Productivity of mixed forests: effect of species diversity in a variable climatic and edaphic context.

Summary Global environmental changes can result in a modification of abiotic conditions and biological diversity. The production function of forest ecosystems has the particularity of being both subject to and regulating these environmental changes. It is therefore essential to understand how species diversity and abiotic factors control this function. In my work, I studied the variation in the effect of species mixing on their productivity in a changing climatic and edaphic context. Using a double approach, based on local data and data from the National Forest Inventory, I focused on five major European forest species in pure and bispecific stands: Quercus petraea, Pinus sylvestris, Fagus sylvatica, Abies alba and Picea abies. In lowland areas, I show that the effect of species mixing varies little with abiotic factors. Moreover, this effect is determined by the shade tolerance of the accompanying species. In the mountains, the effect of species mixing on productivity is stronger and more positive when abiotic factors are the most limiting for growth. These results illustrate the interest of integrating abiotic factors and functional characteristics of species as determinants of the effect of biological diversity on ecosystem functions.