The role of dispersal limitation and site invasibility in the spread of garlic mustard

Background/Question/Methods

The goal of this ongoing study is to determine the relative importance of dispersal limitation and site invasibility as garlic mustard, Alliaria petiolata, spreads through an oak woodland at the Macalester field station, located in Inver Grove Heights, MN.  To date, we have mapped the distribution garlic mustard in a 6 ha area in a steep bluffland habitat and measured a variety of environmental and biological variables to help us begin to understand the extent to which dispersal limitation and invasibility are influencing its spread.  We conducted this study at two spatial scales, 20 x 20 m cells and 1 x 0.5 plots.  At both scales, we measured, slope, elevation, aspect, and photosynthetically active radiation, (PAR), which we compared to garlic mustard cover at the respective scales. On the 1x0.5 m scale, percent herbaceous ground cover by species, bare ground cover, and leaf litter cover were also measured and compared to garlic mustard cover as well as to the number of first- and second-year plants in the plots.

Results/Conclusions

At the 20x20 m scale, the data showed that garlic mustard cover was negatively associated with  elevation (p<0.0001) and positively associated with slope  (p<0.05).  We found no correlation between garlic mustard cover and aspect nor the amount of light in the understory at the 20 x 20 scale. At the 1x0.5 m scale, plots with a south-eastern aspect were more likely to have higher garlic mustard abundance than any other aspect (p<.001).  There was no significant correlation between any of the measures of garlic mustard abundance and shrub, seedling/sapling, or other herbaceous ground cover, nor with litter cover or percent bare ground or species richness.  Our results show that both site-specific factors and dispersal limitation are likely influencing garlic mustard distribution in the oak woodland.  The role of dispersal is especially apparent on the larger spatial scale, where garlic mustard is more common at lower than high elevations, suggesting the role of gravity in moving seeds downhill.    However, site invasibility factors seemed to play a larger role at small scales.  In particular, the findings suggested that microsites facing southeast may favor the establishment and growth of garlic mustard because these sites receive more direct sunlight prior to leaf out by the trees, a major period of growth for garlic mustard.  We plan to test these preliminary findings with additional monitoring and field experiments.