In the summer of 2011, lightning struck a ridge near High Peak Mountain, on the Ouachita National Forest in Arkansas. The High Peak Wildfire began.
“It’s a remote and rugged area, and we were in the middle of a severe drought,” says Virginia McDaniel, a USDA Forest Service forestry technician, who led a study on the fire’s impacts. The study was published in the Natural Areas Journal.
The summer had been hot and dry. Several other lightning ignited fires were burning in the area, but they could be easily reach with a bulldozer and were suppressed. But the terrain on High Peak was impassable and the temperature was reaching 100 degrees daily. In the interest of firefighter safety and hoping for ecological benefits, Ben Rowland, the district fire management officer on the Ouachita NF decided against fully suppressing the High Peak Wildfire.
The wildfire was not threatening human homes or structures and was constrained by a network of roads, trails, and natural barriers. Forest Service fire personnel monitored the fire and installed a bulldozer line where other barriers to spread didn’t exist. Between these containment lines lay 1,800 acres where the fire was free to burn.
In two weeks, rain extinguished the fire. It had burned 1,500 acres. “People were concerned,” recalls McDaniel. “Would the fire kill the overstory trees? What about the planted pine plantations in the burn perimeter?”
McDaniel installed 32 plots in the burned area, as well as two control plots (seven additional control plots were added in 2013). McDaniel and colleagues from the Ouachita NF, The Nature Conservancy, and National Park Service sampled trees in the plots before and immediately after the fire, and then after one, two, and five years.
The five year results show that overstory trees on the High Peak Wildfire were resistant to the fire.
Over 90 percent of the larger, older trees in the overstory survived after the first year. Between one and five years after the wildfire, almost all of the overstory trees were still alive and healthy.
Many smaller trees in the midstory died in the fire – which was good news. Thinning the midstory is a common forest management goal. Historically, many forests were sunnier and more open. Woodlands, savannas, and grasslands were common. Today, closed canopy forests have mostly replaced them.
Fire organizes ecosystems, sorting out the species that can tolerate it from those that cannot.
Plant communities in fire-prone areas were once fire-tolerant. But wildfires are usually suppressed today. Without fire, plant communities blur together. In many areas – including the High Peak area – mesic species (which love moisture and do not tolerate fire) grow alongside more xeric, fire-adapted species.
“The fire top-killed a lot of the mesic trees in the midstory,” says McDaniel.
The fire killed 35 percent of the trees in the midstory. The species that were most likely to be killed were red maple, northern red oak, black cherry, flowering dogwood, winged elm, blackgum, white ash, and serviceberry.
The overstory trees included shortleaf pine, oak, and hickory species in naturally regenerating stands and loblolly pine plantations.
Only one percent of overstory trees in the loblolly pine plantations died. However, mortality in the midstory was much higher – 73 percent of the midstory trees in the pine plantation died
“This indicates that summer burns may be especially effective in the restoration phase of returning dense forests to their historic open condition,” says McDaniel. “The midstory may hold the heat of the fire and prevent it from scorching and killing the overstory.”
The results were surprising – in many cases, trees survive the initial disturbance but die soon after. And after long periods of fire suppression, fires during drought can kill many overstory trees.
McDaniel and her colleagues suspect that the dry conditions before the fire may have caused trees to grow fine roots deep into the soil, which would have protected them from the heat. In addition, the amount of duff on the forest floor was shallow (less than an inch), so it did not smolder for very long.
“Additionally, the High Peak Wildfire was a low-intensity fire that consisted of mostly backing and flanking fire,” says McDaniel. “The results could be different if weather and fuel conditions had allowed it to burn hotter.”
Prescribed burns are only conducted when weather conditions make it safe.
“There are real risks associated with letting a lightning ignition burn,” says McDaniel. “However, naturally ignited fires can sometimes promote forest health, just like a prescribed fire is designed to do. Studies such as this one can support decisions to manage naturally ignited fires with less than full suppression – especially in the dry and dry-mesic pine-oak forests and plantations of the Ouachita Mountains and potentially similar ecosystems.”
Tara Keyser, director of the Center for Forest Management and Restoration, conducted the statistical analyses. Gabriel De Jong, terrestrial ecologist with The Nature Conservancy of Arkansas, contributed to the study, along with many partners who assisted with data collection.
For more information, email Virginia McDaniel at firstname.lastname@example.org.