Herbivory and water availability interact to shape the adaptive landscape in the perennial forb, Boechera stricta

Abiotic and biotic factors interact to influence phenotypic evolution; however, identifying the causal agents of selection that drive the evolution and expression of traits remains challenging. In a field common garden, we manipulated water availability and herbivore abundance across three years, and evaluated clinal variation in functional traits and phenology, plasticity, local adaptation, and selection using diverse accessions of the perennial forb, Boechera stricta. Consistent with expectations, drought stress exacerbated damage from herbivores. Foliar traits exhibited greater plasticity than phenological traits, which displayed more consistent genetic clines. Water availability and herbivory interacted to exert selection, even on traits like flowering duration, which showed no clinal variation and limited plasticity. Furthermore, the direction of selection on specific leaf area in response to water availability mirrored the genetic cline and plasticity, suggesting that variation in water levels across the landscape influences the evolution of this trait. Finally, both herbivory and water availability likely contribute to local adaptation. This work emphasizes the additive and synergistic roles of abiotic and biotic factors in shaping phenotypic variation across environmental gradients.