Species evenness, not richness, has a consistent relationship with productivity in old-field vegetation

The relationship between productivity and species diversity was investigated at the quadrat level for three old-field plant communities that varied in time since the last major disturbance from cultivation. A positive relationship between productivity (estimated by above-ground dry biomass) and the species richness component of diversity was detected only for quadrats from the most recently disturbed community. The communities that experienced longer post-disturbance time showed no significant relationship between productivity and species richness. Quadrat productivity, however, was negatively related to the species evenness component of diversity in all three communities, i.e., regardless of time since disturbance. Furthermore, analyses of covariance using log biomass, species richness and species evenness as response variables, log soil nitrate concentration as a covariate and time since disturbance as a factor revealed that residual log biomass was significantly negatively correlated with residual species evenness, but was not significantly correlated with residual species richness. These results support the view that the productivity of plots within natural vegetation is related more predictably to the relative composition of species (reflected by evenness) than to the number of species present, especially as succession progresses.