Competitive Interactions in Plant Communities of Different Successional Ages

This research was conducted to test two predictions about niche and competitive relationships in the structure of plant communities. It has been demonstrated that early successional species of plants have broad, overlapping niche occupation on many gradients whereas later successional species show more niche differentiation. We predicted that, if the differences in resource use were for contested resources in limited supply, the more niche—differentiated species should: (1) experience less biomass reduction in pairwise interspecific competition than should broadly overlapping species of early succession, and (2) be able to convert relatively more of the available resources into plant matter as a heterospecific assemblage than can the conspecific stands from that assemblage. Six species from each of early (annual), mid— (early perennial), and late— (prairie) successional plant communities were grown in containers singly, in con— and heterospecific pairs, and in pure and mixed stands. Competitive interactions were assessed by comparing biomass production of species under the various treatment conditions. The late—successional community, in which we have previously shown more niche separation on several axes, showed a significantly higher mean ratio for the mass of individuals grown in heterospecific pairs divided by the mass of individuals of the same species grown in conspecific pairs than did the early successional community. All heterospecific pairs in the prairie and early perennial communities had a winner (ratio of mass in heterospecific pair to mass in conspecific pair >l.0) and a loser (ratio <1.0), while for most pairs in the annual community both competitors had ratios of <—l.0. Annuals showed the most reduction in total mass and prairie species the least when the mass of an individual grown in a heterospecific pair was compared to the mass of an individual of the same species grown by itself. Therefore, there was less competitive reduction in the more niche—differentiated community, the prairie. There was some evidence that mixtures of the more niche—differentiated late—successional species may be able to convert relatively more of the available resources into plant matter. The prairie assemblage had a slightly greater ratio of mixed stand to pure stand biomass with more variation among species than did the annual assemblage. There was also a tendency for the pairs of prairie species to have higher relative and absolute yields in mixture than in conspecific pairs. Thus members of the late—successional community experienced less competitive reduction in the presence of heterospecific neighbors than did species of early succession. This is strong evidence that niche reduction and separation are effective in reducing competition within a community and that selection to reduce competition could have been more important in the evolution of late—successional species than for early successional species.