Greenhouse gas emissions from round wood production in Japan

Wood has a long-standing history as an industrial material and fuel. Generic life cycle assessment data is therefore needed to assess the quantitative environmental impact of wood. The aim of this study was to estimate cradle-to-gate greenhouse gas emissions for round wood production of four common Japanese tree species. This study focused on Japanese cedar (Cryptomeria japonica), hinoki cypress (Chamaecyparis obtusa), Japanese larch (Larix kaempferi) and Sakhalin fir (Abies sachalinensis) because these four tree species constitute more than 80% of total round wood production in Japan. This study encompassed all major forestry activities, such as planting, thinning and final felling. In addition to these activities, indirect processes, such as construction of forest roads, were also included. The functional unit used for this study was 1 m³ over bark of green wood (ob). All 47 prefectures were modeled in order to investigate the site characteristics across Japan. Activity data such as yield per area, required seedling numbers, and thinning method were collected for each tree species and for each prefecture. By combining actual surveyed data and simulation data, information collection challenges on variations in sites and time were overcome. The results of this study suggested that greenhouse gas emissions relating to round wood production of Japanese cedar, hinoki cypress, Japanese larch, and Sakhalin fir were 21.7, 25.9, 24.9 and 25.5 kg-CO₂e/m³ ob, respectively. The weighted average value for all of Japan was estimated to be 23.0 kg-CO₂e/m³ ob. The amount of temporal carbon storage for these tree species was calculated as 585–762 kg-CO₂e/m³ ob. The results also revealed large variation between different prefectures across Japan. Regarding hinoki cypress, for example, the largest greenhouse gas emissions and land use values as an inventory indicator among evaluated prefectures were approximately 1.5 times and 2.7 times larger than the smallest emissions and land use values, respectively. A prefecture with a higher share of heavy forestry machinery tended to indicate relatively higher greenhouse gas emissions. A comprehensive analysis of the target geographical area is therefore required to produce values representative of the region of interest. Among the processes assessed, the final felling process contributed to the largest portion of greenhouse gas emissions in all tree species. An improvement of the energy efficiency of this process is therefore needed. Improving labor productivity would reduce greenhouse gas emissions resulting from commuting workers.