The effect of predispersal seed predators on colonization of Aster ledophyllus on Mount St. Helens, Washington

Aster ledophyllus, a wind-dispersed herbaceous perennial of western Cascade subalpine zones, is slowly reinvading subalpine habitats devastated by the 1980 eruption of Mount St. Helens. This aster is subject to heavy predispersal seed losses due to grazing by elk (Cervus canadensis) and to seed predation by a variety of insect larvae. Because reinvasion is slow, we hypothesized that these herbivores may be limiting the colonization rate of this species. We tested this hypothesis with a two-way factorial experiment in which the two classes of seed predators (elk or insects) were either excluded from or allowed access to Aster individuals. We quantified treatment effects using two response variables: (1) seed production during the year of the manipulations and (2) seedling recruitment in the following year. Elk browsing significantly reduced both seed production and seedling recruitment, with larger plants being most affected by the elk. No significant effects, however, were detected for insect seed predators.