Inquiry-based undergraduate learning: Abiotic and biotic controls of garlic mustard (Alliaria petiolata) invasion in Virginia pine and hardwood forests

Background/Question/Methods Garlic mustard (Alliaria petiolata) is an obligatory biennial ephemeral that has become invasive in eastern temperate forests of the United States and Canada where rapid dispersal and dense population growth are reducing biodiversity of native species. Successful invasion is controlled in part by garlic mustard’s competitive abilities belowground, but the role of abiotic factors in population dynamics is less clear. In a novel inquiry-based approach to undergraduate education, students enrolled in a 300-level course in Plant Functional Ecology at Washington and Lee University in Virginia used the garlic mustard system to explore fundamental concepts of plant ecology while performing hypothesis-driven observational field studies. Portions of the mixed-temperate forest behind the campus were clear-cut in the mid-twentieth century and planted with eastern white pine (Pinus strobus), providing a field system to investigate abiotic and biotic factors associated with hardwood versus pine forests that may naturally limit growth of juvenile and adult  forms of this plant. We viewed this as an opportunity to test the effect of the type of dominant tree canopy on the growth of garlic mustard and hypothesized that early spring growth in the open canopy environment of hardwood forests would facilitate higher population densities and fitness than pine forests.

Results/Conclusions We surveyed the gently sloping forest behind Washington and Lee University and chose three paired replicate sites (each with one hardwood upslope and one pine downslope) based on similar canopy cover of overstory trees, slope aspect, and distance from a walking path. We estimated juvenile and adult densities using a stratified random sampling design and measured reproductive output, physical and chemical soil properties, light, temperature, leaf litter and evidence of predation and disease. Our hypothesis was not supported, as garlic mustard juvenile plants were five times more abundant in the pine forest in May of 2008, a result which correlated significantly with lower pH and moisture in the clay soils, a deeper humus layer, and smaller surface areas of leaf litter compared to hardwood forest. Soils in pine-dominated forests may afford developing garlic mustard seedlings with a more hospitable environment for rapid establishment and invasion. Densities of adult plants in their second year of growth were less sensitive to environmental variables. Possible physiological mechanisms for these results will be explored in future undergraduate courses.