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GCB Bioenergy

Cover image for Vol. 9 Issue 11

Edited By: Steve Long

Impact Factor: 4.655

ISI Journal Citation Reports © Ranking: 2016: 2/83 (Agronomy); 2/83 (AGRONOMY); 15/92 (Energy & Fuels); 18/92 (ENERGY & FUELS)

Online ISSN: 1757-1707

Associated Title(s): Global Change Biology

Forest bioenergy use affects atmospheric CO2

Forest bioenergy use affects atmospheric CO2Forest biomass can be used to produce bioenergy. Biomass must be economically and environmentally sustainable in order to be an acceptable choice for replacement of fossil fuels. One facet of environmental sustainability is looking at Greenhouse Gas (GHG) emissions. The most prevalent GHG, CO2, is released by combustion of biomass and also captured by forests, making the determination of net CO2 emissions complex. Recently, several published papers have quantified the effects on increased forest harvest on atmospheric CO2 concentration. While these papers agreed in their estimates of forest productivity, they disagreed in their estimates of CO2 emissions over time. Holtsmark (2013) theorized that the difference in emission estimates could be explained by four different methodological choices or assumptions. He presented rationale for each: 1) Whether a single a or set of repeated harvests were considered – only including a single harvest in data does not allow regrowth and associated carbon capture or the permanently replacement of fossil fuel to be incorporated into the data 2) Whether harvest was conducted during stand growth or at maturation – while some studies only consider the harvest of mature stands, most stands are actually harvested at an earlier stage. 3) How the baseline was determined - to determine the effect of harvest on CO2 emissions, the data must be compared with a baseline scenario which is a similar forest with no harvest in which the trees are still growing, thus capturing CO2 from the atmosphere. If trees are harvested during stand growth, the baseline must include the continued growth of the unharvested trees. 4) If carbon-cycle models have been applied - carbon cycle models can consider CO2 absorbed by the ocean and terrestrial biosphere rather than just the CO2 released during combustion. Holtsmark adjusted previously published models with his preferred assumption and compared the results for each assumption. With this information, he concluded that the modeled simulations showed that the results change fundamentally with some of the assumptions, but not others. When a single harvest approach is replaced with a multi-harvest approach reflecting a cumulative long-term increase in harvest level, wood fuels cease to be carbon neutral (i.e. no CO2 release or uptake). That is to say, the multi-harvest approach revealed a permanent increase in atmospheric carbon. Furthermore, choosing to portray harvest taking place before growth of the trees has culminated, and adjusting the baseline to reflect continued growth of unharvested trees, has a significant effect on the results. In contrast, applying carbon cycle models does not change the results fundamentally. 

Holtsmark, B. (2012). The outcome is in the assumptions: analyzing the effects on atmospheric CO2 levels of increased use of bioenergy from forest biomass. GCB Bioenergy. doi: 10.1111/gcbb.12015

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