Phenotypes of Pinus sylvestris are more coordinated under local harsher conditions across Europe

Plant species that grow across environmental gradients show a range of trait expression, but traits do not vary independently. In fact, phenotypes are integrated expressions of multiple traits that covary due to trade‐offs among functions and processes. Understanding trait covariation structures will ultimately help with predicting species' responses to change and developing management actions. We measured variation and covariation (a proxy of phenotypic integration) among functional traits of Pinus sylvestris from paired populations across its European distribution. Populations within a pair were close enough to be in gene flow contact but located in contrasting environmental conditions across a local gradient. Measured traits represented three axes of variation (groups of traits) related to a tree's competitive ability and the trade‐off between resource acquisition and conservation, namely plant size measures and stem and foliar traits. Results revealed important intra‐ and inter‐population trait variability. In particular, at the population level, trait means shifted across the climatic gradient mainly described by mean annual temperature. Moreover, we found a higher degree of trait covariation in populations under harsher environments (i.e. lower environmental suitability for the species). This pattern was consistent within population pairs, suggesting that higher trait covariation may be adaptive, being more coordinated in sites with harsher conditions. At larger spatial scales, we found a less conclusive pattern with a trend of increasing covariation at the northern edge of the species distribution. This result suggests that at larger scales different processes may be involved in the trade‐off between the adaptive value of phenotypic covariation versus its constraints on trait combinations that may limit plant's response capability. Synthesis . Trait covariation varies at different spatial scales, increasing under harsher conditions, and the robustness and repeatability of this pattern suggests its adaptive role for the species' responses to different environments.