason, we classify the black locust as a non-native tree species (NNT) in Europe. More precisely, we classify it as an invasive NNT because of its characteristics. The northern white cedar, on the other hand, has not developed invasive characteristics and is therefore simply classified as NNT. The presence of various such (invasive) NNT in the European area is a reality since their introduction, and the Alpine region is no exception.

Robinia pseudoacacia L. – The most frequent and abundant non-native tree
species in Europe and a controversial species with positive economic and
negative environmental impacts.

On average, NNT are estimated to cover around 4% of European forests. Most NNT were introduced to the Alpine region intentionally. They were promoted in forested and urban areas for their various positive characteristics: some NNT have higher yield, higher timber value, high aesthetic value, stronger root systems, etc. Only few NNT came to the Alpine region unintentionally or spontaneously. Many species entered the region at some point, but most did not naturalize and thus did not become established. In the course of the ALPTREES project, we have been able to identify more than 530 NNT in the forests and urban areas of the Alpine space. The majority of these NNT can only be found in urban areas and provide ecosystem services not related to forests. Climatic conditions have changed since the first attempts to introduce and promote NNT in our forests and cities. In addition, climate change is occurring faster in the Alpine space than in other areas, making the region more vulnerable.

Climate change has also altered the characteristics of certain established NNT, with some of them becoming more competitive and invasive under the new conditions – in particular when native tree species can no longer thrive optimally in their forest habitats due to the changing climate. Research indicates that certain NNT may be better adapted to changing climatic conditions, and we, therefore, need to consider their presence on specific sites. Future climate conditions and increasing CO2 concentrations are expected to affect site suitability, productivity, species composition, and biodiversity directly as well as indirectly. Whether we see NNT as a threat or a potential, they need to be systematically managed – especially those that are already naturalized and present in our area. Although NNT certainly evoke a range of different associations and emotions in people, careful integration of tested and suitable NNT into future forest management strategies may offer great potential for climate change adaptation and mitigation.


Thuja occidentalis L. – The northern white cedar occurs in all countries associated with the Alpine region.

In critical and vulnerable ecosystems such as the Alpine space, however, potential risks and benefits need to be weighed particularly thoroughly before management decisions are made. In addition, the definition of NNT includes hundreds of different species with diverse traits, shapes, ecological niches, levels of invasiveness, etc. – often even within individual taxa. For this reason, we must be cautious when speaking about the entire category of NNT in general terms and should consider their employment on a case-by-case, site-by-site, and goal-by-goal basis. Whether native or non-native, management measures can increase the ecological, economic, and social value of tree species. The foundation for managing NNT should be a profound awareness of their ecological and physiological characteristics. Most importantly, their management should be based on two fundamental aspects: the experience we already have with NNT, and detailed knowledge of the benefits and risks associated with them. The expected benefits and potential risks of NNT for European geographic regions have polarized the opinions of experts and citizens. The activities within the ALPTREES project – which is part of the Alpine Space program, a European transnational cooperation program for the Alpine region – are designed to bring us closer to achieving the essential goal of providing a transnational strategy for a decision-making support system on responsible use and management of non-native tree species in the Alpine region.

Terms used to classify and define occurring tree species in the Alpine Space.

Further resources:

  • ALPTREES PODCAST – Non Native Tree Species and the Future of the Alpine Space
    The role of non-native trees species in the Alpine Space The ALPTREES podcast series answer the questions of citizens of all ages and educational levels and give insight into our scientific work to build a transnational guideline for the use and management of non-native tree species in the Alpine Space. From the basics “what is a non-native tree species” to specific topics like “women in forestry”, each podcast has its thematic focus.