Agricultural and biofuel implications of a species diversity experiment with native perennial grassland plants

Two primary approaches to perennial biofuel crop production studied so far are fertilized grass monocultures and low-input high-diversity grasslands. While high-yielding perennial grass varieties are being developed in fertilized monocultures, breeding for yield in low-input high-diversity systems would be difficult. Before initiating breeding for low-input systems, it is therefore important to know the minimum number of functional groups and species required for maximum biomass harvest from low-input grasslands. We controlled the number of perennial grassland species in 168 plots in Minnesota, USA. Species were selected at random from a pool of 18, and 1, 2, 4, 8, or 16 were planted in each plot. Aboveground biomass was measured annually, and the plots were burned each spring. We found a strongly positive log-linear relationship between average annual aboveground biomass and planted species number, but a large proportion of plot-to-plot variability remained unexplained. We performed a conditional analysis of the aboveground biomass data to determine whether considering species identity would reduce the minimum number of species necessary in order to achieve yields similar to the highest diversity treatments. A model that accounted for the presence of legumes in general, and for the presence of the legume species Lupinus perennis in particular, showed no increase in biomass yield with increased species number. Over 11 years, average yields of L. perennis/C₄ grass bicultures were similar to those of 16-species (maximum diversity) plots, and both were >200% greater than the average of monocultures. Thus, under low-input conditions, the choice of the appropriate few perennial plant species for each location might result in systems with biomass yields similar to those from high-diversity systems. Because breeding biofuel crops in diverse mixtures would introduce complexity that is unwarranted in terms of maximum biomass yield, the first biofuel crop breeding programs for low-input systems are likely to accelerate progress by focusing on grass-legume bicultures.