Characteristics of expanding and stable populations of garlic mustard in Carolinian parks

Garlic Mustard (Alliaria petiolata) is a short-lived monocarpic perennial introduced to North America in 1868 which reached Canada in 1879. It was first introduced to Point Pelee National Park in 1969 and Rondeau Provincial Park in the 1980s. Both parks have long histories of human-induced disturbance, with most recently high deer grazing pressure. We show that garlic mustard has become (Point Pelee NP) and is becoming (Rondeau PP) a predominant understorey herb from 1994 to the present, and that the two parks are in different phases of colonization. Our research suggests that the garlic mustard population at Point Pelee has stabilized while the Rondeau population is continuing to expand. We found a significant negative relationship between species diveristy and garlic mustard density for both of the parks. However, it is unknown whether garlic mustard is supressing native herbaceous species diversity, or whether low diversity is an outcome of the intense long-term grazing pressure exerted by deer on the native species. Interestingly, the garlic mustard populations at both parks have alternate years of high and low flowering densities, driven by intraspecific density effects. This may expalin the lack of negative association in garlic mustard density and native diversity in some years. Further research will include experiments with native species transplants into garlic mustard dominated sites to determine competitive interactions, and analysis of changes in distribution throughout the transects to determine rate of spread of expanding garlic mustard populations at Rondeau.