Healthy forests to resist invasion: The role of resources, plant traits, and propagule pressure

University of Minnesota Ph.D. dissertation. April 2015. Major: Natural Resources Science and Management. Advisor: Peter Reich. 1 computer file (PDF); ix, 147 pages.Invasive species are a global problem, dominating habitats, negatively impacting biodiversity, and changing ecosystem processes. There is no consensus regarding which nonnative species are likely to become invasive if introduced, nor which habitats are most susceptible to invasion. To investigate these questions, we studied the native and nonnative plants in 68 oak forest stands in Minnesota, USA. Nonnative plants possessed functional traits similar to those of some native species, suggesting that they exhibit similar growth strategies. These traits allow nonnatives and some natives to grow quickly in high resource environments. Among these same sites, we also examined whether there are characteristics of forests that make them more susceptible or resistant to a particularly pernicious invasive shrub, common buckthorn (Rhamnus cathartica L.). We found that buckthorn presence was best predicted by high propagule availability and site light levels, while buckthorn was more abundant in sites with higher soil fertility, lower resident plant diversity, and less leaf litter. Timber harvesting also affected buckthorn abundance, with more buckthorn in sites that were clearcut or unharvested than in those that were selection harvested. Management practices that minimize increases in light levels and soil disturbance or maintain or increase resident plant diversity (e.g., reduce deer populations) may help uninvaded forests resist buckthorn invasion, especially if local propagule pressure is also reduced. Finally, we also investigated the below-ground effects of buckthorn by examining nutrient cycling across a natural gradient of buckthorn abundance along an invasion front. Buckthorn appears to increase soil nitrogen, organic carbon, calcium, and pH through deposition of nutrient-rich leaf litter. Increases in soil fertility may lead to increased forest productivity and potentially facilitate further spread of buckthorn or other invasive species that may be better able than natives to take advantage of abundant resources. Overall, both the traits of invasive plants and the characteristics of receiving systems can play key roles in determining the success of nonnative species and the potential impacts they may have on native ecosystems