Fire exclusion vs. a fire-free interval following repeated prescribed fire: Consequences for forest stand structure and species composition in an upland oak forest

Abstract

In the Central Appalachian forest region, oaks (Quercus L.) and other disturbance-dependent tree species are experiencing widespread regeneration failure after decades of fire suppression. Managers use prescribed fire to decrease basal area with the aim of facilitating oak regeneration and shifting species composition toward pyrophytic species that dominate the overstory. Although fire may set the stage for improved oak regeneration, a fire-free period is necessary for oak seedlings to develop into fire-resistant stems. The effects of prescribed fire have been amply investigated across the region, but few studies have examined changes to stand structure and oak regeneration after cessation of burning. We followed the effects of three treatments, fire-excluded, 3x-burned, and 4x-burned, for 20 years. After a fire-free interval of approximately ten years, burned treatments had significantly lower total basal area and greater relative density of oaks in the subcanopy (10–20 cm DBH) than the fire-excluded treatment. The 4x-burned, which had the greatest reduction in basal area, also had greater relative density of midstory oaks (2–10 cm DBH), and higher stem density of oak regeneration (<2 cm DBH) than the fire-excluded. Despite these trends towards increased regeneration of oaks and other pyrophytes, red maple (Acer rubrum L.) stem density also increased in the midstory of burned treatments, suggesting a legacy effect of decades of fire suppression and the concomitant mesophication that has occurred. We also examined the effects of continued fire exclusion on the fire-excluded reference treatment. Additional evidence of mesophication on this treatment was demonstrated by increased overstory (≥20 cm DBH) and subcanopy red maple stem density as well as midstory (2–10 cm DBH) eastern white pine (Pinus strobus L.) stem density, a prominent mesophyte on these sites. This study supports the idea that repeated prescribed burning followed by a 10-year fire-free interval can alter stand structure and lead to improved oak regeneration by increasing canopy openness. However, the results were variable across burned treatments (3x vs. 4x) and the ingrowth of red maple and other mesophytes after only 10 years of fire exclusion will require additional control measures. Successful maintenance of oak-dominated forests on upland sites in the Central Appalachian hardwoods region may be contingent on a subsequent canopy disturbance that further increases light availability followed by a fire-free interval. Importantly, in the absence of burning or other disturbance, red maple and other mesophytic species will continue to increase in relative stem density.