Linking plant community composition to bioenergy potential in conservation grasslands

Background/Question/Methods

Marginal lands enrolled in state or federal conservation programs and planted with perennial grassland cover can serve as a source of bioenergy. Conservation grasslands are often composed of mixtures of native plants from various functional groups including warm- and cool-season grasses, legumes, and other forbs. These can provide substantial biomass for energy while providing other ecosystem services, including carbon sequestration, soil and water conservation, and wildlife habitat. Harvesting biomass from conservation grasslands could act as a surrogate for other management techniques (e.g. prescribed fire) and possibly fund the maintenance and expansion of conservation grassland programs. But first, research is needed to identify the effects of biomass harvest on plant community composition. Furthermore, identifying relationships between plant community composition and bioenergy potential would help managers maximize yields. We measured biomass yields and theoretical ethanol potential from state-managed conservation grasslands in three regions of Minnesota. We harvested biomass in late autumn of 2009 – 2012 using production-scale methods. Data from vegetation surveys were used to identify relationships between plant functional groups and bioenergy potential. The relative abundance of certain plant species and functional groups was monitored for the duration of the project to determine if changes occurred as a result of biomass harvest.

Results/Conclusions

Harvested biomass yields averaged 2.5 Mg ha^-1, and when multiplied by the average theoretical ethanol potential (L Mg^-1), translated to a land ethanol yield averaging 1156 L ha^-1. The cover of non-legume forbs was a better predictor of biomass yield than warm-season grasses, with a positive correlation in the south and a negative correlation at more northern sites, which was due to the presence/absence of specific forb species. Non-legume forbs decreased the theoretical ethanol potential of harvested biomass, but since most of the variation in land ethanol yield was due to biomass yield, this result is not critical to bioenergy producers. The relative abundance of plant functional groups remained the same in harvested plots from 2009 to 2011. Species richness increased in both harvested and unharvested plots, but there is some evidence that the increase was greater in harvested plots. Data from 2012 will clarify this preliminary result. The frequency and relative cover of non-native species decreased through time in both treatments, but the cover of noxious weeds remained the same. Our results support a practice where land managers could annually harvest conservation grasslands without affecting future yields or altering the species composition of the habitat in the short-term.