Thinning and Burning: The Best Defense Against Southern Pine Beetle

Study validates effectiveness of Southern Pine Beetle Prevention Program treatments

Thinned pine stand on the Oconee National Forest, with understory competition treated with prescribed fire. Photo by U.S. Forest Service.
Thinned pine stand on the Oconee National Forest, with understory competition treated with prescribed fire. Photo by U.S. Forest Service.

A recent study by U.S. Forest Service and university researchers shows that thinning and prescribed fire can protect stands of southern pines on a landscape level from infestations by southern pine beetle. The results, published online in the Journal of Forestry, also provide first-time confirmation of the effectiveness of the treatments supported by the Southern Pine Beetle Prevention Program (SPBPP) to reduce stand susceptibility to the southern pine beetle in the southeastern U.S.

The southern pine beetle (Dendroctonus frontalis Zimmermann) is a native pest which can cause huge damages in the Southeast, where wood product production is dominated by several southern yellow pine species. Major southern pine beetle outbreaks occurred across the region almost every decade until recently. The last multistate outbreak occurred from 1998 to 2002 in the Southern Appalachians, affecting more than 1 million acres of forest in five states and resulting in an estimated economic loss of $1 billion.

In 2003, after that last great outbreak, the Forest Service started the Southern Pine Beetle Prevention Program (SPBPP) to support landowners in reducing the susceptibility of their pine stands to southern pine beetle through silvicultural treatments or by restoring damaged stands with less susceptible species such as longleaf pine. Since then, funds from the program have helped treat more than 1.2 million acres on state, private, and national forest lands in the Southeast.

“It’s long been accepted that thinning is particularly effective in reducing the susceptibility of stands to southern pine beetle,” said John Nowak, the Forest Service Forest Health Protection entomologist who coordinates the SPBPP program with state forestry agencies, and lead author of the research article. “Dense, overstocked stands are inherently stressed. Thinning not only improves forest health but also helps to limit the beetle’s expansion through a stand and the formation of hot spots.”

Adult southern pine beetle. Photo by Erich Vallery, courtesy of Bugwood.org.
Adult southern pine beetle. Photo by Erich Vallery, courtesy of Bugwood.org.

A southern pine beetle outbreak in Mississippi in 2012 provided an opportunity to evaluate at landscape and stand levels the effectiveness of the treatments promoted by SPBPP as well as prescribed fire. The researchers evaluated stands in the Bienville and Homochitto National Forests, comparing thinned versus unthinned stands as well as unburned versus prescribed burned stands.

As expected, there were significant differences between thinned and unthinned stands, with the proportion of southern pine beetle spots much greater in unthinned stands.  Only two of the 910 spots found occurred on stands thinned in the 6 years before beetle activity. “This is good news for our program,” said Nowak. “Previous studies showed that thinned stands might be less susceptible to southern pine beetle, but this is the first to indicate that recently thinned stands can virtually escape attack.”

The researchers also compared differences between unburned stands and those burned with prescribed fire and found that stands with more recent and more frequent prescribed fire had a significantly lower incidence of southern pine beetle infestation.

“Prescribed fire to reduce understory competition has been allowed under the SPBPP, but there’s been a widespread belief that burning could actually make trees more susceptible to beetles, at least in the short term,” said Nowak. “This unexpected result confirms that stands with frequent low-intensity fire, low basal area, and more open growing conditions are more resilient to disturbance factors such as southern pine beetle. Based on these results, there will be an increased focus on burning through the SPBPP, especially in conjunction with thinning.”

The Southern Pine Beetle Prevention Program is now in its 12th year of working with state forestry agencies, private landowners, and national forests to improve the resiliency of southern forests through an “all lands approach” on more than 1.2 million acres. To learn more about the program and how to participate as a landowner, access your state forestry agency website.

Read the full text of the article.

For more information, email John Nowak at jnowak@fs.fed.us

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