Extended leaf phenology as an invasion mechanism for Alliaria petiolata

Background/Question/Methods

Extended leaf phenology is a commonly cited invasion mechanism for exotic invaders of Eastern deciduous forests, yet evidence that extended invader phenology results in an impact on the native plant community is largely anecdotal.  Garlic mustard (Alliaria petiolata) is an herbaceous invader of Eastern deciduous forests that germinates early in the spring, several weeks prior to leaf emergence in native species, and overwinters as a rosette during its first year. High light conditions in early spring and late fall/winter may promote garlic mustard dominance and mediate its negative impact on native diversity. 

We conducted an experiment to determine the role of extended phenology in mediating garlic mustard’s impact on native species.  We established native community mesocosms outdoors in Bloomington, IN.  Community treatments included natives alone (6 species), garlic mustard alone, and both together (invaded community). Using 95% shade cloth, we manipulated light availability throughout the season.  Light treatments included 1) shading year-round to limit garlic mustard’s access to seasonal light, and 2) shading when the local tree canopy was closed to mimic natural forest conditions.  Garlic mustard leaf diameter and number were monitored throughout the experiment.  After one growing season, all plants were harvested, dried, and weighed.  

Results/Conclusions

Garlic mustard leaf diameter and leaf number were consistently higher when plants had access to early spring light than when they were shaded all year.  However, these responses to light treatments converged over the course of the experiment, resulting in smaller differences at time of harvest than in late spring.  For one native species (Eupatorium rugosum), garlic mustard invasion resulted in growth reduction when the invader had access to early spring light, but not when shaded throughout the year.  This indicates that access to early spring light may be an important mediator of garlic mustard’s impact on natives.  However, this pattern did not occur with other species, indicating that the importance of extended leaf phenology as an invasion mechanism may be specific to certain native functional groups.  Future work should examine the importance of extended leaf phenology in mediating impact of other invasive species, as well as the role of interactions between extended phenology and other invasion mechanisms.