Forest disturbance in hurricane-related downbursts in the Appalachian mountains of North Carolina
The authors characterized five 0.2 to 1.1 ha gaps created by downbursts during Hurricane Opal in xeric oak forest at the Bent Creek Experimental Forest, Asheville, NC. Direction of windthrow was nonrandom in four of the five gaps, but differed among gaps, suggesting that each was caused by an independent downburst. Windthrows reduced tree density by 19 to 39 percent and basal area (BA) by 30 to 53 percent within gaps. Most windthrows were uprooted (17 to 38 percent of all trees) versus broken below 1.8 m height (0 to 3 percent). Most species were uprooted in proportion to their abundance regardless of canopy position. Red oaks (Quercus coccinea, Quercus rubra, and Quercus velutina) were disproportionately uprooted, while Nyssa sylvatica and Acer rubrum were resistant to uprooting. As a group, Quercus lost 27 to 47 percent of individuals and 41 to 50 percent of BA. Q. coccinea lost $ 44 percent of trees and > 55 percent of BA in sites where it occurred. Only minor shifts in canopy species dominance were evident. For several species, significantly more large-diameter individuals uprooted than their smaller counterparts. No relationship between d.b.h. and number uprooted was detected for the red oaks, however. Canopy position appeared to have little bearing on this relationship. Uprooting disturbed 1.6 to 4.3 percent of the ground area and displaced 130 to 587 m3 of root-soil-rock masses(rootmasses) per gap. Greenberg and McNab suggest that episodic, high-intensity wind is not uncommon, and has a substantial influence on forest structure, species composition, regeneration, and microtopography of the Southern Appalachian mountains at variable scales.