Forest bioenergy climate impact can be improved by allocating forest residue removal

Abstract

Bioenergy from forest residues can be used to avoid fossil carbon emissions, but removing biomass from forests reduces carbon stock sizes and carbon input to litter and soil. The magnitude and longevity of these carbon stock changes determine how effective measures to utilize bioenergy from forest residues are to reduce greenhouse gas (GHG) emissions from the energy sector and to mitigate climate change. In this study, we estimate the variability of GHG emissions and consequent climate impacts resulting from producing bioenergy from stumps, branches and residual biomass of forest thinning operations in Finland, and the contribution of the variability in key factors, i.e. forest residue diameter, tree species, geographical location of the forest biomass removal site and harvesting method, to the emissions and their climate impact. The GHG emissions and the consequent climate impacts estimated as changes in radiative forcing were comparable to fossil fuels when bioenergy production from forest residues was initiated. The emissions and climate impacts decreased over time because forest residues were predicted to decompose releasing CO 2 even if left in the forest. Both were mainly affected by forest residue diameter and climatic conditions of the forest residue collection site. Tree species and the harvest method of thinning wood (whole tree or stem-only) had a smaller effect on the magnitude of emissions. The largest reduction in the energy production climate impacts after 20 years, up to 62%, was achieved when coal was replaced by the branches collected from Southern Finland, whereas the smallest reduction 7% was gained by using stumps from Northern Finland instead of natural gas. After 100 years the corresponding values were 77% and 21%. The choice of forest residue biomass collected affects significantly the emissions and climate impacts of forest bioenergy.

Time-Dynamic Effects on the Global Temperature When Harvesting Logging Residues for Bioenergy

“…This means that during the remaining years of the forest rotation, the southern forest would emit more CO 2 due to decomposition if the logging residues were not harvested. As a result, the climate mitigation effects per unit of DH produced during one forest rotation from one forest stand would be larger when logging residues are harvested in the south of Sweden, which agrees with the conclusions found in Repo et al [10]. The life cycle impact assessment showed that the average yearly temperature change during the 50 years following the harvest of logging residues was 0.024, 0.030 and 0.032 10 -15 K MJ -1 heat for the southern, central and northern forest stands, respectively, when logging residues were harvested for bioenergy, not including the substitution effect of replacing fossil fuel.…”

“…The GWP 100 of the three forest systems was approximately 3.7-3.8 g CO 2 -eq per MJ fuel, only including non-biogenic GHG emissions from forwarding, chipping, transportation, ash recycling and combustion. Previous studies have shown GWP in the range of 1.8-11 g CO 2 -eq MJ -1 fuel [20,19,55,10]. However, comparing results from different LCA studies are often problematic due to differences in system boundaries, functional unit and choice of allocation.…”

“…This time aspect is especially important to consider when studying conventional forests with longer rotation periods. The decomposition rate also varies between different climate zones, which has been shown to influence the overall climate impact of a bioenergy system [10]. As Sweden covers several climate zones, with warm-temperate zones in the south and cold-temperate zones in the north, these variations are important to consider.…”

Product and Residue Biomass Equations for Individual Trees in Rotation Age Pinus radiata Stands under Three Thinning Regimes in New South Wales, Australia

“…Y pul p = r 11 D r 12 1 + r 11 D r 12Ŷ prod + ε 12 (12) where r 11 = a 12 /a 11 , and r 12 = b 12 − b 11 . In the same way, a system of allocative biomass equations was specified to allocate the estimated total residue biomass in Step 1 to the three subcomponents: res + ε 23 (13) where Y stump , Y branch and Y waste represent for stump, branch, and waste biomass in either fresh or dry weight in kg,Ŷ res represents the corresponding predicted total residue biomass in kg from any of the systems of additive equations, r 21 , r 22 , r 23 and r 24 are coefficients, ε 21 , ε 22 and ε 23 are the error terms.…”

“…Over much of the latter half of the 20th Century, forest harvest residues represented a source of fuelwood for rural communities in many developing countries [1,2], while they were often windrowed, burned and mostly reduced to ashes to return to the soil as part of site preparation for the establishment of the next rotation in developed countries (e.g., [3][4][5]). With the increasing recognition of the role of bioenergy in climate change mitigation since the turn of the 21st Century (see [6][7][8]), forest harvest and timber processing residues have received a renewed focus as a major potential source of woody biomass that can be used to supplement or replace fossil fuels in order to reduce greenhouse gas emissions of energy production [9][10][11][12][13][14].…”