Conclusions
Our study improves the understanding of intraspecific trait variation along environmental gradients, showing that the underlying ecological and evolutionary mechanisms differ between reproductive and vegetative traits of P. lanceolata . The environmental structuring of variation in biomass-corrected reproductive traits was retained in common greenhouse conditions, indicative of genetic differentiation. In contrast, vegetative traits showed strong plastic responses to buffer short-term environmental variation, sometimes in opposition to genetic differentiation. Differences between vegetative and reproductive traits seem to arise due to the different relationship between each type of trait and overall fitness. These results provide a crucial insight into the potential uses and limitations of observational data, which is readily available for a considerable number of species and traits, but which may provide more uncertain information than common-garden experiments. While genetic differentiation was accurately predicted from observational trait-environment relationships for biomass-corrected reproductive traits, it was sometimes masked for vegetative traits by phenotypic plasticity and countergradient variation. Thus, evaluating evolutionary responses to environment from observational data may lead, in the case of vegetative traits or traits not closely related with fitness, to underestimate the capacity of plants to adapt to new environmental conditions. We also advocate for considering biomass dependency in trait variation analyses, as well as the implications of species life histories on trait-fitness relationships. In view of the general call for including intraspecific trait variation in ecological models (Moran et al . 2016, Funk et al . 2017), these considerations are important for a more informed prediction of species responses to global change.