Winter 2008 • ADVOCATE6

URBAN FOREST HEALTH

Trees and Ice Storms:IntroductionEvery year we read or hear in the popular press that an ice storm struck some part of the United States. Is the recent December 2007 ice storm across the central and southern United States a regular event or are ice storms infrequent? The answer is that ice storms occur annually somewhere in the United States, regularly shaping forest ecosystems across rural and urban landscapes. For a kid, they are a great excuse for school to be canceled and to fire the toboggan down the sledding hill. For adults, we tend to see the potential of causing significant damage to trees and property and the likewise death and injury to people. Annual losses from ice storms exceed $225 million in total damage to trees and property. A major multi-state ice storm, such as the 1997 northeastern North America storm, can exceed several billion dollars in losses. Whether localized or widespread, damage to electric distribution systems, blocked roadways, and property damage from fallen trees and limbs pose safety concerns and disrupt normal community functions.

Ice storms visit Minnesota annually with major events occurring on a regular near-annual basis.

Developing Ice Storm Resistance in Urban Tree Populations

Figure 1. Forms of precipitation resulting from a warm air mass advancing over a cold air mass.

For example, 25 severe Minnesotan ice storms have been documented between 1896 and 1953. (State Climatology Office “Severe Local Storms” and anecdotal reports). Areas near Duluth and Lake Benton on the Buffalo Ridge are especially at risk for more frequent and damaging ice storms as a result of their elevation differences with the neighboring areas. A March 4, 1935 Duluth ice storm caused a documented half-million dollars of tree and shrub damage. A 1991 Rochester storm resulted in 16 million dollars in total damage. Ice storms in the 1996/1997 winter were unfortunate precursors to the 1997 spring flooding in western Minnesota. A late 2007 winter storm with ice accumulation and winds followed by snow damaged many trees and utility systems in southeastern Minnesota.

What can you do as an urban forest manager, arborist, horticulturist, elected official, university professor, or concerned citizen? Read on and learn how 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

By Richard J. Hauer

Illustration: Lynn Hawkinson Smith

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Trees and Ice Storms continued on p. 8

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.

How do Ice Storms Form?Ice storms by definition occur when a 1/4 inch or more accumulation of freezing rain on surfaces such as tree branches and electrical wires develops. Most ice storms develop when a moist winter warm front passes over a colder surface-air layer (Figure 1). Rain falls from a warmer layer above freezing through cooler air that’s below freezing without freezing, becoming supercooled. Ice accumulates when supercooled rain freezes on contact with surfaces that are at or below the freezing point. Ice storms may occur over several days and large areas, most storms usually last only a few hours. Ninety percent of these storms occur between December and March, most occur in January. The relative likelihood of ice storms that are most prevalent in the central, northeastern, and southeastern parts of the United States is illustrated in Hauer et al. (2006).

Why Trees Fail From Ice Storms and AccumulationTrees are damaged during ice storms for a number of reasons. Severity of tree damage depends on three factors: amount of accumulated ice, exposure to wind, and duration of the storm. An increased susceptibility of tree species also involves tree characteristics: weak branch junctures indicated by included bark, decaying or dead branches, tree height and diameter, increased surface area of lateral branches, broad crowns, unbalanced crowns, restricted and unbalanced root systems, and shallow rooting habit (Figure 2). Included bark resultsfrom in-grown bark in branch junctures and it enhances a tree’s susceptibility to breakage under ice-loading. The ‘Bradford’ pear, for example, has branches that often break during ice storms where there is included bark in branch junctures. Already weak, advanced decay or dead branches have a high probability of breaking when loaded with ice. Tree branch length, horizontal branching, and inflexibility of the stem, in general, lead to greater susceptibility.

Figure 2. Tree characteristics that increase the susceptibility of trees to ice storm damage. Illustration: Lynn Hawkinson Smith

SUSCEPTIBLE INTERMEDIATE RESISTANTAmerican basswood American beech Amur mapleAmerican elm Boxelder BaldcypressBigtooth aspen Chestnut oak Balsam firBlack ash Choke cherry Bitternut hickoryBlack cherry Douglas-fir Black walnutBlack locust Eastern white pine BlackgumBlack oak Gray birch Blue beechBradford pear Green ash Bur oakButternut Japanese larch CatalpaCommon hackberry Loblolly pine Colorado blue spruceEastern cottonwood Northern red oak CrabappleHoney locust Paper birch Eastern hemlockJack pine Pin oak Eastern redcedarPin cherry Red maple European larchPitch pine Red pine GinkgoQuaking aspen Scarlet oak HophornbeamRed elm Scotch pine HorsechestnutRiver birch Slash pine Kentucky coffeetreeSiberian elm Sourwood Littleleaf lindenSilver maple Sugar maple Mountain ashVirginia pine Sycamore Northern white cedarWillow Tamarack Norway maple Tulip poplar Norway spruce White ash Ohio buckeye Yellow birch Pignut hickory Shagbark hickory Swamp white oak Sweetgum White oak White spruce Witch-hazel Yellow buckeye

ICE STORM SUSCEPTIBILITY OF TREE SPECIES FOUND GROWING IN URBAN AREAS.TABLE 1

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TREES AND ICE STORMS continued from p. 7

Tree architecture plays an important role in ice storm susceptibility. As the surface area of lateral branches increases, more ice can accumulate on lateral branches and greater ice loading 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. Many broad-leafed tree species, when grown in the open, form broad crowns (decurrent branching), that increase their susceptibility to ice storms. Examples include Siberian elm, American elm, hackberry, green ash, and honey locust. Trees with unbalanced crowns (such as at forest edges) are more susceptible to ice damage and increased bending through greater ice accumulation on the side with more branches. In contrast, trees with a conical form (excurrent branching) tend to be resistant to ice storms. Finally, small stature trees such as ironwood, blue beech, service berry, and hawthorn also are infrequently damaged.

