Diversity- and density-mediated allelopathic effects of resident plant communities on invasion by an exotic plant

Abstract

Resident plants can exert allelopathic effects on introduced exotic plants, and resistance to exotic plant invasions usually increases with diversity and density of the resident plant communities. We hypothesize that allelopathic effects increase with increasing diversity and density of the community, and thereby contribute to the community’s resistance against invaders. To test these hypotheses, we conducted two greenhouse experiments in which we grew five individuals of the exotic invasive plant Solidago canadensis in pots without residents and in artificially assembled resident plant communities either with two levels of diversity (4 or 8 resident plant species) or two levels of density (8 or 32 individuals, representing 8 species). We used activated carbon as the allelopathy-neutralizer treatment in both the diversity and the density experiment. In the absence of activated carbon, S. canadensis grew worse in the presence of residents, and when diversity or density of the residents increased. These negative effects, however, largely disappeared in the presence of activated carbon, and at the highest resident density S. canadensis even performed better with than without activated carbon. Low invasibility of species-rich and dense plant communities is likely to be at least in part associated with increased allelopathy. Our study provides the first evidence that increasing allelopathy could be a mechanism underlying the diversity and density effects on community invasibility.