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For non-native tree species with an origin outside of Europe a detailed compilation of enemy species including the severity of their attack is lacking up to now. We collected information on native and non-native species attacking non-native trees, i.e. type, extent and time of first observation of damage for 23 important non-native trees in 27 European countries. Our database includes about 2300 synthesised attack records (synthesised per biotic threat, tree and country) from over 800 species. Insects (49%) and fungi (45%) are the main observed biotic threats, but also arachnids, bacteria including phytoplasmas, mammals, nematodes, plants and viruses have been recorded. This information will be valuable to identify patterns and drivers of attacks, and trees with a lower current health risk to be considered for planting. In addition, our database will provide a baseline to which future impacts on non-native tree species could be compared with and thus will allow to analyse temporal trends of impacts.

Understanding the processes that underlie drought-related tree vitality loss is essential for anticipating future forest dynamics, and for developing management plans aiming at increasing the resilience of forests to climate change. Forest vitality has been continuously monitored in Europe since the acid rain alert in the 1980s, and the intensive monitoring plots of ICP Forests offer the opportunity to investigate the effects of air pollution and climate change on forest condition. By making use of over 100 long-term monitoring plots, where crown defoliation has been assessed extensively since 1990, we discovered a progressive shift from a negative to a positive effect of species richness on forest health. The observed tipping point in the balance of net interactions, from competition to facilitation, has never been reported from real ecosystems outside experimental conditions; and the strong temporal consistency of our observations with increasing drought stress emphasizes its climate change relevance. Furthermore, we show that higher species diversity has reduced the severity of defoliation in the long term. Our results confirm the greater resilience of diverse forests to future climate change-induced stress. More generally, they add to an accumulating body of evidence on the large potential of tree species mixtures to face manifold disturbances in a changing world.

Le réseau européen de surveillance de la santé des forêts (ICP-forests) a été mis en place dans les années ’80 pour mesurer l’impact des pluies acides et comprendre les processus impliqués. En Wallonie, aucun dépérissement massif n’a été constaté sur les placettes de ce réseau et les retombées acides ont considérablement diminué depuis lors. Toutefois, le pH des sols est resté très bas et l’état sanitaire de plusieurs essences s’est considérablement dégradé ces dix dernières années. Ces détériorations peuvent être reliées principalement à des épisodes climatiques extrêmes (chaleur, sécheresse ou excès d’eau en été) ainsi qu’à des attaques d’insectes ou d’organismes pathogènes. La pauvreté des sols en nutriments, accentuée par les apports passés d’acides et la pollution actuelle en ammonium, est susceptible de fragiliser les arbres face à ces stress à répétition. La poursuite de cette surveillance est indispensable pour identifier les nouvelles menaces, bien cerner le fonctionnement des systèmes et proposer des orientations de gestion pertinentes.