What are NNT and what is the situation regarding NNT in Alpine space?
Non-native trees (NNT) also known as “non-indigenous”, “alien”, “introduced”, “allochthonous” or “exotic” trees, refer to tree species, breeds or hybrids outside of area of natural origin, whose presence there is as a result of human activity, due to intentional or accidental introduction. Depending on the time of introduction, they can be separated into Archaeophytes and Neophytes. Archaeophytes include NNT introduced prior to the year 1492, while neophytes include NNT introduced after 1492, when Christopher Columbus arrived in the New World and the Columbian Exchange began. Not all NNT are automatically invasive. (Potentially) Invasive trees refer to NNT whose introduction, establishment and/or spread pose potential or actual risks to the native biodiversity, ecosystem functioning, or socio-economy including human health. So far about 5% of reported NNT have been considered (potentially) invasive in one or several regions of the Alpine Space.
In Work package 1 of our project, we performed a study aimed to perform an inventory of NNT growing in Alpine Space forests and cities, analysing their diversity, distribution, and geographic origin. You can find full report here. The results have shown that at least 526 NNT are present in the Alpine Space. The number of NNT growing in urban areas (352) is much higher compared to woodlands (13). This low number, however, is due to the fact that species in urban areas also include those in parks, cemeteries or even urban forests, where typical forest tree species are often found. The most common NNT in cities of Alpine space are Aesculus hippocastanum (horse chestnut), Robinia pseudoacacia (black locust), and Gleditsia triacanthos (honey locust). Switzerland and Austria had the highest proportion of NNT whilst Slovenia had the lowest.
Most of the reported NNT (90%) have their natural distribution range outside of the European geographical area. The largest number of these species were introduced from Asia (248) followed by North America (180), but the ones that are considered potentially invasive, are mostly originating from North America. The largest number of NNT, which are native to parts of Europe, were introduced from southern or south-east Europe, such as from the Balkans, southern Italy, or from the entire Mediterranean region. From our data we also found out that 35% of the NNT are being cultivated for landscape and gardening purposes, 18% are used as ornamental tree, for example, in parks or arboreta, and 15% are being valued for their potential to sequester carbon. Other commonly mentioned benefits include fuel wood, shading, soil erosion control and timber production.