After a disturbance, establishment of invasive plant species may increase because invasive species sequester unused resources more quickly than native species. Likewise, soil nutrient enrichment may greatly increase the growth and establishment of invasive species if native species respond slowly to increased nutrients. I tested the effects of disturbance and soil nitrogen enrichment on the seedling establishment, rosette growth, and rosette survival of garlic mustard (Alliaria petiolata). I predicted a positive effect of both treatments on garlic mustard growth and survival. Nine blocks were randomly placed within an Iowan forest to capture the variability of vegetation cover, soils, and topography. A block contained 18 0.25 m2 plots. In nine of the plots, one of three nitrogen treatments and one of three disturbance treatments were applied. In 2005, I added 1000 garlic mustard seeds to each of the nine plots. Each plot was paired with a non-seed addition plot that received similar nitrogen and disturbance treatments. From 2005-2007, I estimated percent native species cover, percent garlic mustard cover, and counted individual garlic mustard plants every four weeks during the growing season.
Results/Conclusions
Percent native species cover decreased with increased nitrogen in 2005, was negatively correlated with percent garlic mustard cover in 2006, and decreased with both increased nitrogen and percent garlic mustard cover in 2007. Percent garlic mustard cover was negatively correlated with percent native species cover in all years but also increased with disturbance in 2007. A higher number of new garlic mustard seedlings emerged from plots with no or medium disturbance in 2006. The overwinter survival of garlic mustard rosettes from 2005 to 2006 was highest in control or medium disturbance plots while no rosettes survived in high disturbance plots. Between 2006 and 2007, overwinter survival of rosettes did not differ significantly with disturbance or nitrogen. However, it differed significantly between blocks. Rosette survival over the summer did not differ significantly with nitrogen or disturbance in any year. Location of the blocks affected rosette survival, suggesting that water or light availability may have been a factor. Nitrogen did not have significant impacts on garlic mustard. However, disturbance had a positive effect on rosette and adult size but a negative effect on seedling establishment and overwinter survival. These results suggest that the role of disturbance in plant invasions depends on when it occurs and what plant life stage is affected.
