Abstract:
Natural disturbances play important roles in shaping the structure and composition of all forest ecosystems and can be used to guide silvicultural practices. Disturbance intensity is measured along a gradient ranging from highly localized, gap-scale events to stand-replacing events. High wind storms such as downbursts, derechos, and low intensity tornadoes typically fall in the center of this gradient and result in intermediate-scale disturbances, removing 30-60% of basal area. Despite their frequency and widespread occurrence, little is known about how intermediate-scale disturbances drive stand development. On 20 April 2011, the Sipsey Wilderness Area in Alabama was affected by an EF1 tornado with accompanying straight-line winds. Stands were sampled in a stratified subjective sampling design to evaluate the effects of intermediate-scale wind disturbance on the development of Quercus stands in regard to structure and recruitment. My specific objectives were to: 1) quantify damage severity in basal area reduction and percent canopy loss of this particular disturbance along a gradient of wind disturbance, 2) detect structural acceleration or retrogression of stand development caused by an intermediate-scale wind disturbance, and 3) elucidate compositional acceleration or retrogression for an intermediate-scale wind disturbance. I established 109 0.04 ha plots across a gradient of disturbance, classified as control (undamaged), light, and moderate to inventory the effect of wind damage on development and succession. Basal area was reduced from 25.5 m2 ha-1 to 24.0 m2 ha-1 and 15.5 m2 ha-1 (p < 0.001) for light and moderate damage, respectively. The percent of live and damaged trees for control, light, and moderate was 0.3%, 3.0%, and 10.7%, respectively. PAR was significantly increased within the moderately damaged areas (p < 0.001). Logistical regression showed an increasing probability of mortality during wind disturbance with increasing diameter. Based on my findings, this intermediate-scale disturbance increased intra-stand heterogeneity and accelerated succession, favoring shade-tolerant taxa established in the understory.
