Landscape distribution and characteristics of large hurricane-related canopy gaps in a southern Appalachian watershed
Hurricane-related winds are a major source of disturbance in coastal ecosystems of the southern United States, but their effects on forests in the southern Appalachian Mountains, >400 km inland, have seldom been documented. In October 1995, remnant winds of Hurricane Opal caused windthrow of individual and patches of trees throughout the mountainous region of western North Carolina, USA. The 2-day storm event was accompanied by over 150 mm of precipitation and gusty, predominantly southeasterly winds with peak velocities at low elevations of up to 26 m/s; peak velocities were over 40% greater at high elevations. In a landscape-scale case study, we spatially located the population of large canopy gaps (groups of > 10 windthrown trees) within a 2400 ha watershed to determine frequency of occurrence on basin and highland landscape types, dimensions, association with topographic features, and direction of treefall as a measure of wind vectors. The distribution of large gaps was not random within the watershed and occurred at an average density of I per 39 ha in the basin (elevation ~700 m), which is characterized by hills of low relief and soils with high clay content. In comparison, gap density averaged 1 per 192 ha on the surrounding highlands (elevation >700 m) of high relief and soils with low clay content. Gaps on both landscape types occurred with greater frequency on sites of southeasterly aspect. Sizes of large canopy gaps ranged from 0.1 to 3.9 ha (average 0.7 ha) and were not correlated with landscape type or topography. Gap shape tended to be linear and averaged 2.3 times longer than wide. Direction of mean treefall among gaps was predominantly northwesterly and was strongly associated with aspect on highland, but less so in basin landscape types. Variation in mean treefall direction among gaps suggests that gaps were created by individual gusts of high-velocity, linear winds. A logistic discriminant model based on elevation, azimuth, and slope gradient correctly classified 78% of the study sites. However, gap size and association with topographic features were similar between basin and highland landscapes. Although the southern Appalachian Mountain region is over 400 km from the Gulf of Mexico and the Atlantic Ocean, results from our study suggest that strong winds from hurricane remnants can influence forest structure, with greater impacts on basin than highland landscapes.