Effects of Native Herbs and Light on Garlic Mustard (Alliaria petiolata) Invasion

Abstract The degree to which invasive species drive or respond to environmental change has important implications for conservation and invasion management. Often characterized as a driver of change in North American woodlands, the invasive herb garlic mustard may instead respond to declines in native plant cover and diversity. We tested effects of native herb cover, richness, and light availability on garlic mustard invasion in a Minnesota oak woodland. We planted 50 garlic mustard seeds into plots previously planted with 0 to 10 native herb species. We measured garlic mustard seedling establishment, survival to rosette and adult stages, and average (per plant) and total (per plot) biomass and silique production. With the use of structural equation models, we analyzed direct, indirect, and net effects of native cover, richness, and light on successive garlic mustard life stages. Native plant cover had a significant negative effect on all life stages. Species richness had a significant positive effect on native cover, resulting in indirect negative effects on all garlic mustard stages, and net negative effects on adult numbers, total biomass, and silique production. Light had a strong negative effect on garlic mustard seedling establishment and a positive effect on native herb cover, resulting in significant negative net effects on garlic mustard rosette and adult numbers. However, light's net effect on total garlic mustard biomass and silique production was positive; reproductive output was high even in low-light/high-cover conditions. Combined effects of cover, richness, and light suggest that native herbs provide biotic resistance to invasion by responding to increased light availability and suppressing garlic mustard responses, although this resistance may be overwhelmed by high propagule pressure. Garlic mustard invasion may occur, in part, in response to native plant decline. Restoring native herbs and controlling garlic mustard seed production may effectively reduce garlic mustard spread and restore woodland diversity. Nomenclature: Garlic mustard [Alliaria petiolata (M. Bieb.) Cavara & Grande]. Management Implications: Observations of native herb decline and garlic mustard spread are often presented as evidence of garlic mustard's impacts on woodland plant communities. Alternative explanations include native plants affect garlic mustard invasion, both garlic mustard and native plants influence each other, or neither interact directly but are instead responding in opposite directions to other environmental pressures. Understanding the relationship between garlic mustard and native plants can inform invasion control and woodland management. To test the hypothesis that garlic mustard invades in response to declining herb richness and cover, we planted garlic mustard seeds into experimental native herb communities across a range of light levels and analyzed garlic mustard seedling establishment, survival to rosette and adult stages, and biomass and seed capsule (silique) production. We found that native cover strongly suppressed garlic mustard at every life stage. Higher herb species richness produced greater percent cover and thus contributed indirectly to invasion resistance. Garlic mustard demonstrated a flexible response to light: fewer garlic mustard seedlings established in high light plots, but these individuals were highly productive in the second year, whereas in low-light plots, garlic mustard plants were more numerous but less productive. Managing woodland light levels alone is therefore unlikely to prevent garlic mustard invasion. Woodlands with high resource availability and minimal native herb cover may be most vulnerable to invasion. Restoration of woodland herbaceous communities may play an important role in reducing invasibility, especially following management activities that reduce canopy cover and increase light availability at the forest floor. However, identifying and controlling drivers of native plant decline (e.g., white-tailed deer herbivory and earthworm invasion) may be necessary to restore native herb communities to densities sufficient to suppress garlic mustard. Additionally, garlic mustard's substantial seed production, even in competitive conditions, suggests that propagule pressure may potentially overwhelm biotic resistance to invasion. A comprehensive management strategy that focuses on herb restoration, minimizing environmental stressors, and reducing propagule pressure via targeted control of second-year garlic mustard plants may be most effective at increasing woodland resistance to invasion.