Common buckthorn (Rhamnus cathartica), European earthworms, and ecosystem management: Invasion and restoration in Minnesota’s deciduous forests

University of Minnesota Ph.D. dissertation. May 2015. Major: Natural Resources Science and Management. Advisors: Lee Frelich, Peter Reich. 1 computer file (PDF); viii, 126 pages.Common buckthorn (Rhamnus cathartica) and European earthworms are problematic invasive species in forests of the upper Midwest United States, and it is hypothesized that these two species may have a facilitative relationship. To better understand their invasion, it is necessary to understand how they interact with biotic and abiotic filters, as well as with each other. We established a greenhouse microcosm experiment to investigate the effects of important biotic and abiotic factors on buckthorn establishment and further explored the relationship between buckthorn and earthworms using a 24-plot field study. Using insights from our greenhouse results, we manipulated factors affecting plant colonization in a buckthorn removal experiment in order to improve buckthorn removal and ecosystem restoration efforts. Greenhouse results showed that the presence of earthworms increased buckthorn abundance and biomass across all light and leaf litter treatment levels, supporting the hypothesis that earthworms facilitate buckthorn invasion in upper Midwest forests. Results from the field study, conducted across a naturally-occurring gradient of buckthorn abundance, suggest that buckthorn, in turn, facilitates earthworms in this study system. Plots with higher buckthorn abundance had higher earthworm biomass, with linear regression, mixed model, and path analysis results supporting the directionality of the relationship. Together, these results lend support to a co-facilitative relationship between the two organisms. Co-facilitation my increase the success of both species and strengthen their negative impacts on native species and forest ecosystems. Finally, we tested three buckthorn removal methods (weed-wrenching, cut and paint, and basal bark herbicide application) chosen to differentially affect conditions controlling plant establishment. Removal plots differed in the subsequent cover and diversity of plant regeneration, with methods that disturbed soil and increased available light resulting in the highest species cover and diversity. Non-metric multidimensional scaling ordinations and indicator species analysis demonstrated that the resulting plant communities differed significantly in their species composition, with weed-wrench plots associated with more early-successional community assemblages. Ultimately, removal methods can differentially affect the regeneration of understory vegetation and affect future community succession. Understanding why and how a species invades can encourage a more scientific approach to invasive species management, potentially resulting in improved management outcomes