Diversity, productivity, and stability in perennial polycultures used for grain, forage, and biomass production

The objective of this dissertation was to determine to what extent plant species diversity affects biomass and seed productivity, weed invasion, and stability in perennial herbaceous polycultures across three years, two harvest management regimes, and two locations in central Iowa, USA. Average biomass productivity consistently increased in polycultures with increasing species richness across all environment-years. In most situations, polycultures were more productive than the average of monocultures, but not more productive than the best adapted species in monoculture for each environment. Polyculture overyielding was due to complementarity among species in the community rather than to selection effects of individual species at all richness levels across environments and was likely explained by legume-grass facilitation. Polycultures with high richness had lower variability in yield (i.e., greater stability) than the highest yielding monocultures. Seeds of two perennial grain species were harvested; intermediate wheatgrass produced up to 65.8 +/- 6.5 g m-2 of seed and Illinois bundleflower up to 55.0 +/- 8.1 g m-2. Plant breeding and better agronomic management are needed to increase these yields in the near future to be practical for production situations. The mixture comprising both perennial grains produced as much seed as the best yielding monoculture each year. Polycultures of Illinois bundleflower with C4 grasses and polycultures of intermediate wheatgrass with legumes produced as much seed as the monocultures. Weed biomass decreased exponentially with seeded species richness in all environments. Most polycultures exerted greater weed suppression than perennial grain monocultures but also they had reduced seed yields. Breeding and management of crop mixtures to optimize the trade-off between seed yield and weed suppression is a central challenge for the development of perennial polyculture systems. The dialogue between Ecology and Agriculture provides a basis for designing sustainable production systems.