Prescribed Burning Reduces Wildfire Fuels

In May 2010, Saguaro National Park used prescribed fire to treat about 114 acres of high-elevation ponderosa pine forest. This was the latest in a series of burns in the park going back to 1996. The park was among the first places in the country to manage lightning-caused fires for natural resource benefits. However, many fires were suppressed before and after this policy change, so fuel conditions were not natural.

This specific burn reduced dead surface fuels, such as litter, duff, branches, and logs, by 2.4 tons per acre, equating to more than 273 tons for the whole burn unit. The energy in those fuels was removed and is not available for combustion in a future wildfire. In addition, the prescribed fire removed small trees and brush, reducing live fuel amounts as well.

Objectives of the overall prescribed burning program in the park’s upper elevations include decreasing fuels to reduce the fire behavior of subsequent wildfires, helping to protect a historic cabin and habitat of the threatened Mexican spotted owl, and restoring natural ecosystem function.

Overgrown vegetation
1996: Before first prescribed fire—50 years without fire.
Somewhat open forest with lots of forest litter on the ground.
2002: Before second prescribed fire.
More open coniferous forest with less forest ground litter.
2010: Before latest prescribed fire.
Open forest with grassy forest floor.
2012: Two years after latest prescribed fire.

The area burned in 2010 is near the top of the Rincon Mountains. Scientists from the University of Arizona’s Laboratory of Tree-Ring Research found that the area used to burn frequently. Between the years 1657 and 1893, the mean fire interval was estimated at 6.1 years. After fire suppression was initiated, many fires were successfully suppressed in this area, and that interval lengthened significantly. The last wildfire in this area was a 5,000-acre wildfire in 1943. Fuels accumulated each year there was no fire, increasing the risk of catastrophic wildfire.

Prescribed burning was undertaken to reduce fuels and return the area to a more natural fire regime. This specific area was burned with prescribed fire in 1996 and 2002, then again in 2010. This latest prescribed fire was in the normal fire season, and fire behavior included low, moderate, and high severity, much as a wildfire would under the natural fire regime. Because fire can sometimes kill trees and cause them to fall a year or two after burning, thereby increasing fuel loading, the effects on fuels were checked two years after the prescribed fire.

Burned plots had fuels decrease 17 percent following the 2010 burn. Surface fuel loads within the plots are now 29 percent lower than before the first burn in 1996. The three burns lowered dead and down fuel loads by almost 5 tons per acre.

Small trees and brush also contribute fuel to wildfires, and the 2010 prescribed fire reduced the number of saplings in the plots by an average of 54 percent. There are now about 59 fewer saplings per acre than there were in 2010, and 450 fewer per acre than there were before the 1996 prescribed fire. That equates to about 51,300 fewer saplings in the entire burn unit.

Trees, especially the largest, were less affected by burning. For the four plots, average tree density decreased 10 percent following the 2010 prescribed burn. The area now has 17 percent fewer trees than were growing there in 1996. There are about 23 fewer trees per acre than in 2010, equating to about 2,622 fewer trees for the entire burn unit. Some small trees are killed in each fire, and they fall down or are burned down, then consumed by later fires.

The 2010 prescribed fire, especially in conjunction with the 1996 and 2002 burns, reduced fuel loading in the pine forest of the park. Reducing fuels available for a wildfire makes it more likely that there will be reduced fire intensity and smoke output during future fires. Lower fire intensity reduces the likelihood of stand-replacing crown fires and makes wildfires easier to manage and less costly, and reduces negative impacts such as excessive soil erosion and tree mortality. The forest is now closer to historical conditions, more resilient, and better able to withstand the impacts of climate change.

For additional information please visit https://www.nps.gov/sagu/parkmgmt/firemanagement.htm.

Contact: Perry Grissom, fire ecologist
Email: e-mail us
Phone: (520) 733-5134