How much Smoke will a Prescribed Fire Produce?

Improving Model Estimates in the Ouachita Mountains

Prescribed fire is an important and widely used management tool, but the smoke produced can cause air quality issues and health problems. Before conducting prescribed fires, managers typically model the amount of smoke a fire will produce, which is directly related to the amount of fuel available.

“Most fire-effects models were developed in the western U.S.,” says U.S. Forest Service forestry technician Virginia McDaniel. “Their accuracy has not been well-tested in southeastern forests.”

Prescribed fire in the Ouachita Mountains, AR
A low intensity prescribed fire consumes fuel to promote open woodland habitat in the Ouachita Mountains of Arkansas. Photo by Virginia McDaniel, USFS.

In the southeastern U.S., prescribed fires are used to help restore pine-oak forests to their historic woodland condition. Prescribed fires also consume fuel that could otherwise lead to a catastrophic wildfire. Many plants and animals, such as the endangered red-cockaded woodpecker, depend on these prescribed fires to maintain suitable habitat for their survival.

“Maintaining pine-oak woodlands in the Ouachita Mountains is most efficiently done with large landscape burns because optimal burning days are infrequent,” says McDaniel. “However, there are limits to the amount of smoke that can be emitted on a given day. Balancing the ecological benefit of burning with air quality standards is becoming increasingly difficult for fire managers.”

McDaniel and her co-authors recently tested a fire effects model called the First Order Fire Effects Model, or FOFEM. In addition to McDaniel, the research team included wildlife biologist Roger Perry, statistician Nancy Koerth, and research ecologist/project leader James Guldin. The scientists published their results in the journal Forest Science.

The model McDaniel and her colleagues tested was developed by the Forest Service, and is used by fire managers to estimate the fuel loads and consumption, and smoke emissions that occur during wildland and prescribed fires. However, the model does not currently include data specifically for the Ouachita Mountains of Arkansas and Oklahoma, although the Ouachita National Forest burns over 120,000 acres each year.

McDaniel and her colleagues measured the amount of fallen woody debris, litter, partially decomposed leaf litter (or duff), and other fuels in oak-dominated forests, pine-oak forests, and pine woodlands. The scientists measured fuels before and after 15 prescribed fires on the Ouachita National Forest in Arkansas.

Leaf litter
If the amount of fallen leaves, litter, and duff is over-estimated, the amount of smoke fires produce may also be over-estimated. Photo by Iain Cuthbertson, courtesy of Wikimedia Commons.

“We compared our fuel load and consumption measurements with the model default load and consumption predictions,” says McDaniel. “Default litter and duff loads in FOFEM were greater than our measurements.”

For example, the default duff load was up to 1,307 percent higher than the field measurements. In all three cover types, the model also overestimated the consumption of litter, sticks that were less than a quarter inch in diameter, and sticks that were between a quarter inch and an inch in diameter.

When default fuel loads are over-estimated, the amount of smoke production predicted by a prescribed fire will most likely be over-estimated as well. In particular, duff produces more smoke than other fuels, so its overestimation could have major implications in the amount of smoke predicted by models. The over-estimation could negatively affect fire managers’ ability to conduct landscape burns that are important for regenerating forests and maintaining wildlife habitat.

“Our study shows the importance of using local datasets to improve national fire effects models,” says McDaniel. Others have found that the model’s fuel estimates for Florida, Kentucky, South Carolina, and other southeastern states could be improved.

The findings have implications for restoration, as fire managers may be unnecessarily limiting the size of prescribed burns in order to meet air quality standards. “Predicting the amount of smoke produced by prescribed fires can be challenging,” says McDaniel. “We hope our findings can be used to build better fire-effects models for predicting emissions, so that managers can continue to restore and maintain the diversity of our southern forests.”

Read the full text of the article.

For more information, email Virginia McDaniel at vmcdaniel@fs.fed.us.

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