Effects of defoliation on sandbar willow (Salix interior) chemistry, production, and subsequent overwinter browsing by mammalian herbivores

Abstract

Herbivory can cause changes in plant characteristics, allowing temporally isolated herbivores to indirectly affect one another through their effects on shared host plants. The objective of this thesis was to test how defoliation of the willow Salix interior affects current annual stem production and chemistry, and how changes in these traits may indirectly affect mammalian herbivores. I studied the effect of manual defoliation on S. interior leaf and stem chemistry, and the effect of insect folivory on S. interior stem chemistry, production, and mammal herbivore offtake. Manual defoliation of S. interior affected stem chemistry by significantly increasing stem N concentration and decreasing stem C:N ratio, but did not alter leaf chemistry. Neither stem nor leaf protein precipitation capacity (PPC), a measure of tannin activity, were affected by manual defoliation. In a second field experiment I investigated the effects of natural levels of insect folivory on S. interior stem characteristics, testing the effects of insect herbivore suppression on stem production, chemical composition, protein precipitation capacity, and overwinter mammal browsing. Insect folivory did not significantly alter stem chemistry, but significantly reduced stem production by reducing mean stem diameter the following year. These findings indicate that defoliation of S. interior can improve nutritional quality and reduce availability of stems for mammal herbivores foraging over the subsequent winter.