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An image from a high-fuel burn

Turn up the heat: Longleaf pine survive extreme conditions

A pine tree native to the Southeast may have the ability to withstand extreme drought and fire—conditions that kill pines native to the western United States—according to a new study by a University of Georgia researcher.

 

The study is the first to investigate the ability of longleaf pine to survive these severe conditions and reveals key aspects of the tree’s structure that may aid its resilience. It was recently published in the journal Fire.

 

Once widespread across the Southeast, today only 3% of longleaf remain in its native region, replaced by faster-growing varieties of pine trees or agriculture. But its tolerance of poor soil conditions and its ability to thrive after a burn made researchers at the UGA Warnell School of Forestry and Natural Resources wonder: How much drought and fire could a longleaf pine seedling withstand?

 

Turns out, quite a bit.

 

“I expected some of them to live, but not near the number that actually did,” said Luke Wilson, a master’s student studying forestry at Warnell. When properly managed, stands of longleaf are typically burned every few years and provide good habitat for quail, deer and other wildlife. Regular burns also reduce the amount of pine straw and underbrush to fuel a fire, keeping the heat at a relatively low level—typical of many fires across the Southeast.

 

But Wilson wanted to understand what effect larger, more intense fires might have on the seedlings. Researchers have baseline information on western wildfires and survival of ponderosa and white pines in the western United States, so Wilson used this information to put longleaf to the test.

 

Wilson and his advisor, Warnell associate professor Dan Johnson, started with 80 2.5-gallon pots of longleaf seedlings that had been growing for one year. Wilson first created a drought among the plants. Then, he measured pine straw to use as fuel to create low-, medium- and high-intensity fires, as calculated by ongoing research by the U.S. Forest Service.

 

Using the burn lab at the U.S. Forest Service’s Southern Research Station, also located on the UGA campus, the researchers laid out slabs of plywood cut with holes in intervals to allow the pots to slide in, putting the seedlings flush with the ground and pine straw. Then, they began the fires. Overhead cameras measured the heat as the flames moved across the surface.

 

“With some of the work we’ve been doing out west on Ponderosa pine and western white pine, we have a basic dose-response curve—for example, if you give a plant a certain amount of fire energy, 50% will die; if you give them this dose, 100% will die,” said Johnson. “And that’s for ponderosa pine, which is supposed to be fire-adapted. But at 1.4 megajoules, it kills all of them.”

 

So, Johnson and Wilson started with a fire that produced 1 megajoule. “Then we said, let’s do something crazy and try 4 megajoules—and the longleaf didn’t care. It didn’t bother them at all,” Johnson added.

 

Wilson agreed that the results were surprising.

 

 “The higher fuel loads were exciting; we didn’t expect those results,” said Wilson. “At first the trees looked terrible—they looked just like they were dead.”

 

But they weren’t. While their needles burned to a crisp in the fire, the inner core of the seedling remained intact and full of life. Now, months later, the trees are thriving. Of the 80 seedlings in the study, only two didn’t survive—and Wilson isn’t convinced that the fire is what killed them, given the extreme drought they also experienced.

 

The results shed light on a species of pine that has long been associated with the Southeast, but over the years has been replaced with faster-growing species. As the researchers plan their next steps, they say one lesson is clear: Longleaf is a resilient species that would make a good investment for a novice landowner. 

 

The tree is known for its slow growth and straight trunks; its often grown for use as utility poles. But, Johnson added, it should be known for much more. “It’s more disease tolerant, more fire tolerant, more drought tolerant, more wind tolerant,” he said.

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