Is multifunctionality greater in mixed than in pure forests? A metaanalysis of a latitudinal network of European forest triplets

The biological simplification of managed ecosystems often leads to reduced functionality and greater vulnerability to biotic and abiotic disturbances. By contrast, some mixed forests have proven to be more productive and resistant than monospecific forests. However, different compositions of mixed forests can have contrasting effects on multiple ecosystem services (provisioning, supporting, regulating and cultural), making it difficult to find the best compromise. In the meantime, climate change is likely to affect the ability of forests to provide ecosystem services in a sustainable manner. It is therefore important to better understand how mixed forests can improve multifunctionality under a wide range of climatic conditions. We set up a network of more than fifty forest triplets (monoculture of species A, monoculture of species B and mixture of A+B) from Spain to Scandinavia. To characterize forest functionality, we measured tree productivity, stem quality and resistance to defoliation, as well as tree-related microhabitats as indicators of the capacity to support biodiversity. We produced a quantitative index of multifunctionality and applied a meta-analytical approach to analyse the data synthetically. Overall, multifunctionality tended to be similar in pure and mixed stands. However, the effect of species mixing on multifunctionality varied greatly depending on the composition of the mixed stands. There were trade-offs between functions, in particular between wood production (in quantity and quality) and habitat provision. Finally, we found that climate was also an important driver of the multifunctionality of forests in Europe.