Evolutionary drivers of phenotypic traits in two European tree species – evidence from common garden networks

Evolutionary processes such as phenotypic plasticity and genetic adaptation are key mechanisms that have enabled tree species to cope with major changes in their environments and to colonize new areas over millennia. Forest populations are currently experiencing extremely rapid environmental changes due to anthropogenic climate change, challenging their adaptation and resilience over the coming decades. Tree ecophysiological traits do not vary independently but are rather coordinated; however, our understanding of whether these functional traits are governed by the same evolutionary processes is far from complete. In this study, we assessed the evolutionary drivers of functional traits of two major European tree species: sessile oak (Quercus petraea (Matt.) Liebl.) and European beech (Fagus sylvatica L.). We used multiple common garden experiments (four per species) established in the 1990s within the distribution area of these two species, each comprising 9 to 11 provenances. We measured the following traits: i) tree growth including diameter at breast height, height and basal area increments; ii) specific leaf area; iii) long-term responses to climate including the correlation between annual tree growth and climate; and iv) short-term responses to extreme drought. Individual traits were modelled as a response of environment (sites), genetic identity (provenance) and genetically based plasticity (its interaction). To explore the potential influence of climate conditions at seed origin, both, genetic identity and genetically based plasticity, were correlated with the 19 bioclimatic variables from the seed origin (1961–1990) using Pearson correlations. Associations between the climate of origin and multi-trait genetic effects and genetically based plasticity, as well as associations between the climate of the site and multi-trait plasticity were also explored for both species.

Our results indicate that range-wide variation in the studied traits of oak and beech is markedly driven by phenotypic plasticity. At the individual trait level, sessile oak showed evidence for both genetic and plastic causes of trait variation. In contrast, the variability of traits of European beech seemed to have been mostly shaped by environmentally driven responses with no clear signs of genetic effects and small genetically based phenotypic plasticity. The results of the integral multi-trait phenotypes, however, suggested genetically driven differences along a resource-use gradient governed by temperature conditions in both species. The plasticity of coordinated traits also reflects the ability of all provenances to adjust to new environmental conditions by optimizing the integrated phenotype along a resource-use gradient. Our results suggest that mitigation strategies for climate change could be directed towards seeking provenances that are more plastic in their integral phenotype across the resource-use gradient, rather than typically searching for populations adapted to the current or future conditions at the target site.