An evaluation of goat browsing and other restoration strategies in buckthorn-invaded woodlands and savannas

Abstract

Common buckthorn (Rhamnus cathartica) is a Eurasian shrub that has come to dominate many woodlands, grasslands, and urban areas throughout the Midwestern U.S. Buckthorn has several characteristics that make it harmful to native species assemblages and difficult to control. Because it is so problematic, buckthorn has been the subject of numerous studies investigating control methods. However, few studies to date have investigated the use of targeted goat browsing as a means of buckthorn control, and few studies have holistically assessed multi-component restoration strategies at buckthorn-invaded sites. My research addresses these gaps via: 1) an experimental study testing responses of deciduous-woodland understory plant communities to goat browsing, and 2) a retrospective study investigating multiple possible predictors of restoration outcomes in buckthorn invaded sites and identifying common factors that led to successful or unsuccessful restoration outcomes. I found that goat browsing resulted in immediate reductions in both R. cathartica abundance and native plant diversity and abundance; however, one year later, both R. cathartica and native plants had recovered. Among sites that used varied methods of restoration, I found that buckthorn density at the outset of the restoration and type of implementer predicted both buckthorn and biodiversity outcomes, the use of a diversity of management actions predicted better biodiversity outcomes, and the use of targeted goat browsing did not significantly predict restoration outcomes. Additionally, sites that were restored by long-term site stewards achieved better management outcomes than sites restored by other types of implementers, and longer periods of active management were associated with improved buckthorn outcomes, but not improved biodiversity outcomes. Overall, my findings suggest that targeted goat browsing is not sufficient, by itself, to restore buckthorn-invaded sites, but may be useful as a component of a restoration project, and that good restoration outcomes are more likely to be achieved by incorporating diverse management strategies, ensuring sufficient capacity and time to restore highly invaded sites, and incorporating some form of long-term site stewardship into restoration projects.