When the conditions are right, land managers sometimes allow naturally ignited fires to burn. And new research shows that there can be significant ecological benefits when they do so.
The Mount Trumbull Wilderness Area is a remote piece of ground in Northwest Arizona. In the late 1800s, the forest was wide open, with an estimated 25 trees per acre. And then came more than a century of aggressive suppression policy, which ended the regime of frequent, lower intensity fires that maintained that openness there and in forest ecosystems across the West.
“By 1999, the site was dominated by closed canopy stands, and we saw an increase from … 25 trees per acre up to 495 trees per acre,” said John Paul Roccaforte, a researcher with the Ecological Restoration Institute at Northern Arizona University.
In these overgrown conditions, dense stands of younger, smaller trees can choke each other and other native species out, and can lead to explosive blazes. Officials refer to such trees as ladder fuels because they can help carry fire from the ground up into the crowns of larger trees, potentially leading to severe, often damaging and potentially dangerous incidents.
In 2012 and 2019, managers allowed two fires to burn through the wilderness, and their boundaries included longstanding research sites.
“This lightning-caused fire is being monitored as it burns naturally across the landscape,” the Bureau of Land Management said in a release about the 2019 Trumbull Fire. “This fire is being allowed to play its natural role within the fire-adapted Ponderosa pine ecosystem. Under the right conditions, fire helps break down nutrients and minerals in plants and other debris such as old logs and dense undergrowth and restores them to the soil. The process also improves wildlife habitat and preserves wilderness values while protecting monument objects.”
To Roccaforte and his co-authors, the two incidents also presented a unique opportunity to study the degree to which such burns can help return forest ecosystems to their historic, pre-suppression conditions.
In their wake, tree density fell to around 245 trees per acre.
“These two fires knocked it back in half,” Roccaforte said of the results.
But they also appear to have reduced the risk of damaging blazes.
Beyond shifting “structural conditions toward the more open conditions indicated in dendroecological reconstructions,” the 2012 and 2019 incidents “likely reduced vulnerability to future high-severity crown fires,” the paper reads.
Given the findings, Roccaforte says that such let-burn policies are one of the most powerful fire management tools available to public land agencies, especially in wilderness areas.
“We recommend that they keep using wildland fires when they can do it safely,” Roccaforte said. “To keep pecking away at this problem that we have in terms of the density … and fuel buildup.”
Allowing fires to burn can be politically fraught because of societal expectations that blazes be quickly suppressed, and on rare occasions let-burn fires can turn into serious incidents. Previous research from the Ecological Restoration Institute found that land managers want to know that they have the support of agency leadership when they make the difficult but ecologically sound decision to do so.
This story was produced by the Mountain West News Bureau, a collaboration between Boise State Public Radio, Wyoming Public Media, Nevada Public Radio, KUNR in Nevada, KUNC in Northern Colorado, KANW in New Mexico, Colorado Public Radio and KJZZ in Arizona as well as NPR, with support from affiliate newsrooms across the region. Funding for the Mountain West News Bureau is provided in part by the Corporation for Public Broadcasting and Eric and Wendy Schmidt.
