Hurricane effects on forest understory vegetation in the southern Appalachians

The effects of wind on upland hardwood forest structure and composition have been studied mostly in the context of either small 1 or 2 tree death openings (gap-phase disturbance), or in retrospective studies of ancient disturbances. Larger (>0.1 ha) wind-created openings are common across Southern Appalachian landscapes and can be an important factor in shaping understory colonization, growth, and survival. I investigated the relationships of tree seedling survivorship and growth, tree seedling density, herbaceous species richness, shrub cover densities, and solar radiation received beneath tree canopies in and around large, hurricane-created gaps on the Bent Creek Experimental Forest.; Using logistic regression, I found that seedling survivorship of both the all-species and oak groups declined progressively on a linear distance gradient from gap exterior to gap center. All-species survivorship also declined in the presence of hurricane-created woody debris. Oak survivorship declined as microsite soil moisture increased.; I employed multiple nonlinear regressions and found that basal diameter growth and height growth of both species groups increased towards gap center. Growth was positively related to initial seedling height and predicted soil moisture, and negatively related to canopy cover.; Shrub coverage was positively correlated with time since hurricane-damage and light received at the forest understory; and negatively correlated with gap aperture (a measure of light reaching the gap based on gap size and height of the surrounding tree canopy), and predicted soil moisture. Herbaceous species richness was positively correlated with distance from northern gap edge, gap perimeter, and predicted soil moisture; and negatively correlated with crown debris.; All-species tree seedling density was positively related to predicted soil moisture, eastern gap position, and transformed gap area; and negatively related to hurricane-created crown debris and midstory canopy cover. Oak ( Quercus) seedling density was highest in the south of gaps and on high-energy aspects. Crown and large woody debris cover were negatively correlated with oak densities. Yellow-poplar (Liriodendron tulipifera L.) seedling density was positively related to distance from southern gap edge, predicted soil moisture, and time since disturbance; and negatively associated with crown debris.; Hemispherical photo predicted FPAR (fraction of photosynthetically active radiation received beneath tree canopy) increased in circle-shaped gaps, and declined in elliptical or rectangular shaped openings. Predicted FPAR also increased as midstory canopy cover decreased.; Separate models were developed relating quantum sensor measurements of PAR (photosynthetically active radiation) to total residual canopy cover and distance from gap center. Total PAR was predicted to decrease as total canopy cover increased, and to decrease on a linear distance gradient from gap center to the unaffected forest.