Under the Longleaf Pine Canopy

Silvicultural treatments affect wiregrass and other understory plants

Wiregrass and other understory species carpet the forest floor on the uplands of the Blackwater River State Forest in Florida. Photo courtesy of Dale Brockway.
Wiregrass and other understory species carpet the forest floor on the uplands of the Blackwater River State Forest in Florida. Photo courtesy of Dale Brockway.

Longleaf pine forests once covered over 90 million acres of North America stretching from Texas to Florida to Virginia. However, logging, fire exclusion, and land use change caused the acreage of longleaf pine to shrink to about 2.5 million acres. “Longleaf pine forests are one of the most endangered terrestrial ecosystems in the southeastern United States,” says U.S. Forest Service scientist Dale Brockway. “However, in recent decades, their ecological, economic, and cultural importance has come to be increasingly valued.”

Brockway is a research ecologist at Auburn, Alabama, in the SRS Restoring and Managing Longleaf Pine Ecosystems unit, and principal investigator for a comparative analysis that examined how different methods of timber harvesting and forest regeneration affect understory species such as shrubs, grasses and other flowering plants. The first decade of results from this study were recently published in Forest Ecology and Management.

Brockway and Ken Outcalt, a retired research plant ecologist formerly with the SRS Center for Disturbance Science in Athens, Georgia, studied longleaf pine stands in the flatwoods of the Goethe State Forest and uplands of the Blackwater River State Forest. Both forests are located in Florida, but have different land use and management histories and different combinations of understory species.

Silviculture treatments tested by the scientists included two selection systems and two shelterwood methods. “All four treatments reduced the overstory canopy, improving growing conditions for understory plants,” says Brockway. “The two selection systems mimicked small-scale natural disturbances, while the shelterwood more closely resembled conditions that may result from more severe larger-scale disturbance.” Both selection systems were implemented with Pro-B, an easy-to-learn and apply method for marking stands originated by Ed Loewenstein of Auburn University, and further developed through studies with Brockway and Outcalt.

Prescribed fire in longleaf pine stands is essential to sustainable management, as fire controls undesirable vegetation while the resulting mineral ash becomes part of the nutrient cycle. Fallen needles from longleaf pine, wiregrass, and other understory species help surface fires effectively burn through a stand. In the study, wiregrass responded to the shelterwood treatments very differently, depending on the type of site – in flatwoods, wiregrass became more common after harvest, but in uplands, it declined sharply.

Group selection, which thins forest stands and creates multiple small canopy gaps of a half-acre or less, led to increases in wiregrass on both flatwoods and uplands. After group selection treatment in the uplands, understory plant cover doubled and hardwood tree seedlings, shrubs, vines, ferns, and other species increased significantly.

Single-tree selection, which thins forest stands without the specific goal of creating canopy gaps, also led to increases in wiregrass. In uplands, single-tree selection was also related to increases in oak, dangleberry, broomsedge bluestem, and several forbs. “We need to continue our study of plant communities on differing sites to better understand how various harvest methods interact with understory species,” says Brockway. “Selection silviculture – along with frequent prescribed fire – appears to be a low risk and effective strategy for managing longleaf pine ecosystems.”

Whether single trees or small groups of trees are harvested, selection systems should lead to gradual changes in forests while providing other benefits. “Selection systems can maintain an open, park-like stand structure,” says Brockway. “They can also produce a regular stream of forest products and preserve a greater range of management options for the future.”

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For more information, email Dale Brockway at dbrockway@fs.fed.us.

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