The susceptibility of trees to ice storms is influenced by a variety of factors. Several factors are environmental, dictated by the strength of the storm. These include the amount of accumulated ice, the duration of the storm, and the tree’s exposure to wind. Many other factors are more developmental. These are represented as specific tree features. The following are some of the more common features observed in trees that increase susceptibility to ice storms.
Tree Features That Increase Susceptibility to Ice Storms:
Tree features that increase susceptibility to ice storms include weak branch junctures, dead or decaying branches, broad or unbalanced crowns, an increased surface area of lateral branches, poor root systems, and infectious tree pathogens.
Weak branch junctions are branch attachments indicated by included bark. Included bark forms in the junctions of co-dominant stems that have narrow angle unions. Narrow angle unions are v-shaped formations in the tree. As the tree grows, the narrow union fills with bark, establishing a weak connection. These structural weaknesses increase the susceptibility of the tree to breakage due to ice-loading.
Dead or decaying branches are already weakened, and can easily succumb to breakages when loaded with ice. Decay is a process that occurs when microorganisms enter a tree through an open wound, and feast on the tissue, causing it to decompose. When coupled with other structural weaknesses, decay further increases the susceptibility of trees to ice damage.
The surface layer of lateral branches increases as a crown broadens and spreads. With an increased surface area, more ice can accumulate on lateral branches. The greater ice load results in more widespread branch failure. Many broad-leafed tree species form broad crowns. Some examples include American elm, black cherry, bradford pear, hackberry, silver maple, green ash, and honey locust.
Trees with unbalanced, asymmetrical crowns have weight distributed poorly over their stems. This can lead to increased bending of the stem as a result of ice accumulating on the side of the crown with more branches. This often results in large breakages or total tree failure.
The quality of a tree’s root system greatly influences its susceptibility to ice storms. Diseased and damaged root systems can cause nutrient and water deficiencies, resulting in the tree’s decline. This compromises the tree’s health, making it more vulnerable to ice damage. The tree’s rooting depth plays a vital role as well. Tree species with shallow root systems are more prone to tipping due to ice accumulation. Trees that are rooted close to streams or rivers are also easily toppled. Upon being uprooted, they will often collapse headlong into the water.
Tree susceptibility to ice damage can increase due to the presence of tree pathogens. When infected, trees will often wilt or decline, rendering them more susceptible to ice damage. Decay often develops in trees as a result of disease. This further destabilizes the tree, increasing its vulnerability to insect infestation, and ice damage.
Photograph courtesy of Ed Roberts.