Long and Short-Term Effects of Fire on Soil Charcoal of a Conifer Forest in Southwest Oregon

In 2002, the Biscuit Wildfire burned a portion of the previously established, replicated conifer unthinned and thinned experimental units of the Siskiyou Long-Term Ecosystem Productivity (LTEP) experiment, southwest Oregon. Charcoal C in pre and post-fire O horizon and mineral soil was quantified by physical separation and a peroxide-acid digestion method. The abrupt, short-term fire event caused O horizon charcoal C to increase by a factor of ten to >200 kg C ha−1. The thinned wildfire treatment produced less charcoal C than unthinned wildfire and thinned prescribed fire treatments. The charcoal formation rate was 1 to 8% of woody fuels consumed, and this percentage was negatively related to woody fuels consumed, resulting in less charcoal formation with greater fire severity. Charcoal C averaged 2000 kg ha−1 in 0–3 cm mineral soil and may have decreased as a result of fire, coincident with convective or erosive loss of mineral soil. Charcoal C in 3–15 cm mineral soil was stable at 5500 kg C ha−1. Long-term soil C sequestration in the Siskiyou LTEP soils is greatly influenced by the contribution of charcoal C, which makes up 20% of mineral soil organic C. This research reiterates the importance of fire to soil C in a southwestern Oregon coniferous forest ecosystem.