Functional traits and propagule pressure explain changes in the distribution and demography of non-native trees in Spain

Questions Non-native tree species (NNT) may bring about economic benefits, but also threats to ecosystems, mostly if they show expansive trends. Location A set of 12,000 permanent plots of the second (1986-1996), third (1997-2007) and fourth (2008-2017) Spanish Forest Inventory. Methods We quantified changes over time (1986-2017) of the NNT present in forests of peninsular Spain and we assessed how NNT's traits, propagule pressure and human perception of NNT explain changes in distribution and demography of NNT. We quantified changes in four demographic parameters of every NNT: changes in the occupancy of species (number of plots where the species are present), annual changes in tree density and basal area, and tree growth. To explain the observed species trends, we selected functional traits related to the resource acquisition strategy, and key human drivers. Results Most of the NNT expanded their occupancy in the study area and increased their density, basal area, and tree growth through time. Increases in tree density and growth were greater in NNT with greater tolerance for low water potentials, with low specific leaf area, and with high propagule pressure. Increases in basal area were greater with high height of the NNT. Conclusions The overall increase in occupancy suggests that there is room for expansion of NNT in Spain. This knowledge will help to predict the dynamics of NNT already present in Spain and identify risks for forest biodiversity.

Automated summary

Research has found that non-native tree species are expanding in forests in Spain and their numbers, area, and growth are increasing. This expansion is influenced by the traits of the species, propagule pressure, and human perception. The results of this study highlight the importance of predicting and managing the existing non-native tree species and the risks they pose to forest biodiversity.