Monday, November 13, 2006

Plant Diversity Increases Biomass Productivity in Perennial Polycultures.

Valentin D. Picasso, Iowa State University, 1207 Agonomy Hall, Ames, IA 50010 and E. Charles Brummer, University of Georgia, Athens, GA 30602.

Natural grasslands have shown a positive biodiversity-productivity relationship when manipulating species richness in a given environment. We hypothesized that agroecosystems, with higher soil fertility and species selected for maximizing productivity, may show a different relationship between these parameters than natural ecosystems. Our objective was to measure the effect on biomass productivity of increasing the number of plant species and functional groups in perennial herbaceous polycultures. Perennial species (n=7) were assembled in 49 combinations including all monocultures and selected polycultures of 2 to 7 species. Species were legumes (alfalfa, white clover, Illinois bundleflower), C3-grasses (orchardgrass, intermediate wheatgrass), and C4-grasses (switchgrass, Eastern gamagrass). In April 2003, 4m x 3m plots were sown in two Iowa locations in a lattice design with 3 replications per location. Soils were tilled prior to planting but no fertilizers or herbicides were applied. Plots were mowed twice during 2003 to control weeds. In 2004 and 2005, each plot was split into two management treatments: three harvests (May, July, September) and one harvest (October), clipped with a flail-type harvester. Aboveground plant biomass was sampled from two 0.09 m2 quadrats/plot before each harvest. Samples were sorted by species and weighed. Interactions were found between species combinations and year, location, and harvest management. Plots with alfalfa yielded on average twice as much as others at one location under three harvests, but at the other location or under a single harvest this dominance effect was not observed. Biomass productivity increased with the number of species and functional groups in all combinations of year, management, and location. For the three harvest management, biomass fit a quadratic curve for all locations and years, while for the one harvest management a linear curve fit best. Thus, the biodiversity-productivity relationship in perennial polycultures, while positive overall, is affected by species, environment, and management.

Handout (.pdf format, 123.0 kb)