There is an on-going debate in both the policy and scientific communities about the impacts that forest treatments to reduce high-severity wildfire risk have on carbon storage in forests. Treatments typically involve cutting down trees to break up the fuel that allows fire to move from the forest floor to the canopy and prescribed burning to reduce surface fuels. Both of these management tools remove and emit carbon back to the atmosphere, a fact that is well established. It is also well established that these treatments are effective at reducing high-severity wildfire risk by changing fire behavior. The result, when fire burns through a treated forest, we tend to have surface fires that kill fewer trees and result in lower emissions of carbon back to the atmosphere.
The reason that there is a debate about whether treatments result in more or less forest carbon is because we cannot predict when and where wildfires will burn. This means that we typically have to treat more forest than will burn in wildfire and treated areas that don’t burn store less carbon than untreated forests (see Campbell et al. 2012). Much like in real estate – it’s all about location, location, location.
In our most recent paper we asked the question: how does the unpredictable nature of fire occurrence alter carbon dynamics between treated and untreated forests? With funding from the Department of Defense’s Strategic Environmental Research and Development Program we conducted a study in Ponderosa pine forest at Camp Navajo in Arizona.
We used field data and the LANDIS-II model to run a series of simulations. We simulated three different conditions: 1) no treatment (control), 2) thinning small trees (thin only), 3) thinning small trees and prescribed burning every 10 years (thin and burn). We also simulated two different chances of wildfire occurring, 1 in 50 and 1 in 100; which means that in any given year, there is either a 2% (1 in 50) or 1% (1 in 100) chance that a wildfire will occur.
As we expected and has been demonstrated before, the amount of carbon in the treated forest is lower than the control in the absence of wildfire. However, when we did simulate wildfire, the thin and prescribed burn treatment stored more carbon than the control after several decades. In the 1 in 50 chance wildfire simulation (solid lines), the thin and burn treatment stored more carbon than the control beginning in year 40 and in the 1 in 100 (dashed lines) chance simulation it was in year 51. The treatment that only thinned small trees ended up with about the same amount of carbon as the control in both cases. These results are because of treatment effects on altering fire behavior.
When a fire burns through the forest, we measure its effect using fire severity. Higher severity means that more trees are killed by the fire. When we calculated the mean fire severity for the different simulations, we found that when the forest was thinned and burned, mean severity was consistently lower.
We also calculated the coefficient of variation (CV) for fire severity. CV is a measure of how variable the results are and when the CV is high, fires at any given location ranged from low to high severity. Thinning alone did little to change how variable the results were and had CV values similar to the control. That is because thinning alone doesn’t deal with the surface fuels the way prescribed burning does. When we thinned and burned, CV was much lower. This means that the mean fire severity was lower and that any given fire tended to be less severe in the thin and burn scenario.
While it might be argued that we need to store more carbon now to limit the impacts of climate change and because our study showed that the forest stores less carbon for the better part of five decades that we shouldn’t do anything. However, our wildfire frequencies were on the low end of the range of frequencies for this area. Changing climate is projected to increase the frequency of large wildfires and this presents a bigger risk for storing carbon in forests. In short, treating forests has upfront carbon costs, but stabilizes the remaining carbon over the long-term.