Prescribed Fire and Snags in Shortleaf Pine Woodlands

Managing the forest for the dead trees

“I’ve spent years working in these shortleaf pine woodlands and always wondered about the availability of snags, especially given their importance to bats,” says U.S. Forest Service research wildlife biologist Roger W. Perry.

Perry is talking about 250,000 acres on the Ouachita National Forest in Arkansas and Oklahoma. Efforts to restore shortleaf pine woodlands have been underway for the last two decades — primarily to create and maintain habitat for red-cockaded woodpeckers.

“We’re doing a lot of woodland restoration across national forest lands, which involves thinning trees in the canopy and removing most of the midstory,” adds Perry.

“When there are fewer trees in the forest, there are fewer trees available for snags. Fire can also create or consume snags, so I was interested in how thinning and burning affects snag availability over time.”

Snags are standing dead (or dying) trees that provide roosting, nesting, and foraging habitat for wildlife — woodpeckers, other birds, and bats, to name a few.

shortleaf-pine-snag
Larger diameter snags are favored by many species of wildlife and provide usable habitat for longer periods than small snags. Photo by Roger Perry, USFS.

This restoration work is part of a much larger campaign across the Southern Region — around a million acres of national forest are being restored and managed as fire-adapted woodlands – including longleaf pine ecosystems and shortleaf pine woodlands.

These woodlands are more open than forests, with large pines, few midstory or understory trees, and plentiful herbaceous plants — the open canopy allows for more sunlight to reach the ground and create a lush growth of grasses and flowering plants.

Because prescribed fire is what maintains the open conditions of these woodlands, Perry was curious about its impacts: “Large snags are especially important to wildlife. Are they still readily available, or are we burning them all up?”

Perry, along with SRS wildlife biologist Phillip Jordan and forestry technician Virginia McDaniel, set out to understand if repeated burns over time would reduce the number of snags in the stands being restored to woodlands. Their results were published in Forest Science.

The study examined 24 stands across western districts of the Ouachita National Forest. These stands were second-growth forests that grew up after most of the region was harvested in the early 1900s. Prior to restoration, these stands were mature and relatively unmanaged.

The restoration began with harvesting about 80 percent of the overstory hardwood trees, along with about 36 percent of the overstory pines. Most of the midstory trees were also cut, but a few, like dogwoods, were left for mast production.

Un-harvested buffer strips were left around streams and drainages. The buffers, which were called greenbelts, were between 50 and 165 feet wide. Snags in these areas were assessed separately.

The stands were placed in different burn classes. The classes ranged from one to six, and reflecting the frequency of previous burns. Most prescribed burns were conducted during the dormant season or early spring green-up.

On average, stands with more than one previous burn had been burned within four years. Others had been burned as recently as the year before or as many as eight years before the study began.

Snags were counted, measured, and classified into four decay classes – in the southeastern U.S., snags decay faster than in the West or Northeast because of higher temperatures and humidity.

Some roosting bats, as well as some cavity-nesting birds, will only use snags that are at least six inches in diameter. The scientists considered snags in this size class separately from smaller snags, which were less than six inches in diameter.

shortleaf-pine-woodland
Restored shortleaf pine woodland on the Ouachita National Forest. Photo by Jim Guldin, USFS.

The scientists didn’t find a relationship between the density of large snags and the number of previous burns. However, they did find fewer small snags in woodlands that had been burned frequently. In greenbelts, the number of small snags did not seem to be affected by the frequency of fire.

“Basically, the frequent burning required to maintain these woodlands did not reduce the density of large snags, which is good news for wildlife that prefer large snags for roosting and nesting,” says Perry.

The total density of snags differed between stands and greenbelt areas, with more than three times the number of snags in the greenbelts.

Because the harvest treatments removed most midstory trees, the decrease in small snags was expected. Small snags stand for a shorter length of time and fall after just a few years, whereas large snags can stand for years. Repeated burning also prevents the establishment of new small trees that would have been a source for small snags.

Most of the fires were low-intensity and were not hot enough to kill large trees. Occasional hot spots during a burn can kill a few large trees, but factors other than prescribed fire, such as insects, disease, or lightning, probably create most of the large snags in these woodlands.

“Density of small snags starts to decline after the first burn and then continues to decrease,” notes Perry. “Most forest plans require streamside buffers in riparian areas for water quality protection and storm drainage, so these greenbelts will continue to provide small snags over time.”

Read the full text of the article.

For more information, email Roger Perry at rperry03@fs.fed.us.

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