Differential effects of two dominant plant species on community structure and invasibility in an old-field ecosystem

Aims In this study, we examined the effects of Solidago altissima (hereafter Solidago) and two species in the genus Verbesina, Verbesina virginica and Verbesina occidentalis (hereafter Verbesina), on the structure of an old-field plant community and establishment by an invasive plant species, Lespedeza cuneata (hereafter Lespedeza). Methods We removed Solidago, Verbesina and both Solidago and Verbesina from 4-m(2) plots in an intact old-field community during two growing seasons. We then quantified the effects of these removals on richness, evenness, diversity and composition of the subdominant plant community. We also measured the total aboveground biomass and the aboveground biomass of the subdominant community. To assess how these removals affected establishment by Lespedeza, we planted 20 seeds in each plot and tracked seedling emergence and survival for one growing season. Important Findings Subdominant community evenness and Shannon diversity were higher in plots from which Solidago and Verbesina were removed relative to control plots. However, there were no effects of dominant species removal on species richness or composition of the subdominant community. Total aboveground biomass was not affected by dominant species removal, suggesting that the community of subdominant species exhibited compensation. In fact, subdominant community biomass was greater when Solidago, but not Verbesina, was removed. Light availability was also greater in plots where Solidago was removed relative to control plots throughout the growing season. In addition, removal of dominant species, in particular Solidago, indirectly reduced the emergence, but not survival, of Lespedeza seedlings by directly promoting subdominant community biomass. Taken together, our results suggest that dominant old-field plant species affect subdominant community structure and indirectly promote establishment by Lespedeza.