Life cycle assessment of mulch use on Okanagan apple orchards: Part 2 - Consequential

Wood and bark chip mulch has been shown to reduce net orchard greenhouse gas (GHG) emissions on an Okanagan Valley (British Columbia, Canada) apple orchard. However, this benefit was shown to be outweighed by the (attributional) life cycle impacts associated with mulch production. The current study expanded the scope of prior investigations to perform a consequential life cycle assessment of the impacts of increasing wood chip/bark mulch use in the production of apples on Okanagan orchards. This assessment included the impacts of the orchard system as well as other current alternative uses of wood chip/bark mulch which included bioenergy production and paper manufacturing. Many environmental impact categories were examined including human toxicity, freshwater aquatic ecotoxicity, depletion of abiotic resources (elements, ultimate reserves), photochemical oxidation, ozone layer depletion, terrestrial ecotoxicity, acidification potential, climate change, eutrophication, land use – land competition, and energy use (including non-renewable: fossil, nuclear, primary forest; and renewable: biomass, geothermal, solar, water and wind). One scenario was modelled to represent the case in which no mulch was used on orchards (only used for alternative products). A second model was created to represent the marginal impacts of adding the amount of mulch to an apple orchard necessary to produce 1 kg of apples (0.575 kg bark and 0.144 kg wood chips). These amounts of bark and wood chips were assumed to be taken away from their current alternative uses (co-generation for bark and paper production for wood chips), thereby decreasing the amount of electricity and heat produced by bark by 0.653 kWh electricity and 0.653 MJ heat, and the amount of paper produced by wood chips by 0.144 kg paper. In turn, these amounts of electricity, heat and paper were assumed to be produced by their marginal production technologies – hydro-electric generation for electricity, natural gas for heat, and recycled paper for paper production. Finally, the scenarios were modelled assuming the marginal market for co-generation from bark was in Washington, USA rather than the Okanagan, as a sensitivity analysis. The results did not show a clear environmental benefit to either using or not using mulch on orchards. In the scenario in which bark mulch was assumed to be used either on apple orchards or for co-generation in British Columbia, impacts in 14 categories (including climate change, eutrophication, acidification, all toxicities, land use and some renewable energy use) were lower when mulch was used on the orchard, and results for 5 categories (including some non-renewable and renewable resources/energy use) were higher. When bark mulch was assumed to be used either on orchards or for co-generation in Washington, terrestrial ecotoxicity, land use, biomass and solar energy use were lower when mulch was used on the orchard, and all others (15 categories) were higher. There was a large amount of uncertainty in the model, coming from data variability, data quality and impact assessment uncertainty. Overall, the orchard system played a significant role in the impact assessment results, and was the main contributor to the overall uncertainty. Based on these results, mulch use on orchards cannot be recommended to reduce environmental impacts, but the marginal impacts of using mulch warrant further investigation.