Trees and Ice Storms The

TREES AND ICE STORMS THE DEVELOPMENT OF ICE STORM– RESISTANT URBAN TREE POPULATIONS 15 Contents Summary .............................. 2 Introduction ......................... 3 Historical Accounts .............. 5 Tree Features Influencing Ice Storm Susceptibility ........ 6 Tree Features Influencing Ice Storm Resistance ............. 7 Ice Storm Damage in Wooded Areas ....................... 8 Ice Storm Damage Manage- ment and Prevention ............ 9 References ........................... 12 Richard J. Hauer, Mary C. Hruska, and Jeffrey O. Dawson. 1994. Trees and ice storms: The development of ice storm–resistant urban tree populations. Special Publication 94-1, Department of Forestry, University of Illinois at Urbana- Champaign. Urbana, IL 61801. 12pp. Funding for this publication was provided by the technology transfer program of the USDA Forest Service Urban Forestry Center for the Midwestern States. Graphic Designer: Lynn Hawkinson Smith Illustrator: M.R. Greenberg Technical Editor: Molly Bentsen Photo credits: Cover—Reprinted with permission of Joel Dexter; pages 2, 5, 6, 7, 10, 11—Michael R. Jeffords; page 4—(l. to r.) Mary Hruska, Joel Dexter, Michael Brunk, Jim Skiera; pages 8, 9, 12—Lynn Hawkinson Smith 1 The authors gratefully thank Robert Brush, Gina Childs, Robert Miller, and George Ware for their constructive reviews and comments. SUMMARY evere 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 S 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 mainte- nance 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. 2 Cloud air temperature 35°F 30°F 25°F Cumulus Warm air Stratus Cold air Ordinary Subcooled rain Sleet Snow pellets Snow- Snow Forms of precipitation rain (forms glaze) (frozen rain) (graupel) flakes crystals Figure 1. Forms of INTRODUCTION precipitation that result when a winter warm front slowly advances into ce storms, also referred to as patterns. Ice accumulates when ground layer air at or glaze storms, cause considerable supercooled rain freezes on con- below 0oC and the relationship to cloud air damage every year to trees in tact with surfaces, such as tree temperature (from Lemon, urban and natural areas. They branches, that are at or below 1961). vary considerably in their the freezing point (0oC). This severity and frequency. generally occurs when a winter Figure 2. Ice-loading districtsI in Ice storms are a result of the warm front passes through an the United States for ice ice formation process, which is area after the ground-level accumulation on surfaces (from influenced by general weather temperature reaches or falls National Bureau of Standards, 1948). below freezing (Figure 1). Rain falls through layers of cooler air without freezing, becoming supercooled. 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, as illustrated Loading district Alaska–heavy by the ice-loading districts Hawaii–light Heavy Medium Light 0.50 0.25 0 shown in Figure 2. Radial thickness of ice (in.) 3 Accumulations of ice can potential for damage from ice increase the branch weight of accumulation. trees by 30 times or more. Ice Monetary losses to trees and formation generally ranges from other property can be extensive a trace to 1 inch in additional after an ice storm. In 1990, stem diameter. Accumulations more than a million dollars in between 1/4 and 1/2 inch can damage to parkway trees alone cause small branches and weak occurred as a result of a severe limbs to break, while 1/2-inch ice storm in Urbana, Illinois. to 1-inch accumulations can According to records of the U.S. cause larger branches to break, Federal Emergency Manage- resulting in extensive tree ment Agency, a severe ice storm damage. Branch failure occurs in 1991 in Rochester, Minne- when loading from the weight sota, caused $16.5 million of ice exceeds wood resistance or worth of property damage. In when constant loading further the same year, a widespread ice stresses a weakened area in a storm in Indiana caused $26.8 branch (Figure 3). Strong winds million in property damage. substantially increase the Figure 3. Included bark and woody decay enhance branch breakage and tree damage when ice 4 accumulates on trees (photo by Jim Skiera). HISTORICAL ACCOUNTS n one of the earliest document- of a thunderstorm. Pedestrians ed accounts of an ice storm in kept to the middle of the the United States, Harshberger thoroughfares and many people (1904) reported that there were remained indoors rather than two