Overyielding in mixed forests decreases with site productivity

Abstract

Summary There is a rising interest in the role of species diversity in ecosystem functioning and services, including productivity. Yet, how the diversity–productivity relationship depends on species identity and abiotic conditions remains a challenging issue. We analysed mixture effects on species productivity along site productivity gradients, calculated from a set of abiotic factors, in two biogeographic contexts (highlands and lowlands). We compared the productivity of 5 two‐species mixtures (i.e. 10 cases of mixed species) with that of monocultures of the same species. Five main E uropean tree species were considered: sessile oak ( Q uercus petraea L iebl.), Scots pine ( P inus sylvestris L .), E uropean beech ( F agus sylvatica L .), silver fir ( A bies alba M ill.) and N orway spruce ( P icea abies ( L .) H . K arst). Our data set was compiled from the 2006 to 2010 F rench N ational F orest Inventory data base and covers 2361 plots including pure and mixed stands. Overall productivity of mixtures in highlands, that is E uropean beech– N orway spruce, E uropean beech–silver fir and to a lesser extent, silver fir– N orway spruce, was found to be higher than expected from the productivity of corresponding monospecific stands. Overyielding was mainly due to E uropean beech for the first two mixtures and to silver fir for the third one. No effect of mixture was found for sessile oak– S cots pine and sessile oak– E uropean beech stands in lowlands. Overyielding of sessile oak mixed with S cots pine was not strong enough to significantly increase overall stand productivity. Overyielding of E uropean beech was balanced by an underyielding of sessile oak. The mixture effect changed along site productivity gradients for six cases out of the 10 studied, with a stronger and positive effect on sites with low productivity. The magnitude of this change along site productivity gradients varied up to 89% depending on the tree species. Synthesis . The nature of species interaction in mixtures with regard to productivity changes with species assemblage and abiotic conditions. Overyielding is strongest when species grow in highlands on less productive sites. A negative link between mixture effect and site productivity was found, in line with the stress‐gradient hypothesis.