Forests contain the planet’s largest terrestrial carbon stocks. Wildfires, by burning forests, release a significant amount of this stored carbon into the atmosphere extremely rapidly. This release interrupts a longer cycle where carbon is sequestered by growing trees and then is finally rereleased during the decomposition of the vegetation. Under forest management practices, forests have in many places been “treated” to lessen the effects of wildfires on tree mortality and to be better positioned to have higher survival during dangerous fires. In a study by North and Hurteau (2011), the short term effects of wildfire on carbon stocks were reviewed using field measurements, comparing treated and untreated forest areas in recent burn scars. The authors found that carbon emissions (during a fire) were more than double in treated areas. They further discovered that when the carbon release from the treating process was added to the emissions of wildfire in those same treated areas, the carbon emissions were significantly higher than untreated burned areas (93% tree mortality rate). This however is over a short time period, and that other studies suggest carbon emissions could be up to three times higher over an extended time of natural decomposition as opposed to the instantaneous carbon release induced by wildfire. –Lindon Pronto
North, Malcolm P., Hurteau, Matthew D., 2011. High-severity wildfire effects on carbon stocks and emissions in fuels treated and untreated forest. Forest Ecology and Management 261, 1115–1120.
This study collected data from 12 fire sites in California (Region 5), mostly from recent burn scars in the northern Sierra Nevada. The area was chosen for its extensive use of fuels treatment practices by the U.S. Forest Service, which provided the necessary comparison basis for evaluating carbon emissions for treated and untreated fuels during wildfire events. The objective of this comparison was to assess differences in (1) carbon stocks, (2) carbon loss from treatments and wildfire, and (3) tree survival, mortality, and changes in live tree sizes and species composition. The selection areas were constrained to areas that fell under the practice of ‘thin from below’ prescription, through the use of machinery which creates ‘machine piles’ of slash (often discarded from logging operations) which are burned during favorable weather conditions. The study identified 20 treatment areas that had been treated within the past 5 years; the dominant fuel type was mixed-conifer. Areas where fuel treated projects had not been concluded by the onset of the wildfire (such as unburned machine piles), were excluded from the data.
Using the boundary of fuels treatment projects, 3–6 plots of 0.05 ha for ≥5 cm diameter at breast height (dbh) and 0.1 ha for trees ≥50cm dbh, were selected in both burned/treated and burned/untreated areas, usually measured within 200m from each other for consistency in fuel characteristics. Through a variety of methods, carbon content in treated and untreated stands was calculated as accurately as possible to best represent (estimate) carbon content before the fire to be paired with actual results after the fire. The study assumed that carbon concentration was 50% in woody material and 37% in duff and litter. It was determined that carbon emissions of the fire were 11% of the total stored carbon in treated areas while 25% in untreated areas. North and Hurteau found that on average, fuel treatment removed about 34% of total stored carbon. Additionally tree mortality as a result of the fire was on average 43% and 97% in treated and untreated stands, respectively. The authors determined that if the carbon emitted during the process of treating fuels (i.e. prescribed fire) were added to the wildfire emissions, the treated/burned fuels produced a higher mean net carbon loss (80.2Mg C ha‾1) than the untreated/burned fuels (67.8Mg C ha‾1). However, this is in the context of short term carbon releases, and the same fuels decomposing over an extended period of time will generally produce significantly higher overall carbon emissions. However, if logging operations were used in the treatment process, a part of that carbon store could be subtracted from the overall emissions for that area.
In treated areas wildfire intensity decreased significantly, and carbon loss and tree mortality was lower. Although the authors found that 75% of the forest carbon stocks still remained onsite after severe wildfires, up to 70% of ecosystem carbon became decomposing pools in untreated areas, with only 19% in treated areas. The overall effect was that regardless of fire severity, carbon sinks become carbon pools until the carbon sequestering of the re-growth process became greater than the carbon emissions from the decomposing stocks in the following decades.
In summary, North and Hurteau found that treated areas significantly reduced fire severity and consequent mortality and reduced the carbon emissions during the fire event specifically. However, when the emissions from the treatment process were added to those of the fire, carbon emissions were significantly higher than those produced by severe fire in untreated stands (logging excluded). This study was not intended to be extrapolated to entire fire perimeters due to the extremely variable burn conditions of these different fires; the pairs (treated/untreated) were matched to very small areas of each respective burn. Overall, fuels treatment was found to likely shorten the time until carbon was re-sequestered by stand growth, due to higher survivability. This study suggests that fuels treatment projects that reduce wildfire intensity, successfully reduce carbon emissions during wildfire events and over the long term, by reducing the amount of carbon emitting stocks in long term decomposing stages.