Ice Storm Damage to Urban Trees

Gymnosperms

Little damage occurred to gymnosperms. Only 3 narrow-leafed conifers, 1 jack pine and 2 bald cypress trees, or 0.6% of 501 total conifers growing on Urbana parkways were injured during the storm. Conifers have been observed to resist ice storm damage (1,10,12,14,18,22). However, conifers are not immune to ice storm damage. Pines including eastern white, Virginia, table-mountain, and pitch pine have been damaged severely by ice storms (18,26,27).

Most narrow-leafed conifers have an excurrent branching habit (13). This form features a dominant central leader and short, horizontal branches which may support their weight against gravitational torque greater than branches which approach the vertical position (15). The typical branch habit and the conical crown of conifers may allow them to resist ice damage.

No parkway arborvitae and few baldcypress trees were damaged by the ice storm. Hundreds of arborvitae trees approximately 25 feet tall in an Urbana windbreak suffered little damage from the storm. There are other reports of the apparent resistance of arborvitae and baldcypress to ice storm damage (9,10,12,22).

Ginkgo was not severely damaged. The majority of ginkgo trees on the parkway were between 2 and 8 inches dbh and the small size may have precluded severe ice storm damage. However, there was little damage to larger ginkgo elsewhere in Urbana. One earlier report indicated that ginkgo resists ice storm damage (26).

Angiosperms

Deciduous angiosperm trees, most of which have a decurrent branching habit (13), seem to be generally more susceptible than conifers to ice storm damage (1,7,12). In Urbana, 486 of the 10,100 angiospermous trees (4.8%) suffered severe damage (Table 1).

Of the 25 major street tree taxa in Urbana, the angiosperms sustaining the most severe damage from the ice storm were Siberian elm (41.7%), honey locust (12.7%), Bradford pear (8.8%), common hackberry (8.8%), pin oak (8.4%), sycamore (7.1%), green ash (6.9%), and tuliptree (6.6%). Trees sustaining intermediate damage include white ash (5.9%), silver maple (3.9%), and bur oak (3.1%). Angiosperms least damaged include Kentucky coffeetree (0.0%), swamp white oak (0.0%), silver linden (0.0%), northern red oak (0.6%), white oak (0.7%), littleleaf linden (0.9%), American sweetgum (1,2%), red maple (1.3%), black walnut (1.8%), sugar maple (1.9%), and Norway maple (1.9%).

Siberian elm is a brittle, fast-growing species with rough bark and a fine branching pattern which tends to accumulate more ice than trees with less branch surface area. This tree species sustained the greatest damage in Urbana and also in an ice storm in Colorado (4).

Silver maple, noted for weak crotches, weak wood, and susceptibility to wind damage and ice storms (9,22), was damaged only moderately. This might have been due to their tendency to break at the upper or mid branch position when loaded with ice rather than at major branch junctures. In addition, silver maples in Urbana had been pruned regularly since 1976, possibly contributing to the low incidence of major damage. Pruning to remove structurally weak branches is probably an effective strategy to minimize ice storm damage.

Tuliptree suffered moderate to severe damage from the Urbana ice storm. Severe damage to tuliptree has occurred in other ice storms (7,20,27). Some reports indicate low susceptibility (2) and moderate susceptibility of tuliptree to ice storm damage (18). Plant age, site, and structural integrity probably account for the differing reports (28).

Norway maple and sugar maple sustained little severe damage. Norway maple seems to resist ice storm damage (9,10). Although we observed little damage to sugar maple, other reports indicate this species is intermediate in susceptibility (9,18,22).

Sycamore trees sustained severe ice storm damage in Urbana. Reports indicate that ice damage can range from little to severe for sycamore (2,12,14). Anthracnose (Apiognomonia veneta) infects sycamore annually in Urbana. This disease sometimes causes death of twigs and loss of terminal bud dominance (16). Lateral buds then assume temporary dominance causing a proliferation of shoots along branches. It is possible that the increased surface area which occurs as a result of anthracnose may increase the susceptibility of sycamore.

The small size of Kentucky coffeetree, silver linden, and littleleaf linden in Urbana probably accounted for their avoidance of ice damage. Nonetheless, 15% of the littleleaf lindens (66 trees) were between 13 and 24 inches dbh and none were damaged. Kentucky coffeetree has previously been reported to be resistant to an ice storm (14).

Most trees that are small at maturity sustained minor damage. Examples include ironwood, serviceberry, Eastern redbud, and amur maple. Ironwood has previously been reported resistant to ice storms (3,22).

Bradford pear were severely damaged by ice accumulation. Bradford pears often have narrow crotches with included bark. Included bark is responsible for weak branch attachment. In contrast, the Aristocrat pear has branches that are more horizontal with a wider crotch angle (11) and exhibited greater ice storm resistance than Bradford pear. Of the 21 Aristocrat pear between 3 and 9 inches dbh, none was damaged.

Pin oak suffered greater damage than northern red oak, white oak, and swamp white oak. The majority of pin oak were greater than 12 inches in dbh and this species has a finer pattern of branching than other local oak species. The majority of northern red oak, white oak, and swamp white oak in Urbana were less than 13 inches in dbh. The lack of damage to these species on Urbana parkways may be attributable to their smaller diameter and coarser branching patterns.

Damage in local native hardwood forests occurred at forest edges where longer and lower branches exist. The narrow, high crowns of forest trees were less susceptible to damage than the crowns of open-grown urban trees. Among the common native upland oak species, only a few northern red oaks, which have a shallower rooting pattern than other native upland oak species, were uprooted by the gravitational pull of accumulated ice. Other authors report northern red oak intermediate in susceptibility to ice damage (1,3,18,22).

Cannell and Morgan (5) studied branch breakage under snow and ice loads and found that branching patterns affect the capability of a branch to resist ice damage. Shoots with laterals are more likely to fail under an ice load than shoots of the same midpoint diameter or length without laterals. Observations from this ice storm are consistent with the idea that branch breakage increases with the degree of lateral branching and density of small branches (4,15,22). For example, heavily-damaged Siberian elm and common hackberry exhibit a high density of small branches.

In addition to crown architecture, pre-storm conditions such as an imbalanced crown, included bark, or dead wood may have predisposed a tree to ice damage. The majority of damaged Siberian elm, common hackberry, and honeylocust inventoried in 1988 had been reported to have structural imperfections, disease symptoms, or insect damage. Butler and Swanson (4) suggested that structurally weak crotches were the principal cause for honeylocust damage in an ice storm.

No apparent relationships exist between ice storm damage susceptibility and specific gravity, modulus of rupture, or modulus of elasticity of a tree species (Table 3). Hence, the strength of wood seems less important in ice-accumulation tolerance than the capacity to accumulate ice until it surpasses the bearing capacity of branches.

Cannel and Morgan (6) believe that measures of branch and trunk strength derived from standardized tests such as modulus of elasticity should include intact bark, as branches and trunks naturally contain bark. The bending of tree branches from ice or snow accumulation more closely fits the structural theory for cantilever beams where one end is fixed and the other is free, rather than modulus of elasticity tests in which a sample is supported on both ends (19). Redden (21) states that a tree is bound together primarily by cantilever joints with inherent characteristics that allow bending and twisting in response to loading by wind, snow, and ice.

An ice storm tolerance rating of commonly planted urban trees was developed from the results of this and other ice storm studies (Table 4). Rankings follow the majority findings of published reports and are not totally consistent with our findings from the Urbana ice storm.