Symposium on the F-Scale and Severe-Weather Damage Assessment


Factors Influencing Treefall Risk in Tornadoes in Natural Forests

Chris J. Peterson, University of Georgia, Athens, GA

The Fujita scale of tornado and wind severity is based to a small extent on levels of damage to trees. However, the simple tree-damage characteristics may not be sufficiently precise or may even lead to incorrect estimates of wind speeds, because a wide range of factors influence how badly damaged a tree is from winds of particular velocities. In this presentation, I summarize for a meteorological audience my findings from ecological studies of tornado damage to forest trees in 9 different locations in the eastern United States. The focus is on how easily-identified tree characteristics, such as species and size, influence the level of damage sustained. A clearer understanding of how various factors influence the level of tree damage will allow refinement and improvement of the Fujita scale for estimating tornado intensity.

The clearest pattern that emerges from these field studies is that larger trees are more likely to be damaged than smaller trees, but this seemingly simple relationship is made more complex because the rate of increase in risk of damage or treefall is not constant across sites or species. For example, in two sites that have abundant beech (Fagus grandifolia) and hemlock (Tsuga canadensis), the relative vulnerability of these species reversed. Moreover, relative vulnerability among species is a function of size: in small size classes, hemlock is less likely to be toppled than beech, but in larger size classes, hemlock is more vulnerable. Broad generalizations are emerging that shallow-rooted species are more likely to be toppled by wind than tap-rooted species, and pines are often more damaged than hardwoods but there are clear exceptions. In the most severely-damaged sites, differences among species become obscured, because essentially all trees above some minimum size are damaged or toppled. Finally, within particular sites, anomalous patterns remain (e.g. a large tree of a vulnerable species that is undamaged amidst damaged neighbors) that suggest influences by other non-biological factors, such as soil depth. Much improvement is needed in how tree damage is assessed in the Fujita scale rankings if more accurate assessments of tornado intensity are to be achieved.

extended abstract  Extended Abstract (1.2M)

Session 3, Damage, Winds, and F-Scale II
Monday, 10 February 2003, 1:30 PM-2:30 PM

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