Carbon recovered following different fuel reduction treatments in a Sierra Nevada mixed-conifer forest

Background/Question/Methods:

Forests sequester carbon from the atmosphere, helping mitigate climate change. Fire suppression in many dry, temperate forests has increased stem density and fuel loads, resulting in an increase in wildfire severity, which can kill much of the overstory converting the burned forest from a carbon sink to a source.  Mechanical thinning and prescribed burning can reduce fire severity and carbon loss when wildfire occurs.  However, treatment implementation forces carbon removal (thinning) and carbon emissions (prescribed fire) to reduce high-severity fire risk.  The carbon debt incurred during treatment may be recovered by subsequent tree growth; although much uncertainty lies in the time period necessary to re-sequester the carbon removed through treatments.  To assess the long-term carbon costs and benefits of thinning and burning treatments, we quantified the 10-year post-treatment carbon stocks from a full-factorial experiment involving three levels of thinning and two levels of burning in a Sierran mixed-conifer forest.  Thinning treatments included an understory thinning treatment that removed all trees between 25 and 75cm diameter at breast height (dbh), and an overstory thinning which removed all trees >25cm dbh, except 22 large trees ha^-1left regularly spaced. 

Results/Conclusions:

Our results indicate that less intensively treated forests can quickly recover carbon from tree growth alone, and that as treatment intensity increases the relative portion of carbon recovered in live trees and standing dead trees decreases.  We found that the total carbon gain following treatment was 57.0, 144.1, 41.0, 28.4, 19.3, and 14.5Mg C ha^-1 in the control, burn, understory thin, understory thin and burn, overstory thin, and overstory thin and burn treatments, respectively.  In the burn only treatment, the carbon emitted from prescribed fire was recovered in 10 years, prior to the site-specific mean historical fire return interval (17.3 years).  We conclude that 1) within 10 years all treatments recovered the carbon that was initially lost through burn emission or mechanical removal, excluding the most intense treatment, overstory thin and burn, which had a carbon deficit of 40MgC ha^-1 and 2) while there is an initial carbon loss from implementing fuels treatments, prescribed burning may also reduce resource competition for live trees, leading to an increased rate of carbon sequestration over 10 years, as opposed to implementing no treatment.