To Understand the Growing Wildfire Threat, Look to the Tree Rings

As New Mexicans brace for a potentially catastrophic fire season, forest ecologists explain how we got here—and why the problem isn’t going away.

The Sardinas Canyon Fire in New Mexico, June 27, 2018

Credit: Rogelio Martinez-Ramos/Carson National Forest

Even before the recent Ute Park Fire in northern New Mexico consumed almost 37,000 acres, there were troubling signs of drought everywhere. The Rio Grande had shriveled, the snow had virtually vanished from the state’s mountains by April, and communities were reporting unusual sightings of bears and other wildlife venturing into neighborhoods in search of water and food. New Mexicans are no strangers to dry times, but this drought, and its accompanying fire hazard, are historically extreme—and the consequences potentially irreversible.

New Mexicans’ worries were confirmed in mid-May when some national forests began issuing fire restrictions. Then on June 1 the Santa Fe National Forest (SFNF) closed 1.6 million acres to the public (reopening them on July 9 after heavy rains provided some relief)—not unprecedented, but something that had never happened so early in the year. Later that month, portions of the Cibola National Forest, the rugged and wild backyard of the state’s largest city, Albuquerque, was closed. And most of the Carson National Forest near Taos was closed on July 1.

“The Cibola is a high-use forest, so this is not a decision that we made lightly,” said SFNF fire officer Matt Rau in an official statement issued on June 11. “The forest is tinder-dry and the monsoons may still be a few weeks out. We need to take every action possible to reduce the risk of human-caused fires.”

Lesser Fires

To uncover the roots of today’s fire threat in New Mexico, you need to look back only as far as the 1880s, when human activities changed the fire regimes, explains Ellis Margolis, a tree ring researcher and forest ecologist for the U.S. Geological Survey in Santa Fe.

It happened like this: Before the late 19th century, fires were a lot more frequent but far less destructive. The forest floors were blanketed with grasses and tree litter, which fueled low-intensity fires that, except for a little singeing, left alone the big ponderosa pines and other trees of what is called the mixed conifer zone. The fires also kept the numbers of saplings low so that there was lots of space between trees, Margolis says. In fact, there are five to six times more trees in the forest today than there were in the 1880s.

Then, in the 1880s and 1890s, people brought industrial-scale livestock grazing to the mountains of New Mexico. As the animals gobbled up the grasses, the low-intensity fires ended and saplings and other forest debris started filling in the spaces. The result was denser forests with steeper competition for limited water—a fact made worse by a warming environment.

Equally significantly, beginning around the turn of the century, all fires were actively suppressed and then more aggressively fought after the Big Blowup of 1910 consumed more than three million acres in Idaho, Montana, and Washington. This policy left the timber to pile up. Meanwhile, starting shortly before the first World War and continuing through the second, the state saw massive selective logging of its big “yellow-bellies,” as foresters called the older ponderosas, known for the thick, orange-yellow bark, which had made them fire-resistant.

Tree rings from a Ponderosa pine tree in the Jemez Mountains, showing a fire scar from 1851
Credit: Ellis Margolis/USGS

Margolis and his colleagues have deciphered some of this history and much more in the rings of old New Mexican trees as well as timber from trees cut and used in some of the centuries-old buildings of American Indians and Spaniards that still stand in the state. The tree rings provide a window into much of the fire history of the region for the past 500 years, showing which years were wet or dry and when the fires came through and scarred the trees, among other key data.

“We can even look at the season—early or late in the growth ring,” Margolis says of the fire scars. “So then you have this really cool record of fire over time. We find amazingly similar records in Arizona, New Mexico, and California in the mixed conifer zone.”

The single most conspicuous fact from the tree rings is that the record exists at all. In other words, the fact that trees could have survived the forest fires of the 16th, 17th, and 18th centuries and most of the 19th indicates a kind of fire very different from the explosive conflagrations of our times.

Today when fires start, they no longer run along the ground and singe the trees. They roar up the “fuel ladder” of forest floor debris and thickets of smaller trunks, then explode in the crowns of trees. They jump from treetop to treetop, killing all the trees, leveling the forest, and leaving in their wake an utterly changed landscape.

Greater Fires

Arguably the most powerful influence on today’s New Mexican forests—and all conifer forests of the American West—is climate change. It’s likely to be a key player in fire seasons for the foreseeable future. But divining the future of fire is a challenge very different from reading the past, so to aid in this task, scientists have mated computer climate modeling with the fundamental physics of fire.

“We think of fire on the landscape as kind of like a chemical reaction in the laboratory,” explains Mike Stambaugh, a fire science researcher at the University of Missouri in Columbia, Missouri. He and his colleagues are studying how frequently fires occur across the continental United States, and how they change with environmental conditions.

“We wanted to calibrate with climate,” Stambaugh says. “Is it hot or cold, wet or dry? Warm and wet is like Florida. Obviously, cold and wet areas don’t burn that often.” By assessing the different conditions in different regions, Stambaugh and his colleagues are trying to determine how fire can be predicted across the country.

They have also looked at a wide array of climate models and found that these models vary in their predictions of whether New Mexico will get wetter or drier. But the models do agree on one thing: It will continue to get hotter. Under any conditions, but especially where it’s arid, more heat causes vegetation to dry out more—and drier vegetation is more likely to burn.

But forest ecologists like Margolis say the fire risk comes not just directly from the rising mercury, but also from the increasing evaporation demand it brings. “Even with the same precipitation, you have more moisture stress,” he explains. Scientists call it vapor pressure deficit (VPD). “It’s the atmosphere sucking moisture out,” he says. And here’s the kicker: As temperature increases, the VPD doesn’t just climb steadily with it. That sucking of water into the sky increases exponentially.

“If you look at VPD, the last few decades have been pretty extreme,” Margolis says. When you add more trees, plus a drought, plus a warmer climate, you get New Mexico’s current forest fire time bomb.

The Waiting Game

Most climate scientists and environmental advocates agree that there is still time to slow that ticking time bomb and avoid future threats like this one if we act to reduce carbon emissions now.

“It ought to be an incredible motivation as we see these changes to our forests,” says Noah Long, an attorney for NRDC who lives in Santa Fe and grew up hiking, fishing, and skiing in New Mexico. Long notes that some state leaders have sought to make New Mexico a leader in clean, renewable energy—a vision that former governor Bill Richardson, who served as U.S. Secretary of Energy under President Bill Clinton, was working toward. But Long notes that under Republican Governor Susana Martinez, the momentum has waned.

For now, New Mexicans must brace for the worst. Many are praying for rain. Come late July or early August, the stormy North American monsoon season will arrive, dampening the accumulated fuel on the land, promising—at best—a temporary answer to those prayers.

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