Ice Storms in Central Hardwood Forests: The Disturbance Regime, Spatial Patterns, and Vegetation Influences

Abstract

Freezing rain occurs more commonly in eastern North America than anywhere else on earth. During a major freezing rain event, or ice storm, heavy ice loads accrete on vegetation and strip branches, snap boles, and uproot trees. Freezing rain commonly develops throughout a large area within a winter storm, and therefore a single storm can damage broad swaths of forest. These disturbances greatly exceed ‘background’ gap formation in severity and extent, but they do not generate the catastrophic mortality associated with severe fires or windstorms. Ice storms create disturbances of intermediate severity. A paucity of appropriate weather data obscures freezing rain climatology, but some generalizations are possible. Ice storms occur most frequently in the northeastern and midwestern USA, and a zone of high frequency also extends southward along the Appalachian Mountains and Piedmont. In mountainous landscapes, disturbance severity varies strongly over elevational and topographic gradients. Several studies have investigated ice storm damage at the scale of forest stands. Tree species vary in susceptibility, with ‘pioneer’ species typically sustaining heaviest damage and mortality. Less is known about longer-term consequences for age structure and species composition. The large gaps appear to provide recruitment opportunities for light-demanding colonizers. At the same time, the damage-resistance of slow-growing, stress-tolerant trees such as hemlocks and oaks likely enables persistence in ice-prone forests. The ecological role of ice storms depends, in part, on interactions with fires and other disturbances.