Combustion of Aboveground Wood from Live Trees in Megafires, CA, USA

Mark E Harmon,Dominick A Dellasala,Chad T Hanson

doi:10.3390/f13030391

Mark E Harmon, Dominick A Dellasala + Show 1 more

Open Access

https://doi.org/10.3390/f13030391

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Journal: Forests	Publication Date: Feb 27, 2022
Citations: 8	License type: CC BY 4.0

Affiliation: Earth Island Institute, Oregon State University

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.

Highlights

Combustion of biomass is a major biogeochemical process by which carbon is returned to the atmosphere from terrestrial ecosystems [1,2]
Consistent with Hypothesis 4, we found that, at the landscape level, the very low rates of combustion in low and moderate severity fire patches diluted the effects of higher combustion rates in high severity fire patches
Applied to the Rapid Assessment of Vegetation Condition after Wildfire” (RAVG) fire severity distribution for the Rim Fire, these values suggest a landscape average combustion rate of 45%–57% in contrast to the 1%–1.2% we found when our combustion models were applied to various stand structures

Summary

Introduction

Combustion of biomass is a major biogeochemical process by which carbon is returned to the atmosphere from terrestrial ecosystems [1,2]. Fine twigs are in close proximity to leaves and other twigs (i.e., within tens of centimeters), leading to more exchange of energy than more distant plant parts; they are likely to be combusted. Their small diameter increases their surface area-to-volume ratio, allowing them to dry out and absorb energy faster than larger branches and stems. Tree stems or boles are separated by even more distance (i.e., often >1 m) and have a very low surface area-to-volume ratio when compared to twigs or branches; they are the least likely to be combusted. The smallest diameter woody fuels are theoretically the most likely to be combusted in a wildfire [17]

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