The invasion of the biennial herb garlic mustard (Alliaria petiolata) in North American woodlands has coincided with declines in native plant communities, motivating the question: is garlic mustard driving or responding to ecosystem change? Garlic mustard can affect soil chemistry and arbuscular mycorrhizal fungi (AMF) communities; the extent to which such impacts contribute to woodland degradation will affect the outcomes of invasion control efforts, including biocontrol. Post-invasion plant community recovery is unlikely if garlic mustard is not a primary driver of native plant decline, or if soil legacy effects persist following garlic mustard's removal. In this study, I investigated the impacts and legacies of garlic mustard and the implications for restoration of woodland herbaceous communities in Minnesota. Specifically, in a combination of field and greenhouse studies, I tested the hypotheses that native herbs would have lower germination, establishment, biomass, and mycorrhizal colonization when planted into invaded soils compared to non-invaded soils, and that such impacts would persist after multiple years of complete or partial removal of garlic mustard, indicating a soil legacy effect. Further, I tested the hypothesis that garlic mustard invasion is responding to native herb decline by comparing the performance of garlic mustard plants seeded into field plots of varying species richness and native cover. In two oak woodland study sites, I planted 12 species of native woodland herbs (plug or bare rootstock) into invaded and non-invaded plots subjected to vegetation removal treatments that varied in their degree (full, partial and no removal) and duration prior to planting (two, one or no years of removal). I measured plant biomass after two or three years to test the impacts of garlic mustard presence and history relative to that of non-invaded native vegetation, as well as the persistence of garlic mustard's impacts at low densities or following sustained removal. I tested mechanisms of impact by comparing AMF colonization, and light and nutrient availability in invaded and non-invaded areas. Garlic mustard presence and history did not negatively impact herb biomass, but instead had a facilitative effect resulting in higher biomass in invaded plots. Light availability and AMF colonization were not affected by invasion relative to native vegetation, but soil resource availability was higher in invaded plots. Herb biomass and nitrate availability remained elevated following multiple years of removal, indicating a "positive legacy" due to nutrient enrichment. In the greenhouse studies, I tested garlic mustard's impacts on germination, growth and AMF colonization of 13 native herbs planted by seed into field soils collected from invaded and non-invaded areas. Additionally, I included a fungicide treatment to more explicitly test the AMF mechanism of impact. I found that while garlic mustard did reduce seed germination and mycorrhizal colonization of native herbs, the effects on seedling establishment (herb biomass) varied. Herb biomass was lower in invaded soils in the first experimental replication, when plants were inadvertently subjected to climate stress, and higher in invaded soils in the second replication, under controlled climate conditions; the inconsistent response suggests that garlic mustard's negative impacts on herbs only manifest when combined with additional stressors. To investigate whether garlic mustard invasion responds to native herb decline, I planted garlic mustard seeds into field plots that ranged in species richness and cover and measured the direct and indirect effects of native plants and light availability on sequential life stages of garlic mustard. Light levels had varying impacts on different garlic mustard life stages, resulting in net negative effects on garlic mustard numbers but positive effects on reproductive output per plot. Native plant cover had a significant direct negative effect on all garlic mustard life stages, and native species richness had indirect negative effects mediated through higher plant cover in species-rich plots.
University of Minnesota Ph.D. dissertation. June 2012. Major:Conservation Biology. Advisor: Diane Larson. 1 computer file (PDF); x, 198 pages, appendices A-B.
Garlic Mustard (Alliaria petiolata) Invasion & Impacts: implications for management and restoration of woodland herbs.
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