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Severe ice storms occur every year in the United States, particularly in the Midwestern and eastern regions, resulting in millions of dollars in monetary losses. Tree species vary in their resistance to ice accumulation. Certain characteristics, such as “included” bark, dead and decaying branches, a broad crown, and fine branching, increase a tree’s susceptibility to ice storm damage. Planting trees resistant to ice storms and performing regular tree maintenance to 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 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 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. They vary considerably in their severity and frequency. Ice storms are a result of the ice formation process, which is influenced by general weather patterns. Ice accumulates when super cooled rain freezes on contact with surfaces, such as tree branches, that are at or below the freezing point. This generally occurs when a winter warm front passes through an area after the round-level temperature reaches or falls below freezing. Rain falls through layers of cooler air without freezing, becoming super cooled. Periodically, other climatic events, including stationary, occluded, and cold fronts, also result in ice storms.
Conditions that result in ice storms are most prevalent in the Midwestern and eastern parts of the United States.
Accumulations of ice can increase the branch weight of trees by 30 times or more. Ice formation generally ranges from a trace to 1 inch in additional stem diameter. Accumulations between 1/4 and 1/2 inch can cause small branches and weak limbs to break, while 1/2-inch to 1-inch accumulations can cause larger branches to break, resulting in extensive tree damage. Branch failure occurs when loading from the weight of ice exceeds wood resistance or when constant loading further stresses a weakened area in a branch. Strong winds substantially increase the potential for damage from ice accumulation.
What Makes a Tree Susceptible to Damage?
A number of characteristics increase a tree species’ susceptibility to ice storms: “included” bark, decaying or dead branches, increased surface area of lateral branches, broad crowns, and imbalanced crowns. Included bark (inset) results from in-grown bark in branch junctures. This weak connection enhances a tree’s susceptibility to breakage under ice-loading conditions. For example, ‘Bradford’ pear branches often break during ice storms where there is included bark in branch junctures. In contrast, the ‘Aristocrat’ pear has few branches with included bark and sustains less damage during ice storms.
Decaying or dead branches are already weakened and have a high probability of breaking when loaded with ice. The surface area of lateral branches increases as the number of branches and the broadness of the crown increase. With an increased surface area, more ice can accumulate on lateral branches; the greater ice load results in greater branch failure. Contrary to popular belief, the wood strength of sound branches matters less than the ability of a tree to withstand breakage at branch junctures and the presence of fine branching or a broad crown that enhances ice accumulation.
Examples include Siberian elm, American elm, hackberry, green ash, and honey locust. Trees with imbalanced crowns are also more susceptible to ice damage.
What Makes a Tree Resistant to Ice Damage?
Juvenile and mature trees that have excurrent (conical) branching patterns, strong branch attachments, and low surface area of lateral branches are generally resistant to ice storms. Many conifers have an excurrent branching pattern, and many resist ice storm damage. Some tree species, such as sweet gum, have an excurrent growth habit when young but develop a decurrent growth habit later in life. These species are more resistant to breakage when young than broadleaf trees that do not exhibit a juvenile excurrent branching pattern.
Some tree species that typically exhibit a decurrent branching pattern have clones with an excurrent form, which should have greater resistance to ice storm damage. Tree species with strong branch attachments have greater resistance to breakage than those with included bark. Trees with coarse branching patterns (fewer, thicker branches) and, as a consequence, lateral branches with reduced surface area, such as Kentucky coffee tree, black walnut, and ginkgo, accumulate less ice and typically have little breakage from ice storms. Forest under story tree species such as ironwood and blue beech and trees that mature at small heights, such as amur maple, are also relatively resistant to ice storm damage.
Tree Species Susceptible to Ice Storm Damage
Commonly Planted in Urban Areas
Eastern White Pine
Northern Red Oak
Kentucky Coffee Tree
Swamp White Oak
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