By Dr. Callie Jo Schweitzer, USDA Forest Service Southern Research Station
November 14, 2017 at 1:00 PM central
How to best manage the succession of 20- to 50-year old pine (loblolly, shortleaf, Virginia) plantations to hardwood-dominated mixedwoods is a question the William B. Bankhead National Forest, northcentral Alabama is asking. The pine plantations include a diversity of hardwoods (20 species including the upland oaks, yellow-poplar, red maple and black cherry) that are developing in the understory. We evaluated various prescriptions combining thinning of pine followed by prescribed burning using a randomized complete block design with a three-by-three factorial treatment arrangement and four replications of each treatment. Treatments were combinations of three residual basal areas (heavy thin, 50 ft2 ac-1; light thin, 75 ft2 ac-1; and untreated control) with three burn frequencies (burns once every 3 years; burns once every 9 years, and unburned control). Stands were thinned June through December, and burned January through March. Burning alone altered stand structure by reducing overstory stems density by 15%, thinning alone by 70%; burning coupled with thinning resulted in a 72% reduction in overstory trees. Midstory stem density was reduced by 90% following thinning and burning. Oak and red maple were the most common ground layer after thinning, burning and thin plus burn. Frequent fire had the greatest impact on regeneration. The number of seedling clumps (from basal sprouting) was significantly increased after three burns across all thinning treatments. The number of clumps for oak species in the heavy thin/frequent burn treatment continually increased with successive burns from 240 to 710 clumps ac-1 after three burns. Similar increases in red maple clumps were observed. Currently, greater than 50% of red maple sprouts dominate the regeneration by growing taller than 4 feet, compared to only 8% of the oaks. The treatments overall have begun the transition toward hardwood-dominated mixedwoods; the question remains, which hardwoods will ultimately dominate the canopy?