Dominant plant species affect community structure and control the establishment of an exotic species in an old-field system

A growing number of studies have highlighted how dominant species affect community structure and a community’s susceptibility to invasion by exotic species. In this study, we examined the effect of two co-dominant plant taxa,  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 of an invasive plant species, Lespedeza cuneata (hereafter Lespedeza).  In 2 ´ 2 full factorial experiment, we removed Solidago, Verbesina, and both Solidago and Verbesina from 4 m² plots.  To assess how these taxa affected recruitment by an invasive species, we added 20 Lespedeza seeds to specific locations in each plot.  Evenness and diversity in the community of subdominant species were 20% higher in plots from which either Solidago or Verbesina were removed in comparison to control plots. However, there were no effects of species removal on species richness in the subdominant community. Total community biomass was not affected by dominant taxon removal indicating compensatory responses of subdominant species.  In fact, subdominant biomass was 40% greater and light availability 35% greater in plots from which Solidago but not Verbesina was removed.  In addition, the removal of Solidago decreased recruitment of Lespedeza by 26%.  Taken together, these results suggest that particular dominant taxa can have dramatic effects on the structure and function of plant communities and on invasibility. Moreover, the removal of Solidago promoted compensatory responses from the subdominant community (increase in aboveground biomass) which was negatively related to invasive species establishment.