Composition, structure, and intra-stand spatial patterns along a disturbance severity gradient in a Quercus stand
Natural forest disturbances, which drive succession and development, differ in extent, severity, and return interval and range from frequent, gap-scale disturbances, to infrequent stand-replacing events. Most studies have focused on natural disturbances near the ends of the disturbance severity gradient and relatively little quantitative information is available on intermediate-severity disturbance. On 20 April 2011, an EF1 tornado tracked 5 km through the Sipsey Wilderness in Alabama and resulted in a patchwork mosaic of disturbed areas. To analyze the effects of the intermediate-severity wind event on composition, structure, and intra-stand spatial patterns, we 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 (i.e. basal area of snags, snapped stems, or uprooted stems in decay class 1) by the wind event, we divided the plot into disturbance classes (minimal, light, and moderate) to compare compositional and structural attributes along a disturbance severity gradient. Composition varied little across the disturbance gradient, but diversity was highest in the moderately disturbed neighborhoods. Stems were relatively intermingled by species (i.e. each tree neighbored by trees of different species) in each disturbance severity class. However, some species, such as Fagus grandifolia and Ostrya virginiana exhibited less intermingling than Quercus spp. and stems classed in the "other spp." taxonomic group. Large stems were disproportionately removed by the storm in the light and moderate disturbance categories. In the light disturbance class, O. virginiana was significantly less likely to experience mortality from the storm, which may in part explain the relatively high density of O. virginiana stems in the plot.