Foster, B. 1997. Plant competition and diversity in relation to productivity in old-field plant communities. Dissertation, Michigan State University, East Lansing, Michigan, USA.

I conducted three manipulative field experiments: 1) to examine the effects of living plant neighbors and accumulated plant litter on the recruitment and growth of target plants across gradients of plant productivity and community biomass, and 2) to examine the impact of nutrient enrichment, increased productivity and litter accumulation on species richness in southwest Michigan old-fields.

Plant removal experiments conducted at eleven old-field sites showed that the surrounding plant community suppressed the recruitment and growth of three native grass species (Andropogon gerardi, Schizachyrium scoparium, Sorghastrum nutans) across a wide range of productivity. The net suppressive effect of the surrounding plant community on two of the three target species increased in magnitude with community biomass as predicted by biomass-dependent theories of plant competition. Non-linear dependence of net community effects on community biomass indicated that biomass-dependent theories of competition may be most applicable to narrow ranges of low productivity. The degree to which the effects of plant litter contributed to the net effect of the plant community on target plant performance depended on the life history stage examined. Inhibitory litter effects on seedling recruitment increased with community biomass and contributed to the net effect of the community on recruitment at sites of intermediate community biomass, but litter had little impact on established transplants at any of the sites.

In a two-year experiment carried out in a grass-dominated old-field, nitrogen enrichment increased living plant and litter biomass and reduced species richness by inhibiting the colonization of forb species. Community responses to litter removal and litter addition treatments that were carried out in conjunction with nitrogen enrichment showed that declines in species richness and forb density were due to effects associated with increases in both living plant and litter biomass.

The results highlight the potential importance of effects associated with both living plant biomass and accumulated plant litter in influencing plant recruitment, species diversity, and the distribution of native grasses across old-field productivity gradients. The general significance of these findings are discussed with respect to existing theories of plant community organization.

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