Submitted to: Global Change Biology Bioenergy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/14/2010
Publication Date: 8/1/2010
Citation: Hickman, G.C., VanLoocke, A., Dohleman, F.G., Bernacchi, C.J. 2010. A Comparison of Canopy Evapotranspiration for Maize and Two Perennial Grass Species Identified as Potential Bioenergy Crops. Global Change Biology Bioenergy. 2:157-168. Interpretive Summary: A combination of many factors, most notably increasing demands for energy and a need to lower consumption of fossil fuels, has driven research to investigate renewable energy sources. Of particular interest to the Midwestern U.S. are bioenergy crops that are designed to offset fossil fuel consumption through conversion of biomass into ethanol or other liquid fuels. Currently, a large portion of corn harvest is devoted to ethanol production but next generation species such as switchgrass or miscanthus are identified as suitable replacements provided efficient conversion of cellulose to ethanol is engineered. With the potential for large-scale changes in the landscape to accommodate these cellulosic based bioenergy crops, many key questions related to environmental impacts are unanswered. In this study, we investigate the water use of switchgrass, miscanthus, and maize to determine the total water used during one growing season and how efficiently this water was converted to biomass. Of the three species, miscanthus used the most water, followed by switchgrass, and maize used the less. Most of the differences between maize and the perennial grasses was driven by a longer growing season; however physiological attributes are also likely involved. Despite the large differences in water use, when measured as a function of water use efficiency to biomass harvested, there were little differences between maize and miscanthus. Thus the increased water use is largely offset by the higher yields. If all of the above-ground biomass in the maize were harvested for cellulosic ethanol production, then maize once again becomes more water use efficient, but it is likely that consequences related to sustainability would become more apparent. These results suggest that there might be large scale environmental impacts of altering the landscape to accommodate bioenergy crops and that further, long term measurements are needed to better understand these consequences.
Technical Abstract: Perennial rhizomatous grasses (PRGs) that utilize the C4 photosynthetic pathway are considered one of the most promising vegetation types to accommodate a cellulosic feedstock for renewable energy production. The potential widespread use of biomass crops for renewable energy production has sparked numerous environmental concerns including the impacts of alteration of vegetation at the landscape scale on the hydrological cycle. We predicted that total seasonal evapotranspiration (ET) would be higher for perennial grasses relative to maize as a result of higher leaf area, higher above-ground biomass and prolonged growing seasons. To test this prediction, ET was determined using a residual energy balance approach during the 2007 growing season for three plots each of Miscanthus X giganteus (miscanthus), Panicum virgatum (switchgrass), and Zea mays (maize). When all canopies were completely closed, Miscanthus showed the highest rate of ET at 176 Wm-2, maize and switchgrass were 12.5% and 28% lower, respectively. Water use efficiency was lowest in PRGs due to prolonged seasonal ET without compensating gain in above-ground biomass for much of the growing season. Analysis of canopy differences suggests that changes in ET due to large-scale implementation of PRGs in the Midwestern USA will influence local and regional hydrologic cycles differently than traditional row crops.