Effects of prescribed grazing by goats on non-native invasive shrubs and native plant species in a mixed-hardwood forest.
Invasion of non-native shrubs comprises a serious economic and ecological problem in North American forests. Prescribed grazing by goats may offer an effective alternative to traditional methods of control, but has received little study in forest settings. In a 5-year field experiment, we varied the stocking of goats and number of grazing periods to determine effects of prescribed grazing on both invasive shrubs and native plant species. Specifically, we examined how varied stocking levels and grazing periods affect: (1) the cover and height of invasive shrub and native woody species (2) the cover and diversity of herbaceous species, and (3) the density of native tree seedlings. Data were analyzed with linear mixed-effects models. We found that prescribed grazing by goats significantly reduced the cover and height of invasive shrubs, regardless of goat stocking or duration. Generally, the greatest reductions in invasive shrub cover occurred in treatments with two grazing periods, regardless of goat stocking. While we observed mostly neutral or positive effects on the herbaceous layer, all grazing treatments reduced the cover and height of native understory woody species. While changes were not significant, we observed a general decline in the aggregate height of native tree species regeneration. Our results suggest that prescribed grazing by goats provides an effective and environmentally friendly treatment for heavy invasions of non-native shrubs such as Rosa multiflora, but grazing needs to be followed by focused mechanical or chemical treatments to maintain control while allowing the regeneration of native tree species.