The importance of functional diversity in forest productivity is driven by environmental stress in Italy

The role of forest biodiversity in supporting ecosystem functioning is remarked to be context-dependent, and changes in environmental conditions could alter the shape of biodiversity-ecosystem functioning (BEF) relationships. However few studies have explored the processes underlying shifts in BEF relationships along abiotic gradients, which limits our ability to predict how the BEF relationship will respond to the rapid future changes in environmental conditions caused by climate change. In this study, we sought to test (i) how a water limitation gradient shapes the functional composition and diversity of forests, (ii) how functional composition and diversity of trees relate to forest productivity, and (iii) if functional diversity is more strongly related to productivity in the Mediterranean or temperate climate. To address these points, we created a functional trait data set covering traits of most of the tree species within the Italian peninsula, using forest surveyed data sets from the National Forest Inventory. We found that functional composition varied with water limitation as tree communities tended to have more `conservative` resource strategies (e.g., low specific leaf area and high wood density) under increased water limitation. Functional diversity´s response was highly variable, however, increased water limitation has been found to enhance also functional diversity in the temperate domain. Biodiversity positively influenced forest productivity through a combination of mass ratio and niche complementary effects in Italian forests, but the relative importance of these effects is dependent on the functional trait and bioclimate considered. Niche complementary effects showed a more predominant effect on productivity in the Mediterranean compared to temperate forests. Overall, our results suggest that biodiversity plays an important role in driving forest productivity under more environmental stressed conditions by promoting beneficial interactions between species and complementarity in resources.