Interacting effects of light, native herb cover, and species richness on garlic mustard (Alliaria petiolata) invasion

Background/Question/Methods

The degree to which invasive species drive or respond to environmental change has important implications for management and restoration.  The invasive herb garlic mustard (Alliaria petiolata) is often implicated as a driver of change in North American woodlands, yet its interactions with native herbs are poorly understood. Inverse relationships between garlic mustard and native plants may result from garlic mustard’s impacts, but some native plants have been found to be competitive with garlic mustard, and observations suggest that garlic mustard invasion may respond to differences in native plant cover and resource availability. In this study, I tested the effects of native herb richness, cover and light on the invasibility of garlic mustard in a dry-mesic oak woodland in East Central Minnesota. I planted 50 garlic mustard seeds into blocks of experimental plots that were previously planted with native herbs in a range from 0 to 10 species. I measured garlic mustard seedling establishment, survival to rosette and adult stages, and average (per plant) and total (per plot) adult biomass and silique production.  Using structural equation models, I analyzed the direct, indirect and net effects of light, native richness and cover on successive garlic mustard life stages. 

Results/Conclusions

Native plant cover had a significant negative effect on all garlic mustard life stages. Species richness had no direct effect on garlic mustard but 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 average and total silique production.  Light had a negative direct effect on garlic mustard seedling establishment and a positive effect on native plant cover, resulting in significant negative indirect and net effects of light on garlic mustard seedling, rosette and adult numbers. However, via apparent density dependence in garlic mustard, the net effect of light on total biomass and silique production was positive.  The interacting effects of plant cover, richness and light suggest that woodlands lacking a diverse and robust native groundcover may be more vulnerable to garlic mustard invasion.  High light levels may indirectly accelerate invasion, as the lack of inter- and intra-specific competition increases garlic mustard’s reproductive output.   Garlic mustard invasion may thus be, in part, a response to native plant decline.  Restoring woodland herb communities and addressing other drivers of environmental change may reduce garlic mustard invasion.