Wildfires burn landscapes, but they also sear themselves into memory, and many Big Bend National Park enthusiasts remember the South Rim 4 Fire of April 2021. It began near a backcountry campsite, suggesting a possible human cause, and burned across 1,300 acres of the Chisos Mountains. It was the most intense blaze in the storied range in decades.
In 2023 and 2024, forest ecologist Dr. Helen Poulos, of Wesleyan University, hiked the Chisos to assess the impacts. The fire, she says, was more severe, more destructive, than initially thought.
“We're literally going back to the same exact tree,” Poulos said, “and we're measuring its diameter and vigor over and over again. I always joke that people who study rodents, they have to put traps out and everything. And I'm like, ‘Oh, here it is. It's dead.’”
Poulos has monitored plots of marked trees in the Chisos – as well as the Davis and Guadalupe mountains – for two decades. It’s allowing her to track how the West Texas “sky island” forests are changing.
In 2011 and 2012, wildfire burned almost all the Davis Mountains. And in 2016, the Guadalupes burned in the Coyote Fire. While many Davis Mountains Ponderosa pines were killed, these fires were largely “good burns.” They cleared out dead brush accumulated during a century of fire suppression, freeing up resources for surviving trees and new growth.
Remote sensing suggested the South Rim 4 Fire was similar. But that’s not what Poulos found.
Chisos tree mortality was high even before the blaze. In 2011, an historic freeze, followed by historic drought, decimated oaks, junipers and pinyon pines. Almost half the trees in Poulos’s plots were killed.
Some of the South Rim Fire’s footprint lay outside Poulos’s plots. When she examined these areas, she found a high death rate, even among trees that had survived the freeze and drought.
“We're looking at basically up to 40% mortality of some trees,” Poulos said. “So that's kind of the update in my mind: that fire actually was very severe. And it burned really hot.”
Pinyon pines can survive a moderate blaze, but not a high-intensity one. In its severity, the South Rim 4 was a “stand-replacing” fire, Poulos said.
Some of today’s megafires can be attributed to the effects of fire suppression, or other recent human actions. But the South Rim Fire, Poulos said, was probably in keeping with ancient patterns.
“And probably every couple hundred years there’s a fire like that,” she said, “at least in that pinyon juniper woodland. It’s much more consistent with what we know about other pinyon-pine systems to the north, in Colorado and places like that.”
This stand-replacing fire may have been natural, even inevitable. But the question is whether stands will actually be replaced. In the Davis Mountains, trees began to regenerate quickly after fires. But Poulos hasn’t seen that in the Chisos.
Pinyon pines need moisture to produce seeds. But when they fall two years later, those seeds also need moisture to take root. It takes a “synergistic” set of wet years, Poulos said, for new trees to succeed. And amidst a drying climate, that synergy becomes increasingly unlikely.
The Chisos are Texas’s most widely beloved mountain forest. And while the future of this sky island is uncertain, Poulos said she was encouraged by last year’s strong monsoon.
“The question is, okay, well in the future, what are the ways in which we can keep these forested areas forests?” she said. “And I don't know the answer to that. But I will tell you that 2025 precipitation gives me some hope.”
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