Garlic mustard effects on landscape-scale temperate forest plant assemblages
It is well established that invasive plants can alter species composition and diversity as well as ecosystem function in communities that they invade. Whether and how species of similar ecosystems respond in predictable ways to invasion at the landscape scale is less understood. For instance, there are few studies documenting whether northeastern forest understory plant communities show similar compositional shifts in the presence of invasive plants. Studies that compare invaded and non-invaded communities at the landscape scale are critical for making broader ecological interpretations about the invasion process and its potential to disrupt native community structure. In addition, this approach can help guide invasive plant management, which is frequently done on an ad-hoc basis at a local (parcel) scale. We studied the effects of the widespread invasive plant, garlic mustard (Alliaria petiolata), on eight different parcels of northeastern forest understory plant communities over two years (2013-2014). Plant species composition, functional group identity, and species densities were contrasted with garlic mustard presence and abundance across invaded and similar non-invaded plots using univariate and multivariate analyses. These analyses were used to assess the degree of site-specificity vs. generalizable patterns of plant community assembly in the presence or absence of the invasive.
Garlic mustard presence and density, and parcel, had a strong influence on plant species community structure in both years. Garlic mustard presence was associated with a greater abundance of invasive woody species, which drove a higher Shannon diversity in invaded plots compared to non-invaded. Species richness, meanwhile, was higher in non-invaded compared to invaded plots, due to non-invaded areas containing more forb and native shrub species. When analyzed by plant functional group, plant communities in non-invaded areas were distinct from those in invaded areas. Overall plant species composition, however, was driven by parcel, rather than invasion status. We conclude that garlic mustard invasion can result in recognizable assemblages of plant species that are maintained at the landscape scale. These communities are less species rich than their non-invaded counterparts and also are more likely to host one or more woody invasive species. However, the clustering of plant communities by parcel also suggests the importance of site-specific conditions, and that management practices should reflect these differences as opposed to using a one size fits all approach for managing garlic mustard invasions.