UC Santa Cruz

English ivy has become a common invader in Seattle's urban parks and in forests throughout the Pacific Northwest. Despite a great deal of concern over the potential impacts of this species, no studies have investigated ivy's effects on native vegetation in this region. In this study, paired comparisons between ivy-invaded and adjacent non-invaded plots in three Seattle parks were used to quantify changes associated with ivy invasion in the forest understory. Species diversity, percent cover, and tree regeneration were surveyed. Differences in species diversity, calculated as both richness and evenness, were not significantly different between invaded and non-invaded plots. Ivy-invaded plots did have significantly higher total cover, and significantly lower non-ivy cover, than non-invaded plots. The reduction of percent cover in invaded plots was primarily due to the loss of native shrubs. A plot where ivy had been removed over five years was also surveyed, and percent cover in this plot showed intermediate values relative to invaded and non-invaded plots, for both total cover and cover of native shrubs. The number of trees regenerating in the understory was higher in invaded plots, though this difference was not significant. These results suggest that English ivy invasions have substantial impacts on understory cover, and may influence the species composition and diversity of forest communities over the long-term by increasing vegetative cover and suppressing dominant native shrubs.