exceptionally destructive ice risk the uncertainties of the storms around Philadelphia in public streets.” He also reported The air was 1902. One storm was accompa- that where trees had stood close I nied by high winds and did together, streets became com- filled for hours irreparable damage to numerous pletely blocked and passageways with the rifle fruit, forest, and shade trees. had to be chopped out with The other deposited more ice, axes. Abell (1934) reported that report of snap- but because of the lack of wind forests of the southern Appala- ping branches there was less damage. Twelve chian area had been repeatedly years later in eastern Pennsylva- damaged by ice storms. Refer- followed by the nia and western New Jersey, an ring to a storm in western North crash of ice- area of approximately 600 Carolina in 1932, he quoted a square miles was damaged by an mountaineer to have said, “By laden branches ice storm (Illick, 1916). Actual two o’clock Sunday morning smashing to the counts of damaged trees indi- there was no sleeping at all for cated that 90 percent of the the noise of breaking timber.” ground. forest trees either had their More recently, on Valentine’s crowns broken off entirely or Day 1990, a severe ice storm in were damaged so badly that only Urbana, Illinois, damaged at stubs of branches remained. least 26 percent of the city’s Viewed from a distance, the parkway trees (Hauer et al., forests resembled broken masts. 1993). About 5 percent were Reporting on a severe storm severely damaged and required in southern Wisconsin, Rogers immediate removal or repair. (1924) wrote that “great tree The air was filled for hours with branches ripped from their the rifle report of snapping moorings with startling sudden- branches followed by the crash ness came hurtling downward of ice-laden branches smashing through the air to strike the to the ground. Most of the city ground with such force that the was without power, for as long sounds at times resembled those as eight days. 5 The wood strength of sound branches matters less than the ability of a tree to withstand TREE FEATURES INFLUENCING ICE STORM SUSCEPTIBILITY breakage at branch junctures. number of characteristics breakage under ice-loading con- increase a tree species’ suscepti- ditions. For example, ‘Bradford’ bility to ice storms: “included” pear branches often break bark, decaying or dead branches, during ice storms where there is increased surface area of lateral included bark in branch junc- branches, broad crowns, and tures. In contrast, the ‘Aristocrat’ imbalanced crowns (Figure 4). pear has few branches with A Included bark (inset) results included bark and sustains less from in-grown bark in branch damage during ice storms. junctures. This weak connection Decaying or dead branches enhances a tree’s susceptibility to are already weakened and have a Broad crown high probability of breaking Dead and decaying branches 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 Broken branch enhances ice accumulation. Fine branching Figure 4. Characteristics that increase a tree’s susceptibility Included bark to damage from ice storms. 6 Many broad-leafed tree species, Examples include Siberian elm, when grown in the open, form American elm, hackberry, green broad crowns (decurrent ash, and honey locust. Trees branching), increasing their with imbalanced crowns are also susceptibility to ice storms. more susceptible to ice damage. TREE FEATURES INFLUENCING ICE STORM RESISTANCE uvenile and mature trees that excurrent branching pattern. have excurrent (conical) branch- Some tree species that typically ing patterns, strong branch exhibit a decurrent branching attachments, and low surface pattern have clones with an area of lateral branches are excurrent form, which should generally resistant to ice storms have greater resistance to ice (Figure 5). Many conifers have storm damage. Tree species with J an excurrent branching pattern, strong branch attachments have and many resist greater resistance to breakage Narrow crown ice storm damage. than those with included bark. Some tree species, Trees with coarse branching Coarse branching such as sweet patterns (fewer, thicker gum, have an branches) and, as a conse- excurrent growth quence, lateral branches with habit when young reduced surface area, such as but develop a Kentucky coffee tree, black decurrent growth walnut, and ginkgo, accumulate habit later in life.

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