Plant neighborhood diversity and production

We examined the role of species and functional-group diversity in the perfo rmance of model plant assemblages at the level of the plant neighborhood (a central, focal, or "target" plant and its neighbors). Three hundred and si xty plant neighborhoods were constructed with one to four species (drawn fr om a pool of six) and one to three functional groups. Our assemblages were modeled after prairie grassland communities typical of the Midwestern regio ns of North America. The three functional groups were legumes (Vicia villos a and Astragalus canadensis), grasses (Panicum virgatum and Bouteloua graci lis), and non-leguminous forbs (Rudbeckia hirta and Achillea millefolium) k nown to co-occur in nature and common to Cedar Creek Natural History Area, Anoka County, Minnesota. We measured total above- and belowground biomass, soil moisture, canopy cover, CO2 photosynthetic drawdown, and rate of decom position. Performance of neighborhoods, as measured by these six variables, showed significant associations with both species and functional-group ric hness, with greatest magnitude of response variables occurring when both sp ecies and functional-group richness were high. Specifically, all variables except decomposition showed significant associations with both species and functional-group richness. We suggest that spatially well-mixed, functional ly diverse, and species-rich grassland assemblages may outproduce aggregate d, depauperate (poor in either functional groups or species) assemblages.