Incorporating Uncertainty into a Life Cycle Assessment (LCA) Model of Short-Rotation Willow Biomass (Salix spp.) Crops

Abstract

To estimate fossil fuel demand and greenhouse gas emissions associated with short-rotation willow (Salix spp.) crops in New York State, we constructed a life cycle assessment model capable of estimating point values and measures of variability for a number of key processes across eight management scenarios. The system used 445.0 to 1,052.4 MJ of fossil energy per oven-dry tonne (odt) of delivered willow biomass, resulting in a net energy balance of 18.3:1 to 43.4:1. The largest fraction of the energy demand across all scenarios was driven by the use of diesel fuels. The largest proportion of diesel fuel was associated with harvesting and delivery of willow chips seven times on 3-year rotations over the life of the crop. Similar patterns were found for greenhouse gas emissions across all scenarios, as fossil fuel use served as the biggest source of emissions in the system. Carbon sequestration in the belowground portion of the willow system provided a large carbon sink that more than compensated for carbon emissions across all scenarios, resulting in final greenhouse gas balances of −138.4 to −52.9 kg CO2 eq. per odt biomass. The subsequent uncertainty analyses revealed that variability associated with data on willow yield, litterfall, and belowground biomass eliminated some of the differences between the tested scenarios. Even with the inclusion of uncertainty analysis, the willow system was still a carbon sequestration system after a single crop cycle (seven 3-year rotations) in all eight scenarios. A better understanding and quantification of factors that drive the variability in the biological portions of the system is necessary to produce more precise estimates of the emissions and energy performance of short-rotation woody crops.