CARBON FOOTPRINTING A life cycle greenhouse gas inventory of a tree production system

Purpose This study provides a detailed, process-based life cycle greenhouse gas (GHG) inventory of an ornamental tree production system for urban forestry. The success of large-scale tree planting initiatives for climate protection depends on projects being net sinks for CO2 over their entire life cycle. However, previous assessments of urban tree planting initiatives have not accounted for the inputs required for tree production in nurseries, which include greenhouse systems, irrigation, and fertilization. A GHG inventory of nursery operations for tree production is a necessary step to assess the life cycle benefits or drawbacks of large-scale tree planting activities. Methods Using surveys, interviews, and life cycle inventory databases, we developed a process-based life cycle inventory of GHG emissions for a large nursery operation in California, USA. Results and discussion The inventory demonstrated that 4.6 kg of CO2-equivalent is emitted per #5 (nominally a 5gallon) tree, a common tree size produced by nurseries. Energy use contributed 44% of all CO2-equivalent emissions, of which electricity and propane constituted 78%. Electricity use is dominated by irrigation demands, and propane is used primarily for greenhouse heating. Material inputs constituted the next largest contributor at 36% of emissions; plastic containers contributed just over half of these emissions. Transport emissions accounted for 16% of total nursery GHG emissions. Shipping bamboo stakes from China (43%) and diesel fuel consumed by nursery delivery trucks (33%) were the largest transport emission sources. Conclusions GHG emissions from the tree production life stage are 20% to 50% of mean annual CO2 storage rates based on urban tree inventories for three California cities. While considering nursery production alone is insufficient for drawing conclusions about the net climate change benefits of tree planting initiatives, the results demonstrate that nursery production emissions are modest compared with CO2 storage rates during tree life. Identifying key sources of emissions in the nursery tree production system can help operators reduce emissions by targeting so-called hot-spots. In particular, switching to renewable energy sources, capitalizing on energy and water efficiency opportunities, container light-weighting, and sourcing bamboo stakes from producers closer to the point of use are potential avenues for reduced emissions.