Abstract
Forest harvest residues are important raw materials for bioenergy in regions practicing forestry. Removing these residues from a harvest site reduces the carbon stock of the forest compared with conventional stem-only harvest because less litter in left on the site. The indirect carbon dioxide (CO 2 ) emission from producing bioenergy occur when carbon in the logging residues is emitted into the atmosphere at once through combustion, instead of being released little by little as a result of decomposition at the harvest sites. In this study (1) we introduce an approach to calculate this indirect emission from using logging residues for bioenergy production, and (2) estimate this emission at a typical target of harvest residue removal, i.e. boreal Norway spruce forest in Finland. The removal of stumps caused a larger indirect emission per unit of energy produced than the removal of branches because of a lower decomposition rate of the stumps. The indirect emission per unit of energy produced decreased with time since starting to collect the harvest residues as a result of decomposition at older harvest sites. During the 100 years of conducting this practice, the indirect emission from average-sized branches (diameter 2 cm) decreased from 340 to 70 kg CO 2 eq. MWh ―1 and that from stumps (diameter 26 cm) from 340 to 160 kg CO 2 eq. MWh ―1 . These emissions are an order of magnitude larger than the other emissions (collecting, transporting, etc.) from the bioenergy production chain. When the bioenergy production was started, the total emissions were comparable to fossil fuels. The practice had to be carried out for 22 (stumps) or four (branches) years until the total emissions dropped below the emissions of natural gas. Our results emphasize the importance of accounting for land-use-related indirect emissions to correctly estimate the efficiency of bioenergy in reducing CO 2 emission into the atmosphere.
Highlights
Bioenergy, i.e. energy derived from renewable biomass, is used to replace fossil fuels in energy production in order to decrease greenhouse gas emissions into the atmosphere
The indirect CO2 emissions, caused by combusting logging residues after harvesting instead of letting them decompose at the harvested site, were equal to the CO2 emissions from combustion, 340 kg CO2 MWhÀ1, when the practice was started but these emissions decreased over time as a result of decomposition of the harvest residues (Fig. 2)
Using logging residues as a source of bioenergy causes net CO2 emissions into the atmosphere and a great majority (85–97%) of these emissions are indirect emissions resulting from a decline in the carbon stock of harvest residues in forest
Summary
I.e. energy derived from renewable biomass, is used to replace fossil fuels in energy production in order to decrease greenhouse gas emissions into the atmosphere The rationale behind this practice is that bioenergy does not cause any net carbon dioxide (CO2) emissions since the amount of CO2 released into the atmosphere in combustion is taken up again by the generation of growing plants (Wihersaari, 2005; Stupak et al, 2007; Lattimore et al, 2009). Union (EU) adopted a directive on the promotion of renewable energy, including bioenergy This directive set targets to produce 20% of the final energy consumption using renewable energy sources in the EU by the year 2020. During the reference year of the directive 2005, renewable sources represented already 28% of the total energy production in Finland and 39% in Sweden, while the EU-average was 11%
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