The scourge of the Midwest: Does introducing invasive garlic mustard affect understory diversity at sandy pine plantations in central Illinois?

Background/Question/Methods

As invasive species continue to spread and become more abundant outside their native range, land managers and researchers need to focus on potential management techniques that are cost effective and less-labor intensive. The goal of our research was to examine garlic mustard (Alliaria petiolata (M. Bieb) Cavara and Grande (Brassicaceae)) population invasion success and the effect that its presence may have on native understory diversity. Garlic mustard is a weedy biennial species not native to the United States. Most notably, garlic mustard roots, leaves, and stems exude glucosinolates, a secondary metabolite that has the ability to disrupt mutualistic relationships between soil microorganisms and native plants potentially altering understory diversity. We experimentally introduced garlic mustard seed and mature rosettes at two nitrogen-limited pine sites in Sand Ridge State Forest (SRSF; Mason County, IL). Our experimental design allowed us to observe changes in population density of garlic mustard and the effects that experimental introductions have on diversity at two invasion intensities. Forced introductions resulted in more rapid and heavier infestations early on than our plots that were naturally invaded at lower densities. Plots were then subjected to either nitrogen additions or management techniques.

Results/Conclusions

Prior research conducted at our study sites found that N-availability is particularly limited which we believe may affect the invasbility of SRSF and the success and invasiveness of garlic mustard. Surprisingly, the outcome of our study found that introducing garlic mustard, regardless of the life stage, results in successful invasions that ultimately reduced species richness, evenness, and/or diversity in pine forests where soil-N is in short supply. Additionally, the techniques used here were not effective at preventing invasions however we did find that certain management techniques can mitigate against these declines at different times during the invasion. Particularly, the N-immobilizing effects of added sawdust to plots was found to be the most effective management technique of our study by potentially offsetting the effects of the invasion. Additionally, removal of the pine litter layer from plots not subjected to invasions resulted in increases in species richness and equitability; a trend not seen in seeded plots. The results of our study suggest that invasion by garlic mustard is inevitable, however the degree to which the invasion occurs and the resultant effect on diversity can be lessened with certain management techniques.