Effects of understory prescribed burning on shortleaf pine (Pinus echinata Mill.)/mixed-hardwood forests
We examined the effects of a single dormant season fire on overstory and understory species diversity and composition and tree seedling regeneration patterns the first and second years following a prescribed burn in the Conasauga River Watershed of southeastern Tennessee and northern Georgia. We asked: Can a single dormant season fire initiate a trajectory of overstory and understory species change consistent with restoring Pinus enchinata/mixed-oak/bluestem (Andropogon gyrans and Schizachyrium scoparium)-grass community types? Six sub-watersheds (similiar in begetation, soil type, stream size, and disturbance history) were located within the Conasauga River Watershed; four of the sites were burned in March 2001, and two sites were designated as controls. Within each site, vegetation was measured in layers: the overstory layer(trees ≥ 5.0-cm DBH), the midstory layer (woody stems < 5.0-cm DBH and ≥ 0.5 m height), and the ground flora layer (woody stems < 0.5-m height and all herbaceous species). All plots were sampled before the prescribed burn (Sept. 2000) and after the burn in July of 2001 and 2002. Consistent with the goals of the land managers, all the prescribed fires resulted in low-to-moderate intensity and low severity fires. However, we found no significant change in the overstory, midstory, or ground flora species diversity after burning. We found no regeneration of P. echinata seedlings after the prescribed fire. Although fire reduced basal area of woody species in the midstory, prolific sprouting from hardwoods resulted in higher density of fire-sensitive hardwoods such as Acer rubrum, Oxydendrum arboretum and Nyssa sylvatica. density of Pinus strobus, an undesirable species, was reduced by 20% and its basal area was redued by 50% after the burn. Overstory mortality occurred in small size class hardwoods as a result of the fire, but most of the mortality occurred in P. echinata and P. virginiana Miller due to infestation with pine bark beetles. The prescribed fires were not of sufficient intensity to: reduce overstory basal area, prepare a seedbed for successful pine germination, affect diversity of any of the vegetation layers, or promote A. gyrans and S. scoparium recruitment. Thus, additional fire treatments or a combination of fire and thinning treatments will be necessary to restore these ecosystems to P. echinata/mixed-oak/bluestem grass community types.