Doctoral Dissertations

Date of Award

8-2000

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geography

Major Professor

Sally P. Horn

Committee Members

Michael Huston, Carol Harden, Ken Orvis

Abstract

Major ice storms deposit heavy loads of freezing rain on trees, causing intense disturbances in eastern North American forests. Two ice storms that affected southwestern Virginia in 1994 caused heaviest damage on windward mountain slopes (those facing southeast or east). Several processes may have contributed to aspect-related variations in ice accretion, including orographic effects on rainfall and influences of wind on twig-surface and raindrop thermodynamics. These topographic patterns in ice storm disturbance appear to be typical in the Appalachians. Another characteristic pattern is the confinement of damage to specific elevation zones. Elevational zonation was evident in forests of Virginia and New York that were affected by ice storms during 1998. Several factors, including tree size, wood strength, and canopy architecture, influence tree damage characteristics within stands. Small trees typically suffer bent or broken stems, and large trees usually sustain canopy damage. Toppling is most common in medium-size trees. Canopy damage is common in species with weak wood and straight boles, whereas toppling is more frequent among trees with stronger wood. Site factors, such as slope and soil depth, also affect damage patterns. Steep slopes and thin soils contribute to higher rates of toppling. Frequency of ice storm disturbance influences how significant these events are for vegetation dynamics. In the southern Appalachians, ice storms occur most frequently in eastern parts of the region, where subfreezing surface air becomes trapped against the mountains and creates conditions favorable for freezing rain. A dendrochronological analysis indicates that ice storms produce tree-ring signatures that may be useful for detecting fine-scale spatial variations in ice storm frequency. Forest modeling results suggest that periodic ice storms have significant long-term consequences for Appalachian forest dynamics. These disturbances may reduce species richness on xeric ridgetops and enhance richness on mesic sites, predictions consistent with theoretical expectations. They are also predicted to reduce the degree of compositional zonation along a topographic gradient of soil moisture and to promote increased Quercus importance over much of the landscape. Ice storms may be especially significant for promoting the maintenance of shade-intolerant species, such as Robinia pseudoacacia and Liriodendron tulipifera, on Appalachian landscapes.

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