Altered structural development and accelerated succession from intermediate-scale wind disturbance in Quercus stands on the Cumberland Plateau, USA
Natural disturbances play important roles in shaping the structure and composition of all forest ecosystems and can be used to inform silvicultural practices. Canopy disturbances are often classified along a gradient ranging from highly localized, gap-scale events to stand-replacing events. Wind storms such as downbursts, derechos, and low intensity tornadoes typically result in disturbance that would fall near the center of this gradient and result in intermediate-scale disturbances. Despite their frequency and widespread occurrence, relatively little is known about how intermediate-scale disturbances influence stand development and succession. On 20 April 2011, the Sipsey Wilderness in Alabama was affected by an EF1 tornado with accompanying straight-line winds. In the third growing season after the disturbance, stands were sampled in a stratified subjective sampling design to evaluate the effects of intermediate-scale wind disturbance on structural and successional development of Quercus stands. We established 109 0.04 ha plots across a gradient of disturbance grouped into three classes, control (considered to represent pre-disturbance conditions using a space-for-time substitution), light, and moderate categories, to examine the effect of the intermediate-scale wind disturbance. Basal area was reduced from 25.7 m2 ha1 to 23.7 m2 ha1 and 15.3 m2 ha1 for light and moderate disturbance classes, respectively. Logistic regression revealed an increasing probability of mortality during wind disturbance with increasing tree diameter. This intermediate-scale disturbance increased intra-stand heterogeneity and altered the developmental pathway. The stands did not structurally resemble one of the four widely accepted stages of stand development. The disturbance also accelerated succession and released shade-tolerant taxa that were established in midstory and understory strata prior the event.