Light, Wind, and Touch Influence Leaf Chemistry and Rates of Herbivory in Apocynum cannabinum (Apocynaceae)

Simply visiting and manipulating plants in a way consistent with measurement in typical ecological studies influences the amount of leaf herbivory experienced by some plant species. We examined the mechanistic basis for why Apocynum cannabinum is particularly responsive to such visitation and manipulation. In a field experiment, we manipulated both visitation and shading by neighboring plants and measured the resultant changes in plant chemistry, growth, and herbivory. In a greenhouse experiment, we manipulated touch and wind exposure while holding light constant, allowing us to directly test whether the handling causes changes in the plant that might also occur in response to wind exposure. Visitation and neighbor tie back both increased herbivory, shoot biomass, and cardenolide concentration. These changes appear to be mediated by changes in light environment with each treatment. Leaf N and C were also highly responsive to visitation, neighbor tie back, and touch. The strong and similar responses to visitation and neighbor tie back suggest that in this species, visitation acts by reducing aboveground competition through trampling of neighbors; that growth, plant chemistry, and herbivory are extremely sensitive to visitation effects associated with basic ecological measurement; and that competition between plants for light can influence plant-insect interactions. Of even greater importance is the identification that some species are extremely sensitive to even minor changes to their local environment. Such sensitivity may have significant implications for growth in natural communities.