Longleaf pine and oak responses to hardwood reduction techniques in fire-suppressed sandhills in northwest Florida

Restoring fire-suppressed longleaf pine (Pinus palustris Mill.) sandhill co mmunities often includes reducing hardwood structure before re-establishing maintenance fire regimes. Using a randomized complete block design, we com pared the effects of three hardwood reduction techniques (spring burning, a pplication of the ULW (R) form of the herbicide hexazinone. and midstory ch ainsaw felling/girdling) and a no-treatment control on oak and longleaf pin e densities in fire-suppressed sandhills at Eglin Air Force Base. FL. Treat ments were applied in the spring and summer of 1995. Felling/girdling and h erbicide plots were also burned for fuel reduction from March to April in 1 997. Frequently burned, high-quality sandhill plots were sampled to establi sh reference conditions. Pre-treatment diameter distributions of oaks follo wed a negative-exponential curve in all treatments, but were Aat with low t ree densities in reference plots. Oak densities were significantly reduced in the herbicide and felling/girdling plots in 1995. Compared to the contro ls, growing season fire topkilled up to 20% more hardwoods among smaller tr ees in 1995, but this value increased to approximately 50% after 1996. In a ll pears, the greatest reduction of oak juvenile density (<1.4 m high) was caused by herbicide application. Control plots contained significantly fewe r oak juveniles than the burn and felling/girdling plots. Reference plots c ontained the lowest and most variable oak juvenile densities. Size distribu tions of longleaf pine across all plots were bimodal with modes at 0-4.9 an d 25-29.9 cm in diameter. The highest mode was at 0-4.9 cm in treatment plo ts and at 25-29.9 cm in reference plots, Only fire quantitatively changed t he distributions by the attrition of the smallest trees >1.4 m high in all years. Fire caused approximately 50% decreases in longleaf pine juvenile (< 1.4 m high) density in 1995 and 1997. By 1997, median juvenile densities co nverged to 5-6 stems/200 m(2) in all treatments, including the control. Juv enile densities were slightly higher and more variable in reference plots t han in treatments. In 1997, fuel reduction burns in the herbicide and felli ng/girdling plots decreased densities of recently germinated longleaf pines to <5 seedlings/20 m(2), a 90% decrease compared to 1996 densities. Seedli ng densities dropped by approximately 50% in control and burn plots, althou gh these sites received no manipulations after 1995. Seedling densities onl y decreased by 22% in reference plots (205 seldlings/20m(2) in 1996), which did experience some fires, (C) 2001 Elsevier science B.V. All rights reser ved.