Population and plant community dynamics involving garlic mustard (Alliaria petiolata) in a Minnesota Oak Woodland: a four year study1

Abstract

Garlic mustard, Alliaria petiolata (M. Bieb.) Cavara & Grande, a European biennial, has become common in many Midwestern and eastern North American forests. Researchers have sometimes documented a two-year (two-point) population cycle in which the vegetative and reproductive phases alternate in abundance. However, little research has been conducted on the scale dependence of the two-point cycle and the extent to which the cycle is dependent on the density of A. petiolata. In addition to this interest in A. petiolata's population dynamics, researchers have investigated A. petiolata's effects on other plant species, often concluding that it displaces native herbs and inhibits tree seedling growth and survival through the production of allelochemicals. In order to examine both the population dynamics of A. petiolata and its possible effects on North American native plants, we conducted a four year study in which we monitored A. petiolata in a 6.2 ha portion of a Minnesota oak (Quercus) woodland at two spatial scales (0.5 m2, 400 m2). We also conducted an indoor seedling pot study to investigate whether any soil inhibitory effects associated with A. petiolata may be exacerbated under drought conditions. We found that a two-point cycle was not sustained during the four year study throughout the entire study site. However, the two-point cycle was exhibited in high A. petiolata density areas in the study site, providing support for the hypothesis that the cycle is driven by competition between first-year (rosette) and second-year (stem) plants. Both the monitoring study and the pot experiment yielded little evidence that A. petiolata is a major agent of ecological change in the woodland. Rather, the data indicate that over the four years of this study A. petiolata and the other plant species in the woodland we studied are changing in abundance largely independent of one another.