Cradle-to-gate inventory of wood production from Australian softwood plantations and native hardwood forests: Carbon sequestration and greenhouse gas emissions

Abstract

Life cycle assessment provides a useful means of estimating environmental impacts associated with different materials and production systems and is increasingly being used to assess net greenhouse gas (GHG) emissions. Here we report on a cradle-to-gate inventory of wood production, focussing on GHG emissions associated with wood from softwood plantations and regrowth hardwood native forests across Australia. The inventory was primarily based on data provided by forest growers, managers and contractors across seven large case study regions. Modelling was used to combine data from different operations, account for upstream processes associated with the production of fuel and materials used, and to estimate total GHG emissions associated with the production of high- and low-value sawlogs, pulplogs, woodchips and other logs. We considered only the case of managing established forests for wood production, where the C stock of these forests could be considered to be constant over the long-term. Emissions were expressed in terms of m3 product (i.e. functional unit) and were allocated to products on an economic basis. Total GHG emissions associated with wood production, harvest and transport were 25.9 (SD 8.0) kg carbon dioxide equivalent (CO2-e) m−3 for an average plantation softwood log and 70.6 (SD 1.8) kg CO2-e m−3 for an average native hardwood log. The largest contributors to total emissions per unit wood production in softwood plantations were log haulage (37%) and harvesting (21%), whilst in native hardwood forests they were slash burning (46%) and log haulage (23%). Sensitivity analyses indicated that the amount of forest residues burnt in native hardwood forests had a relatively large influence on CO2-e emissions while wood density had a large impact on net carbon sequestration for both native hardwood and plantation softwood logs, indicating that improving estimates of these should be a priority. Estimated net carbon sequestered in an average plantation softwood log was 761 (SD 27) kg CO2-e m−3 and in an average native hardwood log it was 902 (SD 162) kg CO2-e m−3. Total net GHG emissions (assuming no net biogenic emissions) represented 3.3% and 7.3% of the estimated carbon content of an average log for plantation softwood and native hardwood, respectively. Thus, logs from sustainably managed forests delivered to downstream processing plants represent significant net stores of carbon and have potential to substitute for more GHG-intensive raw materials.