Species Identity Drives Geographic Variation in Leaf Traits and Palatability to a Generalist Insect Herbivore of a...

Introduced plants represent unique opportunities to understand how diverse phenotypic traits vary across broad geographic scales. However, clear generalizable principles about genetically based variation in key functional plant traits across geographic clines remain elusive. For example, functional plant traits often vary predictably across climatic gradients, yet these emergent patterns may be obscured by other factors that may collectively shape trait expression across a species' range, such as species-level differences or understudied non-climatic factors that operate across latitude. To address this knowledge gap, we leveraged an established common garden of 213 individuals of the octoploid Reynoutria japonica and the hybrid polyploid R. × bohemica initially collected over a wide latitudinal range across the eastern United States. Specifically, we assessed how heritable variation in Reynoutria spp. resistance to herbivory, measured as performance of the generalist herbivore Spodoptera exigua and leaf traits affecting herbivory, may be explained by climate, latitude, or interspecific differences. Interspecific differences, such as those mediated by hybrid polyploidy, were the main factors explaining variation in resistance to herbivory: R. japonica was generally more resistant and less palatable than R. × bohemica. Spodoptera exigua performance and leaf trait variation were also better explained by models that included climate rather than latitude at original collection locations. Herbivore survival to adult increased when feeding on R. japonica and decreased when feeding on R. × bohemica as climates at collection locations became colder and more seasonally variable. Plants originating from wetter and more seasonally consistent climates also had tougher leaves than plants from more seasonally variable climates. Our findings implicate the critical role of interspecific differences, potentially mediated by linked evolutionary mechanisms, in shaping heritable variation in resistance to herbivory across species' introduced ranges. We found that climate also mediates trait expression and herbivore performance, but its effects were less prominent than those associated with interspecific differences between two closely related congeners.