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  • (2022) Combustion of Aboveground Wood from Live Trees in Megafires, CA, USA


    Michelle Connolly
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    By Mark E. Harmon, Chad T. Hanson and Dominick A. DellaSala

    Abstract: Biomass combustion is a major biogeochemical process, but uncertain in magnitude. We examined multiple levels of organization (twigs, branches, trees, stands, and landscapes) in large, severe forest fires to see how combustion rates for live aboveground woody parts varied with tree species, size, and fire severity in Ponderosa pine (Pinus ponderosa Dougl. ex Laws.) and mixed conifer-dominated forests of the Sierra Nevada, California, USA. In high severity fire patches, most combustion loss was from branches < 2 cm diameter; in low to moderate severity patches, most was from bole charring. Combustion rates decreased as fire severity declined and with increasing tree size. Pinus species had little branch combustion, leading them to have ≈50% the combustion rate of other taxa. Combustion rates could be 100% for small branch segments and up to 57% for small tree aboveground woody biomass in high severity fire patches. However, combustion rates are very low overall at the stand (0.1%–3.2%) and landscape level (0.6%–1.8%), because large trees with low combustion rates comprise the majority of biomass, and high severity fire patches are less than half of the area burned. Our findings of low live wood combustion rates have important implications for policies related to wildfire emissions and forest management.

    (2022) Combustion of Aboveground Wood from Live Trees in Megafires, CA, USA.pdf

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    This study has interesting implications for the BC ministry of forests' policies around salvage logging of burned forests. An explanatory article at phys.org included an interview with Mark Harmon, the lead author:

    "We suggest that researchers and policy makers avoid using combustion rates not based on field study as they appear to overstate the wildfire emissions used in carbon emissions reporting; this can potentially misdirect climate mitigation policy," he said.

    Dead trees decompose slowly as new vegetation grows and absorbs atmospheric carbon, the scientists point out. If fire-killed trees are allowed to remain in place, the natural decomposition process might take decades to hundreds of years to release the trees' carbon.

    On the other hand, if those trees are logged to serve as energy-producing biomass, that same carbon could potentially enter the atmosphere much faster. More study is needed, the researchers note, to determine the degree to which post-fire forest management influences the carbon release time frame, including how biomass energy might offset the burning of fossil fuels and how wood products release carbon as they are used and disposed.

    "The effects of salvaging and putting some of that wood into durable wood products need to be fully investigated," Harmon said. "More fires need to be examined using our type of approach to determine how variable the combustion rates are at different levels for different forest types and ages."

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