The effect of thinning and burning treatments on charcoal formation and carbon storage in a mixed-conifer forest, Sierra Nevada, California
Fire suppression, increasing temperature, and prolonged drought have resulted in increased wildfire frequency and severity in the western United States. Large and severe wildfires impact the carbon cycle both through direct emissions and reduced sequestration resulting from tree mortality. Forest thinning and prescribed burning reduce high-severity fire risk, but require removal and emissions of carbon. However, during each fire event not all combusted biomass is emitted to the atmosphere. A fraction of the burning vegetation and soil organic matter is converted into charcoal, a stable carbon form. The objective of our research was to quantify charcoal carbon resulting from prescribed burning as a function of pre-fire fuel load. We quantified post-treatment charcoal formation in the O horizon and first 5cm of the A horizon. Samples were obtained from downed logs (> 30cm diameter) that were present before treatment, from a full factorial experimental design. The experimental design involved three levels of thinning and two levels of burning in a Sierran mixed-conifer forest. Treatments included understory-thinning that removed trees between 25-75cm diameter at breast height (dbh) followed by burning and overstory-thinning which removed all trees > 75cm dbh, except 22 large trees ha-1left regularly spaced followed by burning.
Treatments that included burning produced significantly more charcoal carbon than treatments that did not include burning. This increase in charcoal production was significant in both the O horizon and first 5cm of the A horizon. On average one log in the burn-only treatment produced 20.1 g of charcoal carbon m2, one log in the understory-thin and burn treatment produced 16.7 g of charcoal carbon m2, and one log in the overstory-thin and burn treatment produced 7 g of charcoal carbon m2 from a single prescribed fire. In the burn-only, understory-thin and burn, and overstory-thin and burn treatments, 0.40-0.68 Mg of charcoal carbon ha-1 was produced from logs greater than 30cm diameter, compared to a background amount of 0.03 Mg charcoal C ha-1 in un-burned treatment units. Compared to live tree carbon, which ranged from 78.4-259.1 Mg C ha-1, this is a relatively small fraction of total ecosystem carbon. However, charcoal carbon is long-lived and will likely continue to accumulate with repeated burning, leading to additional increases in long-term soil carbon storage. Given increasing efforts to reduce high-severity wildfire risk with thinning and burning, our results help improve our understanding of the effects these treatments have on ecosystem carbon flux.