By Richard J. Hauer, Jeffrey O. Dawson, and Les P. Werner
Stevens Point, WI (January 1, 2009)- Severe ice storms occur every year in the United States and Canada, particularly in the Midwestern and eastern regions of the United States. Ice storms annually result in millions of dollars in loss, and potentially billions of dollars in losses for extreme and widespread ice storms. Damage to electric distribution systems, blocked roadways, and property damage from fallen trees and limbs pose safety concerns and disrupt normal community functions. Tree species vary in their resistance to ice accumulation. Certain characteristics, such as weak branch junctures indicated by included bark, dead and decaying branches, a broad crown, and fine branching, increase a tree’s susceptibility to ice storm damage.
Planting a diverse urban forest that includes trees resistant to ice storms and performing regular tree maintenance to avoid or remove structural weaknesses will reduce damage caused by severe ice storms. Management plans for urban trees should incorporate information on the ice storm susceptibility of trees in order to: limit potential ice damage; to reduce hazards resulting from ice damage; and to restore urban tree populations following ice storms. Susceptibility ratings of species commonly planted in urban areas are presented in this publication for use in developing and maintaining healthy urban tree populations.
Ice storms, also referred to as glaze storms, cause considerable damage every year to trees in urban and natural areas within the United States. They vary considerably in their severity and frequency and are one of the most devastating winter weather events. Every year at least one major ice storm is expected. Glazed roads and pathways, fallen power lines, power outages, and falling trees and branches result in deaths and injuries to people. Monetary losses typically are tens to hundreds of millions of dollars. In extreme cases that occur once every 10 to 20 years, ice storms have the potential to cause losses in the billions of dollars.
Conditions that result in ice storms are most prevalent in the central, northeastern, and southeastern parts of the United States, as illustrated through maps of ice accumulation and ice storm frequency. Accumulations of ice can increase the branch weight of trees by a factor of 10 to 100 times.
Monetary losses to forests, individual trees, utility lines, agriculture, commerce, and property can be extensive after an ice storm. Between the years of 1949 and 2000, insured property losses from freezing rain were $16.3 billion U.S. dollars (adjusted to the value of year 2000 dollars). Actual losses are even greater as this total excludes non-insured losses. As an example, losses from a 1998 ice storm covering the northeastern United States and southeastern Canada were estimated at $6.2 billion with less than one-half of this amount insured.
Other effects include more than four million people without power and more than 40 deaths attributed to the ice storm. Tree damage to electrical systems are the primary cause of outages. In 1990, more than a million dollars in damage to parkway trees alone occurred as a result of a severe ice storm in Urbana, Illinois documented by a $12 million federal disaster declaration.
According to records from the U.S. Federal Emergency Management Agency, a severe ice storm in 1991 in Rochester, Minnesota, caused $16.5 million worth of property damage. In the same year, a widespread ice storm in Indiana caused $26.8 million in property damage. On average, ice storms account for more than 60 percent of winter storm losses within the United States at a mean total annual cost of $226 million.
Ice storm frequency and severity within the eastern United States necessitates the incorporation of ice storm information into the urban forestry planning process. While we cannot stop ice storms from occurring, we can take steps to reduce the impact of this major forest disturbance on urban forests and the interface between forests, buildings, and infrastructure.
Trees and Ice Storms: The Development of Ice Storm-Resistant Urban Tree Populations, Second Edition