Thomas Wohlgemuth1, Thomas Campagnaro2, Pilar Castro-Díez3, Martin M. Gossner1, Elisabeth Pötzelsberger4, Joaquim Sande Silva 5 and the NNT-invasiveness working group
1Swiss Federal Research Institute WSL, CH-8903 Birmensdorf, Switzerland
2Università degli Studi di Padova, I-35020 Legnaro, Italy
3Universidad de Alcalá de Henares, Faculty of Sciences, E-28805 Alcalá de Henares, Spain
4University of Natural Resources and Life Sciences, Vienna, A-1190 Vienna
5Polytechnic of Coimbra, P-3045-601 Coimbra, Portugal
Most non-native tree (NNT) species in Europe were introduced more than 100–150 years ago, mostly to increase timber production, to plant them in parks and yards as ornamental trees, or to stabilize slopes or restore land, e.g. in post-mining landscapes. Today, proportions of alien tree species in European countries range between 0.2 and over 30% of the total national forest area (Fig. 1). Here we question whether higher proportions of these trees cause negative effects on native ecosystems and their functions and services, beyond considerations on the invasiveness status depending on life history traits (e.g. Richardson & Rejmánek 2004) and unequal regulation in different countries (Hasenauer et al. 2016). We therefore evaluated the effects of the 15 most abundant NNT species – nine broadleaved and six conifers – on forests in European countries in the frame of the EU-COST-Action “NNEXT” (Hasenauer et al. 2016). We found 550 published papers with information on multiple pairwise comparisons of ecosystem responses in stands with native (control) and NNT species (test). To quantitatively assess ecosystem responses, we used (i) soil parameters such as nutrient availability and pH, and (ii) measures of biodiversity such as richness of vascular plants, arthropods and fungi. Soil conditions were affected in both positive and negative ways, probably depending on leaf traits, in particular to litter decomposition. For biodiversity, a majority of the analysed papers showed negative effects of NNT species on species richness, partly due to altered micro-environmental conditions near ground (e.g. reduced light transmission) and a lack of time for adaptations to the new host. Here, broadleaved NNT species affected native ecosystems more often than coniferous NNT species. The results of our comprehensive review provide a unique basis for evidence-based comparisons of the ecological significance of NNT species in native forest ecosystems in Europe.
Hasenauer H., Gazda A., Konnert M., Lapin M., Mohren G. M. J., Spiecker H., Van Loo M. & Pötzelsberger E. (2016) Non-native tree species for European forests: experiences, risks and opportunities. COST Action FP1403 NNEXT Country Reports, Joint Volume. Ed. 2. University of natural Resources and Life Sciences Vienna, Vienna.
Richardson D. M. & Rejmánek M. (2004) Conifers as invasive aliens: a global survey and predictive framework. Diversity Distrib. 10: 321–331.