ICE STORMS: A NATURAL DISTURBANCE
The great ice storm of 2009 was a nasty one. Everywhere devastation: downed trees, roads snarled, power lost. Because our infrastructure is so vulnerable to these storms, our lives are upended. Ice storms are seen, perhaps justifiably, though a negative lens. But nature doesn’t operate through such lenses. In the natural world, hidden by the obvious damage, are an ice storm’s positive outcomes.
Ice storms form when there is a layer of warm air trapped between layers of cold air. Snow falls from a high, cold air layer and descends through a warm layer where it melts and becomes rain. Closer to the ground, raindrops reenter a cold layer where they are super-cooled (cooled below 32°F but not frozen), and upon impact, freeze instantly. Thus we get “freezing rain” or, more simply, ice.
The amount of ice build-up varies by the intensity and duration of the storm. In 2009, Louisville received about one inch of ice, while portions of western KY received two inches or more. The amount of ice, and the winds and temperatures that follow the storm, determine the damage to human and natural landscapes. While the northeastern US receives the majority of ice storms, a 1961 storm in northern Idaho set the record for greatest thickness of ice at eight inches.
Trees have varying vulnerabilities to ice based on the strength of the wood, ability to flex, the structure of the branching, and the strength of the branch junctures. Tree size and age determine variability within species. Dead branches on trees readily slough off with the added weight of ice, which is part of the tree’s natural self-pruning process. Most breakages occur at branch junctures—where branches divide. These junctures are more important than wood strength in determining damage by ice loading.
Trees found in The Parklands that are particularly susceptible to ice storm damage include American elm, hackberry, honey locust, black cherry, and silver maple. Trees with intermediate resistance to ice glazing include American beech, box elder, tulip poplar, red oak, sycamore, sugar maple, and white ash. Trees resistant to ice damage include black walnut, bur oak, white oak, most hickories, and eastern red cedar.
With enough ice-loaded fine branches fall, horizontal limbs break, and trees with major divisions (forks) split. The immediate after-effects are little; cold weather prevents fungus and decomposing agents from working. It will be spring before nature’s clean up crew leaps into action.
But before decomposition of fallen limbs occurs, the branches need to be cleaned of nutritious living tissue. The buds—where next year’s leaves and flowers would have come from—are a ready target. Buds are calorie-packed, full of nutrients and are generally safe, high above the ground, from plant-eating mammals. Deer will quickly browse these buds. But before they can get to the buds they must navigate the ice-crusted snow than can leave their shins bloody. Most deer will wait days for a thaw before moving far. Aspen, cottonwood, oaks and other fat-bud species are particularly sought after.
With deer, small mammals will also take advantage of the fallen bounty. In addition to the buds, small mammals will strip twigs of their bark to get the inner cambium of the wood.
By spring the forest is beginning to respond. If the ice was heavy enough and tree damage high, sunlight hitting the forest floor will increase significantly. This means, potentially, a good year for spring wildflowers, and thus for the ants, small mammals, and box turtles who feed on their fruits. Plants in the shrub layer will also benefit from increased light. Sunlight energy on the forest floor can be just 1% of the sun energy in the open. So a canopy trimming can be a huge energy boon to ground plants.
The trees will respond as well. Heavily damaged trees become stressed, and some species will sprout epicormic buds from beneath the bark. Sprouting will be especially vigorous on oaks, and others if there is increased sunlight.
Within a year, as decomposition ensues, birds will begin excavating cavities in damaged tree trunks. Arboreal mammals will take advantage of openings and cavities. For many bird species, the availability of cavities for nesting is the main limiting factor to their populations. With the increase in cavities created by an ice storm the populations of those species can be expected to increase—if only temporarily (a decade or two).
In ecology we call events such as ice storms, fire, and logging adisturbance. Certain species of sun-loving species (of plants, trees, and animals) require periodic disturbance. A forest will maintain maximum levels of biodiversity (numbers of species) with intermediate levels of disturbance. Periodic ice storms, which typically affect a forest every 20 to 100 years, create opportunity for many species.
With only an inch of ice, the 2009 ice storm had little effect on the forests of The Parklands. Affected most were the old growth beech trees in Pope Lick Park. Their canopy branches suffered a major pruning. But this is how we know those trees are old; the shape of the canopy branches shows us, through bends, twists, and crooks, that the trees are well storied and have survived many ice storms over their 300 years.
After the storm, Louisville restrung the lines, processed the downed trees, and got on with the business that is our society. Ecosystems function similarly: decomposers act on fallen wood, plants take advantage of changes in light conditions, and animals respond to plant growth and changes. Ecosystems, however, respond much more slowly than human societies to such events. Most changes won’t begin until spring, and the subsequent repercussions are measurable for decades. The forest will never look as it did before a heavy ice storm.
About the Author
Michael Gaige
Michael Gaige became involved with 21st Century Parks in 2007 on a recommendation from his graduate adviser, Tom Wessels, author of Reading the Forested Landscape. His first project was to groundtruth the nearly 4,000 acres around Floyds Fork to discover and document interesting places park users will experience and learn about. He then compiled a Natural Areas Plan to ensure that the parks’ forests and meadows are well-tended, and park infrastructure is designed in accordance with the landscape’s history and ecological detail. Michael now works as a freelance ecologist and educator and lives in upstate New York. He returns to Louisville periodically to share with others his favorite places in The Parklands, and to visit his cherished old trees.