Diversity effects in early- and mid-successional species pools along a nitrogen gradient

Biodiversity experiments with grassland species have shown that plant productivity commonly increases with species richness and that this increase can be stronger in nutrient-rich than in nutrient-poor environments. It has been suggested that these effects are due to functional diversity among species. To investigate this, we established five early- and five mid-successional pools of six species each, expecting stronger effects in the latter. The 10 six-species mixtures and the 60 corresponding species monocultures were grown at four levels of soil nitrogen. We used five species traits (plant length, leaf length, leaf tissue density, specific leaf area, leaf nitrogen concentration) to calculate a functional diversity index weighted by relative species densities in mixture (Qd/m). This functional diversity index and aboveground biomass production (standing crop at end of experiment) in mixture were similar between early- and mid-successional communities but varied among communities within successional status. Mixture biomass was between the mean and maximum monoculture biomass, indicating that the latter species did not dominate mixtures and complementarity was not strong enough to cause transgressive overyielding. As expected, high mixture biomass was positively correlated with Qd/m. Also as expected, net biodiversity effects (mixture-mean monoculture biomass) were stronger in mid- than in early-successional pools. Net biodiversity effects increased with nitrogen level in some pools but decreased in others. The increase was consistent with a marginally positive effect of nitrogen level on functional diversity and a positive relation between functional diversity and net biodiversity effect.