Ice Storm Management and PreventionUrban tree populations in areas subjected to ice storms should have management plans to incorporate ice storm resistance. Regular maintenance of tree populations to develop greater resistance should occur. Removing structurally defective branches, training trees from a younger age, and proper branch removal will go far in developing ice storm resistance in the urban forest. The old adage you can pay me now or you can pay me latter is true with tree maintenance. Regular and budgeted infusions of money into an annual tree care budget is better than an unbudgeted and often large expenditure to recover from an ice storm. See Burban and Andresen (1994) for more information on storm damage planning.

Tree species vary in their susceptibility to ice storms (Table 1). Planting a diverse financial portfolio is a wise investment strategy. Planting a diverse urban forest is a likewise sound investment to increase your odds of long-term benefits that urban trees provide communities. Rather than avoiding susceptible tree species to ice storms, avoid creating the majority of your tree population with susceptible species. Also, minimize

planting susceptible species near locations that damage to property and infrastructure would occur if a tree failed from an ice storm.

ConclusionIce 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.

Richard J. Hauer is an Assistant Professor of Urban Forestry at the University of Wisconsin – Stevens Point. You can reach him at rhauer@uwsp.edu or 715-346-3642.

Additional information on next page

Table: Richard J. Hauer

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FOR ADDITIONAL INFORMATION ON ICE STORMS AND TREES:Burban, L.L. and J.W Andresen. 1994. Storms over the Urban Forest: Planning, Responding, and Regreening - A Community Guide to Natural Disaster Relief. Second Edition. USDA Forest Service, Northeastern Area. 152 pp. http://www.na.fs.fed.us/spfo/pubs/uf/sotuf/sotuf.htm

Hauer, R.J., J.O. Dawson, and L.P. Werner. 2006. Trees and Ice Storms: The Development of Ice Storm-Resistant Urban Tree Populations, Second Edition. Joint Publication 06-1, College of Natural Resources, University of Wisconsin-Stevens Point and the Department of Natural Resources and Environmental Sciences and the Office of Continuing Education, University of Illinois at Urbana–Champaign 20 pp. http://web.extension.uiuc.edu/forestry/publications/pdf/urban_community_forestry/trees_and_ice_storms_2006.pdf

WHAT’S UP continued from p. 5

Photo 11: “pretty” stock.

Photo 12: an elm at planting in 1909.

Photo 13: a mature elm.

For more information about the TRE Nursery, visit their website at www.tre.umn.edu

Chad P. Giblin is a Scientist in the Department of Horticultural Science and a Graduate Student in the Department of Forest Resources at the University of Minnesota.

Jeff Gillman is an Associate Professor in the Department of Horticultural Science, University of Minnesota, where he specializes in nursery management and production.

Gary Johnson is an Extension Professor of Urban Forestry in the Department of Forest Resources, University of Minnesota.

stock, why did we stop growing and planting them as such? I doubt that any modern urban forester, arborist, or nursery grower could get away with planting, maintaining, or selling trees that look like that! As stewards and advocates of urban and community forests, we are obliged to educate people about the changing face of arboriculture and tree health. In the case of elms, a compromise between aesthetics and structural success must be reached. To that end, replicated research is underway at the University of Minnesota and several off-campus locations to take a close look at how different pruning practices affect these trees long-term. We’ll keep you posted.

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The Minnesota Shade Tree Advisory Committee’s mission is to advance Minnesota’s commitment to the health, care and future of all community forests.

Inside ThIs Issue2 Perspectives Column

3 Mystery Tree

6 Trees and Ice storms

10 Cold Temperature Tolerance and Dormancy in Woody Plants

12 What happened to the Oaks this summer?

15 Calendar

16 Important MNsTAC updates

Visit MnSTAC on the Web at www.mnstac.org

COMMUNITY FOREST PROFILE

ADVOCATE • Winter 2008

Winter 2008

VOl. 10, NO. 1

Square Roots for Round Holes Since the presentation and publication of our original work on root pruning extremely pot-bound container stock, we’ve received a lot of requests and questions regarding further work. One of the biggest concerns was the time length of the original study and the number of species used. To broaden our understanding, we planted a new study in the autumn of 2005. This new research includes four different species: Techny white cedar (Thuja occidentalis), Red Splendor crabapple (Malus ‘Red Splendor’), Sienna Glen Freeman maple (Acer x freemanii ‘Sienna’), and Deborah Norway maple (Acer platanoides ‘Deborah’). Now in the second full year of study, many observations are being made.

The results from our first study showed that root pruning techniques had no positive or negative effect on rooting out. Indeed, doing nothing was as effective as cutting, scoring and teasing roots. Even cutting the container in half in the often-recommended “butterfly-cut” had no positive effect. Therefore, in this new study, we decided to get a little more aggressive with root disruption techniques. Again, we maintained a control treatment that was planted as-is directly out of the container (see photo 1). Next, we modified the root-scoring treatment used before, this time cutting a little deeper into the root system, both on the sides and bottom of the root ball (see photo 2). Finally, we added a new treatment, dubbed “The Box-Cut” where all visible portions of circling roots were removed using a pruning saw (see photos 3, 4, and 5). Even though this treatment appears extremely aggressive, consider that amount of root loss when transplanting a typical B&B or bare-root tree; this method probably retains even more roots!

WHAT’S UPat the TRE* Nursery?

By Chad P. Giblin, Jeff Gillman, and Gary Johnson

Photo 1: pot-bound control sample.

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WHAT’S UP continued on p. 4*Teaching, Research, Education