Hardwood-Pine Mixedwoods Stand Dynamics following Thinning and Prescribed Burning

Restoration of hardwood-pine (Pinus L.) mixedwoods is an important management goal in many pine plantations in the southern Cumberland Plateau in north-central Alabama, USA. Pine plantations have been relatively unmanaged since initiation, and thus include a diversity of hardwoods developing in the understory. These unmanaged pine plantations have become increasingly vulnerable to insects, and management activities were initiated to facilitate transition towards hardwood-pine mixedwoods. We evaluated a combination of thinning and prescribed fire prescriptions in a randomized complete block design with a 3 × 3 factorial treatment arrangement and four replications of each treatment. Treatments were combinations of thinning to three residual basal areas (no thin, light thin to 17.2 m2 ha−1, heavy thin to 11.5 m2 ha−1) and three prescribed burn applications (no burn, one burn to be repeated every 9 years, three burns repeated every 3 years). Burning without thinning altered stand structure by reducing overstory stem density by 15%, whereas thinning without burning reduced density by 70% and burning coupled with thinning resulted in reduced overstory trees by 72%. Frequent fire had the greatest impact on midstory structure and regeneration. Midstory stem density was reduced by 90% following thinning and burning. Oaks (Quercus L.) and red maple (Acer rubrum L.) were the most common understory species after thinning only, burning only, and thinning combined with burning. There were more oak and red maple seedling sprouts following frequent burning. Currently, >75% of red maple sprouts dominate the regeneration, compared to only 40% of the oaks. Although the treatments have accelerated the transition toward hardwood-pine mixedwoods, the fate of oak and which hardwood species will be dominant in the future remains uncertain.