Healthy Forests to Resist Invasion

PROJECT OVERVIEW
Invasive plants cause considerable ecological and economic damage in Minnesota and their control is often difficult to achieve in a long-term cost-effective manner. Although not immune from invasion, healthy forests may be somewhat resistant to invasion; therefore management aimed at maintaining, restoring, or enhancing key forest characteristics might be a useful strategy for slowing forest invasion. Scientists from the University of Minnesota's Department of Forest Resources will use this appropriation to study 80 different forest sites in order to determine the links between forest attributes and plant invasion. Findings will be used to make recommendations for how to best manage forests to resist invasive species.

OVERALL PROJECT OUTCOME AND RESULTS
The primary project goal was to identify forest characteristics effective as deterrents to invasive plants. Healthy forests are likely more resistant to invaders, so management to enhance these key characteristics might slow the spread of invaders.

Invasive plants sometimes form dense thickets that affect recreation and wildlife and exclude native plant species. To determine how various site characteristics affected the abundance of common buckthorn and other invaders, we surveyed plant diversity in 67 sites in central and southern Minnesota. At each site, we measured environmental characteristics to simultaneously account for other factors that might influence invasibility. Buckthorn was most abundant in sites with sparse leaf litter, where seed availability was high, and where native plant diversity was low. Both a greenhouse experiment and a second field study indicated that introduced earthworms also benefit germinating invasive plants by eliminating leaf litter.

We propose the idea of "preventive environmental care" that, like preventative medicine, manages forests to maintain "wellness". Although not a panacea for reducing invasion, it is worth considering given the challenges of controlling established invasive species. We suggest managers enhance the competitive challenge to invaders by increasing the diversity of native species by seeding natives and/or reducing the density of white-tailed deer, a species that severely impacts native forest plants. Furthermore, timber harvests should be limited to the winter season and trail maintenance should be done in a way that limits disturbance. This will help maintain intact native understory plants and litter layers, important deterrents to invasive plant establishment. However, none of these approaches are likely to be successful without a strong effort to control landscape level seed availability. Collaborative management with neighboring landowners is crucial to any effort that hopes to reduce invasibility.

PROJECT RESULTS USE AND DISSEMINATION
To summarize results from the project and provide guidelines for management, we prepared a pamphlet that included all aspects of the research, as it pertains to the invasion of buckthorn. The pamphlet also provides suggestions for pre-invasion management to reduce invasibility, the main focus of the "Healthy Forests" research project. We distributed the pamphlet to all participants at a symposium held on August 14, 2013. The pamphlet is available as a pdf from the project website, http://forestecology.cfans.umn.edu/Research/Buckthorn/index.htm.

We presented talks at the Upper Midwest Invasive Species conference (a regional meeting focused on invasive species) and the Ecological Society of America conference (an international conference focusing on all aspects of ecology) in 2012 and 2013. The talks focused on measuring propagule pressure, the greenhouse study, the relationship between earthworm and buckthorn buckthorn, and the effects of native species diversity on buckthorn abundance.

On August 14, we hosted a symposium on the St. Paul campus that brought together managers, researchers, and private landowners to share the latest information on invasive plants in Minnesota forests. In addition to talks based on this LCCMR project, other speakers presented information about buckthorn invasion on the prairie-forest border in west central Minnesota, garlic mustard (another common plant invader in Minnesota's forests) as a driver of species invasion, management of buckthorn from a forester's perspective, and management efforts to control other common invasive plants. The symposium was attended by 100 people. The project website has links to recordings of all the symposium talks, as well as links to the MS Access database, species lists from all survey sites, and a photo gallery.

We have published one paper ("Community phylogenetic diversity and abiotic site characteristics influence abundance of the invasive plant Rhamnus cathartica L.") in the Journal of Plant Ecology. A second paper based on results from our greenhouse experiment (Native plant diversity and introduced earthworms have contrasting effects on the success of invasive plants") has been submitted to the peer-reviewed journal Biological Invasions. More papers are in preparation including one focusing on propagule pressure and another that documents the relationship between earthworms and buckthorn abundance.