Two Projects Studying Aftermath of Tornado

Peterson, Chris

On 27 April 2011 a tornado outbreak spawned three long-track tornadoes that passed across densely forested and mountainous landscapes (including one in the park), providing an exceptional opportunity to characterize this important but poorly studied type of natural disturbance, determine how disturbance characteristics are influenced by physiographic features, and test models that predict forest recovery trajectories from levels of disturbance severity. In the Smokies, an F4 tornado destroyed powerline pylons in Chilhowee Lake and moved into the park, creating extensive destruction along the Hannah Mountain, Hatcher Mountain, and Beard Cane Trails especially, moving in a NE direction. Disturbance severity at the landscape scale is derived from aerial photos of the tornado swaths. PI will monitor regeneration for several years in plots wherein the severity has been fully characterized. It will be possible for the first time to directly test models that both describe the severity of impact and the regeneration post-event. A second study area is outside the park on National Forest land in Georgia and will be subject to post-tornado salvage logging. In July 2011, aerial photos were taken along the entire length of both tornado tracks. The geographic coordinates were determined for over 448,000 standing and fallen trees, and treefall orientation for > 130,000 of the fallen trees. The damage path of the track shows that the tornado crossed Abrams Creek from left to right and ascended another ridge. As the winds ascended the draw from Abrams Creek, the flow became impeded by the steep slopes on either side and dramatically accelerated upslope. Several additional examples appear to indicate that topographic features that align within a few degrees of the path of the parent storm can influence the path of the tornado at ground level. In summer 2012, damage surveys were initiated in the park, establishing 22 plots where every tree with a dbh >10cm is identified and its damage described. Another 10-12 plots are planned for 2013. The wind disturbance in these locations did not appreciably change species composition. Overall, in these survey plots, density (trees > 10 cm dbh) was reduced by 41.5%, and basal area was reduced by 57.6 %. However, severity of damage varied substantially among plots: severity of basal area lost ranged from 19% in Hatcher 9 and 21% in Hatcher 4, to 99% in Beard Cane 1 and 100% in Cooper Road 7. Shifts in relative abundance by two measures were minimal. Comparing the amount of damage experienced by the major trees, in terms of amount of pre-disturbance basal area lost, eastern hemlock and white pine suffered the least losses (28.6% and 46.2%, respectively), while red maple and scarlet oak suffered the most losses (66.8% and 75.3%, respectively). There is a significant trend for treefall risk to increase as tree size increases.