Are mixed forests more resilient to environmental change? Analysis of recent growth trends along a gradient of tree species diversity.

Forests provide important ecosystem services to humans. The EU renewable energy target can only be reached by massive use of forest biomass. This implies productive forests, while recent studies show climate change related decreasing growth trends. This project aims at evaluating the role of biodiversity to support the production function of forests. The hypothesis that mixed forests are more resilient to environmental change is tested by setting up the Tree Diversity Observatory, a unique platform of plots along a tree diversity gradient in Flanders and Wallonia. These plots consist of target trees of oak and beech with their neighbour trees of different species. In the Observatory tree ring cores of target trees from different age class are taken and their  eighbourhoods described. Tree rings widths are measured and provide a database of wood basal area increment. Tree rings further provide samples to analyse stable isotopes, used to build a  ata series of drought stress and nitrogen deposition. In order to evaluate overall recent growth trends and to separate effects of species diversity and functional diversity from other covariates,  Fixed models are fitted to the tree ring series. To test for higher resilience to environmental changes in forests with higher tree diversity, mixed models for growth response to these stress factors are built for separate pointer years, comparing extreme and consequent years with normal years. The detected diversity effects are then further interpreted in terms of their ecological drivers, and recommendations for forest management are formulated.