Variation of multiple plant defenses and herbivory reveal different patterns of adaptation across a productivity gradient.

Plants with large geographic distributions experience varying biotic and abiotic conditions across their geographic range that can influence plant defense traits among plant populations. It is often the case that more productive regions support higher herbivore numbers. Yet whether this results in greater herbivore pressure and stronger plant defenses in these populations compared to populations inhabiting less productive regions is uncertain. Here we quantified defense traits and herbivory of Solanum carolinense L. across a large productivity gradient that spans its entire north-south range (29-44 N), using both field observations (33 populations; ~14 plants per population) and a common garden experiment (15 populations; ~4 plants per population). We examined the effects of productivity on 1) plant defense traits (specific leaf area, trichome density and glycoalkaloid concentration), 2) herbivory, and 3) the correlation among traits and herbivory in field and common garden plants. Trichome density and herbivory were higher for S. carolinense at the center of its range, while glycoalkaloid concentrations were negatively associated with productivity both in the field and in the common garden. In the field, plants with higher glycoalkaloid production experienced reduced herbivory, but there was no association with plant defense traits and herbivory in the common garden. Overall, these findings suggest that different types of defensive traits within a single species may follow different ecological and evolutionary trends and highlight the need for trait-specific considerations when applying plant defense hypotheses at the intraspecific level.