| Alliaria petiolata (garlic mustard) is a non-native, shade tolerant forb that has successfully invaded the understory of eastern North American forests, causing a variety of ecological changes. Little is known, however, about the consequences of this invasion for native plant communities and soil processes. I hypothesized that A. petiolata would decrease microbial activity and slow nutrient cycling by releasing cyanogenic glycosides to the soil. I also hypothesized that native plant diversity and the growth rate of native plants would decline in the presence of A. petiolata. To assess the impact of A. petiolata on native plant communities I measured native plant diversity and tree seedling growth in the field and plant growth in greenhouse competition experiments. To analyze the changes in microbial activity, soil nutrient availability, and pH in soils invaded by A. petiolata I performed a series of field studies and laboratory experiments.; The richness and diversity of native plant communities declined with increasing abundance of A. petiolata in the field. The growth and survivorship of tree seedlings was significantly lower in plots invaded by A. petiolata than in adjacent uninvaded plots. Alliaria petiolata had no clear impact on the activity of soil bacterial communities, but significantly altered fungal community composition and the abundance of mycorrhizal fungi on plant roots. I found that A. petiolata invasion increased the availability of nitrogen, phosphorus, calcium and magnesium in the soil, and soil pH. The increase in soil nutrient availability and the decrease in diversity and growth of native plants by A. petiolata are likely caused by the reduction in the abundance of mycorrhizal fungi. Importantly, A. petiolata is non-mycorrhizal and grew better in soils it previously inhabited, suggesting a positive feedback between site occupancy and continued proliferation. |
