Abstract
Climate change modifies ecosystem processes directly through its effect on environmental conditions, but also indirectly by changing community composition. Theoretical studies and grassland experiments suggest that diversity may increase and stabilize communities’ productivity over time. Few recent studies on forest ecosystems suggested the same pattern but with a larger variability between the results. In this paper, we aimed to test stabilizing diversity effect for two kinds of mixtures (Fagus sylvatica–Quercus pubescens and F. sylvatica–Abies alba), and to assess how climate may affect the patterns. We used tree ring data from forest plots triplets distributed along a latitudinal gradient across French Alps, adapting NBE approach to study temporal stability. We found that diversity effect on stability in productivity varies with stand composition. Most beech-fir stands showed a greater stability in productivity over time than monocultures, while beech-oak stands showed a less stable productivity. Considering nonadditive effects, no significant trends were found, regardless of the type of mixed stands considered. We further highlighted that these patterns could be partially explained by asynchrony between species responses (notably to variation in temperature or precipitation), overyielding and climatic conditions. We also showed that the intensity of the diversity effect on stability varies along the ecological gradient, consistently with the stress gradient hypothesis for beech in beech-oak forests, but not for beech-fir forests. This study showed the importance of the species identity on the relationships between diversity, climate and stability of forest productivity. Better depicting diversity and composition effects on forest ecosystem functioning appears to be crucial for forest managers to promote forest adaptation and maintain timber resource in the context of ongoing climate change.