Greenhouse gas balance of Eucalyptus biomass fuel production from converted Japanese cedar and cypress forests: A temporal carbon balance assessment

Although the climate benefits of woody biomass are being questioned because of concerns over temporal carbon dynamics, Japan is considering the introduction of fast-growing Eucalyptus species by replacing cedar ( Cryptomeria japonica ) and cypress ( Chamaecyparis obtusa ). This study examined the greenhouse gas (GHG) balance of Eucalyptus biomass production through the conversion of 56-year-old cedar and cypress forests over a 56-year evaluation period. Two scenarios were evaluated: using cedar and cypress as cross-laminated timber (CLT) construction materials (Scenario 1) and as fuel (Scenario 2), with Eucalyptus as biomass fuel in both scenarios. Carbon dioxide emissions from cedar and cypress were counted as GHG emissions, whereas temporary carbon sequestration by Eucalyptus was treated as emission reductions. Scenario 1 considered the substitution effects of concrete, steel, and kerosene for lamina drying, whereas Scenario 2 evaluated only fossil fuel power generation substitution. Carbon dioxide emissions from cedar and cypress biomass in the initial year dominated GHG emissions (180.36 and 271.44 t-CO 2 /ha in Scenarios 1 and 2, respectively). Although the substitution effects from cedar and cypress utilization alone could not achieve net GHG reductions in any case, net reductions were achieved within the evaluation period in all cases through Eucalyptus fuel substitution. The timing of achieving emission reductions varied depending on the substitution target of Eucalyptus fuel. In comparison with conventional reforestation, Eucalyptus achieved earlier GHG reductions in all substitution cases. Although converting cedar and cypress forests to Eucalyptus requires careful consideration of temporary GHG increases, Eucalyptus introduction can serve as a viable climate-mitigation option. • Temporal GHG balance of Eucalyptus biomass conversion in Japan evaluated. • Combined life-cycle and t-year method captures biogenic carbon dynamics. • Eucalyptus introduction achieves earlier GHG reduction than reforestation. • Timber use of cedar–cypress mitigates initial emission surge post-harvest. • Framework aids policy on fast-growing species and carbon-neutral forestry.