How Fire Helps a Forest Stand the Test of Time

"Alright, let's head out," says Duke Forest Supervisor Tom Craven.

It's a crisp March morning in the Durham Division of Duke Forest, just west of campus, and Craven leads a 7-member team, dressed in yellow fire gear and hard hats, down to the south end of a plot of widely spaced pines.

This story is part of the Climate Research Series

There, a rare stand of once-ubiquitous southern trees rises up skyscraper-like from the ground, their straight trunks climbing 90 feet towards the clouds.

This towering tree, the longleaf pine, once blanketed much of the southeastern U.S., but only scattered remnants remain.

A few stands of longleaf pines can be found in Duke Forest. But to preserve them, the Forest and its partners do something that may seem counterintuitive — they set the forest floor alight.

Using a special canister called a drip torch, Craven drizzles a thin stream of fuel onto the ground and lights it. The first flames of the day begin to creep through the underbrush.

Planning a burn is complex work. Before setting fire on the ground, staff consider dozens of variables, from relative humidity and wind speed to soil moisture.

Craven and Duke Forest Executive Director Sara Childs lead the fire upwind of where it started, steadily working their way back and forth with their drip torches to draw lines of fire across the 150-meter-wide stand.

Soon, orange flames crackle and white smoke billows up on the breeze.

The rest of the crew, meanwhile, works to make sure the fire stays within certain boundaries. Some rake pine needles and sticks away from areas they don't want to burn to starve the fire of fuel. Others watch out for unruly sparks or flying embers that might cause flare-ups away from the main fire so they can quickly put them out with a water truck they have on hand.

"The longleaf pine community is fire-dependent," Craven says. "We don't have that much longleaf in Duke Forest, so it's really important that we burn to maintain these rare communities and promote the regeneration of new longleaf trees."

Prior to European settlement, naturally-occurring fires sparked by lightning periodically swept through southern forests. Indigenous peoples burned the underbrush to flush out game and clear areas for crops.

When longleaf pine forests go unburned, leaf litter piles up on the forest floor and the seeds can't take root. Competing plants such as hardwoods and other pines overshadow the seedlings and they can't get the sunlight they need to grow.

The trees once stretched more than 90 million acres across the South's coastal plain, from Virginia to Texas. Centuries of harvesting, development and fire suppression have shrunk the pine's extent such that, today, less than 3% of its original acreage remains.

Throughout the 1800s, longleaf pines provided tar, pitch and turpentine for North Carolina's naval stores industry, hence the nickname "the Tarheel State." The tree is so entwined with North Carolina history that the state is referred to as "the land of the longleaf pine" in the official state toast.

These Duke Forest pines were planted in the 1930s, in the sandy soils of land that had been cleared for farming.

"This stand is iconic," Craven says. "Generations of students have worked in this stand to take tree measurements, to learn about forest ecology. It's a treat to keep that going."

A team led by Duke environmental science professor Jim Clark, for example, has been using this and other sites to study how tree reproduction -- seed production and seedling success -- will be affected by climate change.

As climate warms, the regions where young trees can comfortably sprout and make it through the winter months are shifting northward.Duke Forest lies just northwest of the longleaf pine's historic native range in North Carolina. But now, with climate change re-shaping where species can live, it's become a testing ground to see how this tree might fare in the future, says Clark lab manager Jordan Luongo.If this stand is going to sustain itself, however, it's going to need human help.As the burn progresses, Childs points out a dozen or so baby longleaf trees growing in a sunlit gap in the forest, noting that they’ve sprung up since the last burn. They’re not much to see yet — just clumps of green grass-like needles poking up from the ground. But to Childs, they’re a sign that the next generation of longleaf trees is taking root.Within two hours, the fire has swept through the 9-acre site and the flames have died down, leaving the forest floor carpeted in black ash.

Young plants such as loblolly pines and sweet gum saplings will be killed by the blaze. But the longleaf seedlings and trees will survive and rebound, protected by their tufts of needles and armor of thick bark.

More sun will fall on the forest floor, and the bare mineral-rich soil left behind by the fire will open up space to help sprout new trees, Childs says.

After putting out the last smolders, Craven gathers the crew in a circle to debrief and review the day's events.

"We’re going to have a more open stand for a few years, and give the little longleaf seedlings their best chance," Craven says.

"Good job everyone," Childs says.

Series: Campus as a Research LabA Climate Commitment Series

This series aligns with the Duke Climate Commitment, which unites the university's education, research, operations and public service missions to address the climate crisis.