Global change affects biodiversity, which may in turn affect ecosystem functioning, eg. productivity, and associated ecosystem services. The interaction between these diversity-related effects and direct impact of climate change on ecosystem functioning still remains mostly unknown. Focusing on forests, former studies have shown that tree species richness affects forest productivity, which may mitigate the impact of environmental variations through i) a performance enhancing effect, ie. an increase in productivity’s temporal average, and ii) a buffering effect, ie. a decrease in productivity’s temporal variance across years. Previous results also suggested that productivity of more diverse ecosystems was more resistant to extreme climatic stresses, and more resilient (with a stronger recovering ability after a stress). However the examples remain scarce regarding forest ecosystems.In this study, we tested the relationship between diversity and stability in natural forests’ productivity in French Alps and Provence, and tested how climate may affect this relationship. So we used three metrics to quantify stability: temporal stability during last 20 years, resistance and resilience. This study relied on 250 km-long latitudinal gradient of forest, distributed in 6 sites. We sampled 1000m²-plots with various tree species richness (monospecific vs. plurispecific), and focusing on beech, silver fir and pubescent oak.
Results/Conclusions
We found that climate has an important effect on stability, regardless the metrics considered. The more stressful the climate, the more instable the productivity over time.
Our results are more ambiguous regarding the effect of diversity. Regarding temporal stability, the diversity effect varied with the species and mixtures considered. No significant effect was found for fir, beech-oak stands were more stable than monospecific oak ones, and the mixing had different effects for beech, depending on the identity of the accompanying species.
We further showed that this trend can be explained by asynchronous responses in species productivity between different species in mixed stands. We found that diversity did not affect resistance and resilience of productivity. We noticed, however, that pubescent oak did not show the same resilience/resistance pattern than the two other species.
Our results suggested that community composition may stabilize ecosystem productivity in the long term, but has no effect directly after an extreme climatic stress (either cold or dry). We can conclude that climatic stresses have a strong effect on stability of forest productivity, that this pattern can be buffered by species composition in long term although depending on species identity.