Effects of an intermediate-scale wind event on forest composition, structure, and structural complexity
Forest disturbances alter environmental conditions, influence species composition and stand structure, and affect successional and developmental pathways. Natural disturbances differ in magnitude, severity, and return interval and range from frequent, gap-scale disturbances, to infrequent stand-replacing events. Disturbances at the gap and intermediate scale increase structural complexity and intra-stand heterogeneity. On 20 April 2011 in Lawrence County, AL, an EF1 tornado tracked 5 km, leaving a patchwork mosaic of disturbed areas. To analyze the intra-stand spatial patterns of tree morality and biological legacies after an intermediate-scale wind disturbance, I established a 100 × 200 m (2 ha) rectangular plot perpendicular to the path of the storm within an affected Quercus alba stand. Based on the basal area removed by the wind event, I divided the plot into disturbance classes (minimal, light, and moderate) to compare compositional and structural attributes across areas of increasing disturbance severity. I analyzed species- and size-specific mortality trends within each disturbance class. In addition, I quantified the structural complexity of each disturbance class and described the effect of the intermediate-scale disturbance on stand development. Composition was not substantially affected by the disturbance, but large stems were disproportionately removed by the storm. Structural complexity increased as a result of the wind event. However, the spatial distribution of stems was more uniform after the disturbance. The intermediate-scale wind event altered the stand size class from a mature stand to a mature–sapling mosaic stand. This size class characterizes a stand in the mixed stage of development. Results from this thesis contribute to the understanding of the compositional and structural attributes of upland Quercus stands after an intermediate-scale wind event. Quantitative descriptions of this stand may be used as references to inform silvicultural systems intended to enhance structural complexity and minimize the disparity between natural and managed stands.