Fertilization influences overyielding through dominance of species with high specific leaf area in young tree mixtures

In the context of anthropogenic eutrophication of the biosphere, understanding the impact of nutrient addition on plant diversity–productivity relationships remains a major challenge. In particular, the indirect effect of nutrient addition on productivity in tree mixtures via changes in the functional structure of tree communities has never been experimentally quantified and may play an important role in mediating diversity–productivity relationships. Here, we present results from young experimental tree communities 4 years after planting, including monocultures and two- and four-species mixtures of six European species (Acer platanoides, Betula pendula, Larix decidua, Picea abies, Pinus sylvestris and Quercus robur) with or without 4 years of mineral fertilizer addition. Using a response-effect trait framework that links plant functional traits with ecosystem processes, we specifically examined how functional diversity (FD) and community weighted means (CWM) of tree height (H) and specific leaf area (SLA) were influenced by fertilization and to what extent they mediated the fertilization effect on above-ground productivity in woody tissues. The overall fertilization effect on the degree of community-level overyielding was marginally positive, yet the influence of fertilization varied significantly depending on species composition. Fertilization affected community functional trait structure primarily by increasing CWM of SLA through dominance of species with high SLA, rather than through intraspecific trait variability. Interestingly, the fertilization-induced change in CWM of SLA reduced community-level overyielding. Irrespective of fertilization, CWM of H and SLA influenced community-level overyielding, while FD had no significant effects. Our results indicate that changes in nutrient availability can influence the magnitude of overyielding indirectly through dominance of species with high SLA in young tree communities. This result improves our understanding of the context dependency of diversity–productivity relationships, which strongly depends on nutrient availability influencing the competitive relationships among tree species. The occurrence of such effects early after tree planting could potentially produce long-term consequences for tree community composition, and even more so in naturally regenerating forests with less control on tree species selection. Read the free Plain Language Summary for this article on the Journal blog.