Relating soil fertility and plant competition to Rhamnus cathartica L. (common buckthorn) invasion success

Abstract

Rhamnus cathartica (common buckthorn) is a shrub or small tree that is invasive in the northern and central United States and southern and eastern Canada. Buckthorn invades a wide variety of habitat types, including open areas and forests. Impacts of buckthorn invasion include the loss of native species, alterations to soil nutrient cycling and decay rates, and increases in non-native earthworm abundances. Although it is shade-tolerant, buckthorn grows rapidly in high light. However, the effects of competition on buckthorn growth are more pronounced in high light environments. Therefore, buckthorn may be particularly adapted to succeed in habitats with intermediate light regimes, which provide ample light for growth and decreased competition. Habitats with low soil fertility may also be especially vulnerable to buckthorn invasion due to buckthorn’s ability to increase soil nitrogen while limiting the growth of herbaceous competitors. An observational and experimental study was performed to determine how habitat, competition, and soil nutrient status affect buckthorn success. I hypothesized that seedlings would have greater success in 1) shrubland habitats than meadow or forest edge habitats and 2) plots with decreased soil fertility than plots with increased or unaltered soil fertility. For the observational study, I measured vegetation and soil characteristics and leaf litter decomposition rates in one meadow, shrubland, and forest edge habitat at six sites in western New York. Meadows were different from shrub and forest habitats in vegetation, but not soil characteristics. Meadows had more herbaceous vegetation and fewer seedlings and saplings than the shrub and forest habitats. There were no differences in herbaceous vegetation cover, woody vegetation abundances and diameters, or in soil characteristics 2 between the shrub and forest habitats. Buckthorn leaf litter decayed faster than a native species mix. For the experiment, I measured the effect of habitat, competition, and soil nutrients on the growth and photosynthetic rates of transplanted buckthorn seedlings at the six sites. Although photosynthetic rates and light levels were greater in the meadows than the shrub and forest habitats, buckthorn seedlings displayed increased height loss in the meadows that was likely due to seedling herbivory and desiccation. While herbivory and desiccation likely exerted the strongest effects on seedling growth in the meadows, light availability exerted the strongest effects in the shrub and forest habitats. In contrast with my hypotheses, competition had no effect on seedling growth, and soil fertility affected only a small subset of seedlings. As meadow habitats were less susceptible to invasion than shrub and forest habitats, management efforts should prioritize shrublands and forests for buckthorn removal. As the competition and soil nutrient treatments had no effect on buckthorn growth in the shrubs and forests, management practices based on manipulating soil fertility or planting native competitors to inhibit buckthorn are not indicated. Instead, buckthorn monitoring and removal practices in forest and shrub habitats should focus on areas with increased light availability, which may create an invasion window for buckthorn.