Rhamnus cathartica L. (common buckthorn) is a shrub or small tree that is invasive in North America. Rhamnus invasion affects communities by decreasing fitness and survival of native plants and ecosystems by altering nutrient cycling and decomposition rates. While competition decreases Rhamnus survival, the inhibitory effects of competitors may vary with light availability and soil fertility. Rhamnus is more sensitive to competition and desiccation in high light environments, while low soil fertility tends to decrease the competitive effects of herbaceous species, offering a potential invasion window. Our objective was to determine how habitat, competition, and soil fertility influence Rhamnus growth. We transplanted Rhamnus seedlings into experimental plots (n = 972) in meadows, shrublands, and forests in six sites across western New York and applied competition treatments (aboveground and belowground, belowground only, no competition) and soil nutrients treatments (nitrogen fertilizer added, sawdust added, control) in a factorial design. We measured height and diameter relative growth rates (RGR) and biomass ratios over one growing season. The results were analyzed using three-way ANOVAs. The entire dataset was first analyzed, then the subset of seedlings exhibiting positive growth rates was analyzed separately to determine how the treatments affected biomass accumulation versus biomass loss.
Seedling height RGRs were greater in the shrublands (-0.0058 cm/day) and forests (-0.0057 cm/day) than the meadows (-0.0214 cm/day, p > 0.001). For the positive values subset, diameter RGRs were greater in the meadows (0.00179 cm/day) than the shrubs (0.00074 cm/day) and forests (0.00086 cm/day) within the nitrogen addition treatment (p = 0.001). Biomass ratios were lower in the meadows (0.8973) than the shrubs (1.0388, p = 0.019) and forests (1.0316, p = 0.028), lower in the “no competition” (0.9052) than the “aboveground and belowground” competition treatments (1.0824, p = 0.002), and greater in the nitrogen addition (1.0881) than the sawdust addition (0.9386, p = 0.012) and control (0.9410, p = 0.014) treatments. Greater height loss in the meadows was likely due to predation and desiccation. Based on positive diameter RGRs, only the meadows were nitrogen-limited. Our findings support the use of different management practices in different habitats. In environments with high light and soil nitrogen levels, such as anthropogenically disturbed meadows, decreasing soil fertility may decrease Rhamnus’s success. Management based on altering soil fertility or planting native small-statured competitors to reduce Rhamnus dominance are not indicated in shrub or forest habitats.