One of the biggest scientific challenges for modern ecology is to accurately predict the response of ecosystems to changing climate. To achieve this, however, we require a higher understanding on what drives tree response to their environment across large temporal and spatial scales. Many environmental variables can potentially interact with trees to create adaptation patterns (biotic and abiotic, above- and below- ground). By contrast, a large majority of the local adaptation literature have focused on abiotic factors, mostly on climate, while adaptation to biotic factors and interactions between variables are still poorly explored for most species. Strong biotic adaptation to herbivores or soil conditions have been previously reported in some tree species, but large scale comparisons of biotic and abiotic drivers of adaptation are still largely lacking. In the combined results presented here, we used phenotypical and genetic approaches to study the patterns of genetic differentiation in relation to climate and microhabitat conditions in 5 species of dominant trees in 6 European countries within the FunDivEurope project. We present here the joint results of 3 PhD chapters, which combined a diverse arrange of methods, including observational and manipulative experiments, common gardens, and genetic methods.
We found consistent patterns of local advantage on sapling survival across our species and countries. This advantage, however, appeared only under natural conditions in the forest plots but not under common garden conditions. This suggests that factors other than climate (such as biotic interactions or microhabitat) could drive tree local adaptation. This is further supported by the genetic analyses on a subset of countries, which showed small genetic differentiation between populations. On the other hand, biotic and abiotic differences between plots, significantly affected allelic turnover, stressing the role of microhabitat and biotic selection. Lastly, a soil transplant experiment showed no difference in biomass increment related to local mycorrhizal communities, suggesting no role of specialized mycorrhizal interactions on local adaptation, at least under experimental conditions. Our results support a more complex and nuanced vision of tree adaptation than usually considered, with an important role of biotic and microhabitat selection. This questions the singlehanded use of average climate values to describe tree adaptation. Consequently, we question the wholesale use of adaptation strategies that rely on strong climatic adaptation, such as ‘assisted gene flow’. Our results also suggest that experiments under common garden conditions may greatly underestimate the strength of local adaptation observed.