Native perennial grassland species for bioenergy: Establishment and biomass productivity

Proposed perennial bioenergy cropping systems include both native grass monocultures and polycultures of grasses and forbs. We determined the effect of species richness and composition on establishment and initial biomass production of native plant polycultures. Twelve treatments with varying levels of species richness (1-24 species) were established. Establishment success and yield varied over eight locations. The number of species established in polyculture increased linearly as the number of species seeded increased. Average biomass yield ranged from 1.2 to 6.0 Mg ha^-1 with the highest yielding treatments being grass monocultures or an eight species grass-legume mixture. An increase in species richness from one to eight species increased yield an average of 28%, but increasing species richness from 8 to 12 or 24 species had no yield advantage at most locations. Early successional species, Canada milkvetch (Astragalus canadensis L.) and Maximilian sunflower (Helianthus maximilian Schrad.), were dominant in mixtures and contributed a majority of the biomass to the yield. Even in high diversity plots, biomass was from only a few plant species with a single species dominating the mixture. Our results suggest that selected low diversity mixtures (one to five species) likely offer the best combination of species establishment and high yield during stand establishment. However, we expect that early successional species that were dominant during the establishment phase of our experiment will contribute less biomass as stands mature and later successional species will become dominant and provide greater biomass.