Abstract
AbstractBioenergy 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 CO2 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.
Highlights
Biomass-based fuels may be used to replace fossil fuels in order to reduce greenhouse gas (GHG) emissions of energy production and mitigate climate change
To estimate the CO2 emissions resulting from the reductions in forest carbon stock, we simulated the decomposition of the forest residues collected for energy using the dynamic soil carbon model Yasso07 (Tuomi et al, 2009, 2011; www.environ ment.fi/syke/yasso)
The decomposition rate of the forest residues had a crucial effect on the GHG emissions of bioenergy production from the residues over time. This was because, firstly, this rate determined how much the carbon stock of forest was reduced as a result of removing the harvest residues, and, secondly, the emissions from the bioenergy production chain, e.g. from machines used, were relatively small (Fig. 2)
Summary
Biomass-based fuels may be used to replace fossil fuels in order to reduce greenhouse gas (GHG) emissions of energy production and mitigate climate change. It has been common to think that bioenergy is a carbon neutral energy source because carbon emitted into the atmosphere through combustion will be absorbed again by the generation of growing vegetation (Schlamadinger et al, 1995; Wihersaari, 2005b; Stupak et al, 2007) Following this philosophy, the member states of the European Union are increasing bioenergy production as part of their climate and energy policies (EU energy and climate package 2008; Beurskens & Hekkenberg, 2011). Deforestation and a consequent conversion of the forest to an energy crop plantation may reduce the carbon stocks of biomass or soil or both, and cause GHG emissions into the atmosphere These emissions are not limited to land use changes but bioenergy production may reduce the carbon stocks within the same land use. This may happen, for example, when residues of forest harvests are removed from forests for bioenergy production in addition to removals of industrial round wood (Schlamadinger et al, 1995; Palosuo et al, 2001; Repo et al, 2011)
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