Garlic Mustard (Alliaria petiolata) in a Minnesota oak woodland: A four year study of population dynamics and ecological impacts

Background/Question/Methods

Garlic mustard  (Alliaria petiolata), a European biennial common in many Midwestern and eastern North American forests, has typically been characterized as an invader that displaces native herbs and inhibits tree seedlings through the production of allelochemicals.  However, several recent studies have questioned the extent to which A. petiolata is an important agent of change in these forests.  To help resolve this uncertainty, we conducted a four year study in which we monitored a population of A. petiolata in a Minnesota oak woodland at three spatial scales: the size of the monitored population (6 ha), 20 x 20 m cells in the 6 ha study grid, and 1 x 0.5 m plots.   We also monitored the cover of all other vegetation in the small plots.  In addition, we conducted pot experiments to test whether field-collected soil ‘trained’ by garlic mustard negatively affected native tree seedlings.  These data enabled us to determine if this population of A. petiolata was exhibiting alternating years of high reproduction, which has been documented elsewhere for this species, whether the population was stable over the four summers, and whether changes in its abundance were associated with changes in abundance of native herbs and tree seedlings.

Results/Conclusions

The monitoring results showed that the A. petiolata population is in a state of dynamic equilibrium.  Garlic mustard abundance was relatively stable over the four summers at the scale of the woodland while commonly exhibiting substantial increases or decreases in abundance at smaller scales.  The population exhibited an alternate year demographic cycle, with first-year basal rosettes dominating one year and second-year flowering stems dominating the next.  We found little evidence that A. petiolata is negatively affecting other plant species, including tree seedlings, in any substantial way.  Rather, the data indicate that over the four years of this study A. petiolata and other plant species in the Ordway woodland are changing in abundance largely independent of one another.  Our results to date indicate that A. petiolata is not likely a major driver of ecological change in this forest.  These results, along with those from other recent studies, question the justification and value of management efforts to eradicate or reduce the abundance of A. petiolata.