Population differentiation in functional traits shapes plant–consumer interactions along an elevational gradient

1. Understanding how intraspecific variation in functional traits shape species interactions is central to predicting ecological responses to environmental change. The relative contribution of genetic differentiation and phenotypic plasticity to trait variation and their consequences for species interactions remain poorly understood. 2. Here, we conducted a reciprocal transplant experiment using Plantago lanceolata along an elevation gradient in the Eastern Swiss Alps to disentangle genetic differentiation and genotype-by-environment interactions underlying trait variation along a steep climatic and edaphic gradient. We measured several functional traits as well as arthropod herbivory and fungal infection prevalence. 3. We found strong evidence for trait differentiation between populations and weaker evidence for genotype-by-environment responses to transplant elevation or soil conditions. Herbivory showed little variation among transplant sites, whereas fungal disease decreased strikingly with increasing elevation and disease also varied according to population origin. Importantly, several traits showing population differentiation were linked to variation in plant consumer damage. 4. Our results suggest that variation in functional traits contribute to variation in plant-consumer interactions. More broadly, this study highlights the usefulness of identifying the sources of trait variation when predicting plant-consumer dynamics under environmental change.