Effects of boreal forest management practices on the climate impact of CO2 emissions from bioenergy

In Life Cycle Assessment (LCA), carbon dioxide (CO2) emissions from biomass combustion are traditionally assumed climate neutral if the bioenergy system is CO2 flux neutral, i.e. the quantity of CO2 released approximately equals the amount of CO2 sequestered in biomass. This convention is a plausible assumption for fast growing biomass species, but is inappropriate for slower growing biomass, like forests. In this case, the climate impact from biomass combustion can be potentially underestimated if CO2 emissions are ignored, or overestimated, if biogenic CO2 is considered equal to anthropogenic CO2. The estimation of the effective climate impact should take into account how the CO2 fluxes are distributed over time: the emission of CO2 from bioenergy approximately occurs at a single point in time, while the absorption by the new trees is spread over several decades. Our research target is to include this dynamic time dimension in unit-based impact analysis, using a boreal forest stand as case study. The boreal forest growth is modelled with an appropriate function, and is investigated under different forestry regimes (affecting the growth rate and the year of harvest). Specific atmospheric decay functions for biomass-derived CO2 are then elaborated for selected combinations of forest management options. The contribution to global warming is finally quantified using the GWPbio index as climate metric. Results estimates the effects of these practices on the characterization factor used for the global warming potential of CO2 from bioenergy, and point out the key role played by the selected time horizon.