Experimental assessment of tree diversity-ecosystem functioning relationships in young forest plantations

Mixed forest plantations, delivering multiple ecosystem services, are generally recognized as more sustainable than monocultures. Mixing different tree species in forest plantations can increase the stand productivity, provide resistance to pests and diseases, and improve nutrient cycling processes. The more heterogeneous habitat created by mixed forests also provides diverse niches for associated biodiversity, such as arthropods. Although numerous studies looked into diversity effects in mature forest, mixing effects in the early stages of mixed forest plantations are still poorly understood. Tree species diversity experiments, such as the FORBIO experiment in Belgium, are a model platform to study the effects of tree species diversity and composition on forest ecosystem functioning throughout the different stages of forest development. The design of FORBIO allows unequivocally exploring the effects of tree species identity and diversity on three ecosystem functions. The main objective of this thesis was to study early effects of tree species diversity and composition on three related groups of functions: primary production (tree growth), resistance to crown damage (crown damage and crown arthropod community composition), and nutrient cycling (litter decomposition). We focused on 9 different tree species in two study sites of FORBIO, which were planted in 2009 and 2010. We measured the saplings’ increment between 2 years, assessed crown damage in 2 seasons, captured crown arthropods in 1 season, and studied litter decomposition rates by using litterbags for 60 weeks. In our results, tree species identity explained the highest amount of variance in each function. A sapling’s growth was additionally related with the characteristics of its local neighbourhood (notably phylogenetic diversity, relative size asymmetry, and ground vegetation cover). The crown damage of a sapling was influenced by the site characteristics and the timing of the assessment. The crown arthropod community differed between the landscapes, sites, and was related to sapling apparency and the phylogenetic diversity of the sapling’s local neighbourhood. The decomposition rates of mixed litter were affected more by the identity of the litter species within the mixture than by the diversity of the litter per se, but the variability in litter decomposition rates decreased as the litter diversity increased. We expect that the diversity effects will become more apparent as the trees start to interact and develop a closed canopy. Interesting patterns were already developing and some management guidelines can be formulated