Ungulate herbivores reduce fruit production of shrubs in dry conifer forests of the interior Pacific Northwest, USA

Abstract

Ungulate herbivores can affect food webs by reducing fruit production of browsed plants. In this study, we evaluated the effects of herbivory by large mammalian herbivores (cattle, elk, and mule deer) on fruit production of three common, widely distributed shrub species, Amelanchier alnifolia, Ribes cereum and Rosa woodsii that produce fleshy fruit consumed by wildlife in dry conifer forests of eastern Oregon, U.S.A. Specifically, we evaluated herbivory effects on fruit count, shrub height and fruit density in stands exposed to ungulates and in adjacent stands where ungulates have been excluded for 15 years. We measured shrub height, density and fruit production within each herbivore treatment using belt transects. Data was collected for two years for A. alnifolia, and one year for R. cereum and R. woodsii. For A. alnifolia, herbivory decreased the probability that an individual shrub would produce fruit by 80%. Herbivory reduced overall fruit set of A. alnifolia by ~90%; there was weak evidence of a decline in R. woodsii fruit set by ~31%; and we found no significant herbivore effect on R. cereum. Reduced fruit production in combination with herbivore impacts on shrub density resulted in large differences in estimated fruit density across treatments, with the greatest effects found for A. alnifolia; estimated fruit production per hectare was 97–99% lower when exposed to herbivores as compared to the control treatment. Results support the hypothesis that ungulate herbivores influence standing fruit crops of at least some browsed shrubs within this forest ecosystem. This has implications for food webs because forest understory shrubs are a primary provider of fleshy fruits that are consumed by a wide range of insects, mammals, and birds in these forest systems.