Impact of garlic mustard (Alliaria petiolata) on forest floor decomposition rates

Background/Question/Methods

Garlic mustard is an invasive non-native species known to have allelopathic impacts on plant communities. Garlic mustard is also thought to out-compete other species for nutrients such as nitrogen and soil water. Thus, it is likely that garlic mustard could alter carbon and nutrient cycling. We investigated the impacts of garlic mustard on decomposition in an oak forest (Quercus alba) and a sugar maple dominated forest (Acer saccharum). We compared decomposition rates of oak and sugar maple leaves, garlic mustard leaves and stems, as well as filter paper over 24 months in two forest stands at the Edward Lowe Foundation property, Cass County, MI. Within each forest type five plots invaded by garlic mustard and five uninvaded plots were established. The experiment began in May 2008; three replicate samples were collected from each plot every six months through May 2010.

Results/Conclusions

Overall, the presence of garlic mustard accelerated decomposition rates in the sugar maple, but not in the oak forest. Across materials, there were minimal differences in decomposition between forest stands in the presence of garlic mustard. However, decomposition was greater in the oak compared to the sugar maple stand without garlic mustard. The greatest mass loss of filter paper, garlic mustard leaves and stems occurred within the first six months in all plots. In the oak stand the presence of garlic mustard decreased the decomposition of garlic mustard leaves. In the sugar maple stand the opposite occurred for garlic mustard stems. Overall, garlic mustard stems decomposed more slowly compared to leaves. Over two years there was between 15-45% mass loss of oak and sugar maple litter across plots.  In the absence of garlic mustard sugar maple litter decomposed more rapidly than oak litter in both forest stands. In the oak stand garlic mustard decreased the decomposition of oak and sugar maple litter. However, the opposite occurred in the sugar maple stand. Our results support previous suggestions that garlic mustard invasions can alter litter decomposition rates but the magnitude and direction of these impacts may be affected by forest type.