In late January while traveling through ice devastated northern New York, I spent my evenings reading Bernd Heinrich's excellent new book, "The Trees in My Forest," published by Harper Collins. This remarkable study provides interesting insights into the dynamics of that destructive storm.
Heinrich is a University of Vermont biology professor whose earlier "Bumblebee Economics" considered how insects combat environmental stresses. In his new volume he addresses similar concerns about his own Maine woodlands. Here are some of his observations about trees and ice:
"Trees have...been tested throughout history and they are constructed to stand up to specific stresses, but not to exceed them.... Unlike bridges and skyscrapers, they have a smaller margin of safety because, as in all construction, safety is bought at a price. In the fierce competition among trees, compromises have to be made. Trees cannot afford to insure themselves against all possible stresses. Their optimal strategy is to pay for Žjust enough' to ensure survival given exposure to Žaverage' stresses."
Heinrich carefully measured the weight of ice collected on his trees after an ice storm. "On average," he says, "a birch twig carried a load of ice eight times its own weight, American ash had the lowest ratio of ice weight to branch weight, while sugar maple, red maple, and apple twigs had an intermediate ratio.... Birches... have bushier and more numerous twigs, so per weight, they have more surface area to adhere to. Moreover, when birch twigs are partly ice-laden, they tend to droop outward, away from the stem of the tree. Surface tension holds water droplets on the twigs longer, and the slow flow down the twig allows a thin film of ice to form. In contrast, the relatively stiff, horizontal twigs of apple trees, for example, generally hold little ice. The water simply drips off....
"A vertical rod has only its tip exposed to rain. Laid horizontally, its whole side is exposed. This principle is important in the architecture of the tree. Ash, maple, and poplar trees (especially younger ones) have upward-pointing twigs, and this arrangement of twigs has two effects with respect to ice. First, the more a twig points straight up, the less surface it presents to falling rain (that freezes to ice on the twig), and the less ice will collect on the twig surface. Furthermore, the branches form rough Vs relative to the trunk, so rain that does land on the twigs runs inward toward the trunk (rather than outward as on the gray birch) and freezes where it can do the least harm.... The closer the ice forms to the trunk, the less leverage it can exert." But he adds, "Mature trees generally tend to have large spreading crowns, and as one might expect, they lose many limbs to ice storms while the thinner and more upward-pointing trees go unscathed."
My own observations in the northern Adirondacks corroborate Heinrich's principles. Birches did indeed suffer most, closely followed by mature maples. Of course, this storm was much more than an above average event. Even man-made constructions like those giant power poles, which were designed to meet stresses that exceeded anything previously recorded, were overwhelmed. No wonder the trees suffered so badly.
Professor Heinrich was in Maine during the January storm where he listened for days to the crashing of falling ice. Fortunately, his forest appears not to have suffered as badly as ours in northern New York. For example, he told me in a phone conversation that his willows are already rising from their deep bows.
To gain an understanding of forest ecology, I can imagine no better guide than Bernd Heinrich.