Huberty, L. 1994. Dominance, diversity, and resource competition in old-field plant communities. Ph.D. Dissertation, Michigan State University, East Lansing, Michigan, USA.

I conducted species removal and resource addition experiments in a first- and second-year old-field in southwestern Michigan to test two mechanistic hypotheses about the relationship between diversity and productivity: 1) plant diversity declines with increasing fertility due to increasing competition intensity from the dominant species, and 2) the time of resource addition within a season influences the magnitude of competition intensity, and consequently the effect of a dominant species on community structure.

In the first-year annual old-field the effect of the dominant species, Chenopodium album, on subordinate community biomass, evenness, diversity, and nitrogen acquisition depended on the time of nitrogen addition. When nitrogen was added early in the growing season (early July) both the dominant species and the subordinate community increases in biomass and total nitrogen content relative to the unfertilized control. However, when nitrogen was added mid- or late-season, the dominant species pre-empted the added nitrogen and exhibited a disproportionate increase in biomass relative to the subordinate community. The subordinate plant community had increased tissue nitrogen concentrations but did not increase in biomass. The degree of canopy development appears to determine the ability of dominant and subordinate species to acquire added nutrients and allocate them to increased growth rather than storage, and thus, influences the magnitude of the effect of the dominant species on diversity and community structure.

In the second-year old-field community the composition and abundance of species in the subordinate community varied seasonally, while Conyze canadensis was the clear biomass dominant. Biomass evenness and diversity but not richness declined with increasing C. canadensis biomass. Nitrogen addition reduced evenness and diversity but not richness, but the magnitude of this effect did not vary with the time of nitrogen addition. Rather the biomass of life-history/growth form groups and individual species varied with time of nitrogen addition. Light penetration was lower and nitrogen availability was higher in nitrogen addition plots early and mid-season, but by mid-August, resources declined to the same low level in control and nitrogen addition plots.

Finally, I present a general graphical and statistical approach to evaluate the absolute and relative response of a species biomass hierarchy to experimental treatments or environmental perturbations.

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