The utility of high-diversity prairie mixtures as bioenergy feedstocks

Two of the more pressing, yet opposing, ecological challenges that we face at the global-scale are the loss of biodiversity and rising demand for energy. Many ecological experiments have shown the importance of biodiversity for ecosystem services and functions, but simultaneous demand for energy has led to greater conversion of natural landscapes to low-diversity energy crops (e.g., corn for ethanol). One potential solution to these seemingly opposing issues would be to grow diverse native vegetation for bioenergy. Native tallgrass prairie produces large amounts of aboveground biomass but also provides great habitat for wildlife and other ecosystem services. In this study, we compared the productivity, yearly biomass variability, and invasion resistance of four potential bioenergy feedstocks with contrasting diversity: 1 species - a switchgrass monoculture; 5 species - a mix of C4 grasses; 16 species - a mix of grasses, forbs and legumes; and 32 species - a mix of grasses, sedges, forbs, and legumes. Each diversity treatment was replicated four times on three different soil types (clay loam, loam, and sandy loam) for a total of 48 plots (0.33-0.56 ha each). We compared productivity by harvesting all plant material in 10 randomly placed 0.3m2 quadrats per plot. Species composition was compared using basal area sampling of 10 randomly placed 0.1m2 quadrats per plot. Across soil types, the 1-, 16-, and 32-species treatments produced the same amount of aboveground biomass over the 5-yr study, with the 1-species treatment producing more biomass than the 5-species treatment. Although the overall productivities of the 1-, 16-, and 32-species treatments were similar, the relative ranking of the four diversity treatments varied between soil types. Weed biomass was higher in low-diversity treatments than high-diversity treatments. Year-to-year variation in productivity did not differ between treatments. My results show that diverse mixes of native perennial vegetation are as productive as a switchgrass monoculture over a 5-yr period suggesting that they are reliable source of biomass for bioenergy. However, differences in the relative ranking of the four diversity treatments, between soil types suggest that bioenergy seed mixes must be tailored to site characteristics to maximize productivity and stand success