How To Save Your Trees From The Next Isabel
Research shows that pruning can reduce the risk of windthrown trees

ANNAPOLIS, MD - Many of the trees that came down during Hurricane/Tropical Storm Isabel were trees that looked sound, but they caused problems by uprooting rather than by breaking. Is there a way to treat trees to minimize the risk of uprooting in future storms? The short answer is yes.

Researchers studying biomechanics (the study of how living organisms move and grow in relation to mechanical principles) at the Institute for Material Research in Karlsruhe, Germany, have found that intact trees with smaller height-to-diameter ratios are less likely to be windthrown. Field observations of thousands of single trees subject to significant wind (trees found in landscaped settings rather than in forests) revealed that high uprooting (windthrow) rates exist only for trees with a height-to-diameter ratio greater than or equal to 50. This means that if a tree is 50 feet tall, it is highly likely to be subject to windthrow if the diameter of the tree is less than or equal to 1 foot.

Although the diameter of a tree cannot be changed, its height can. Crown reduction (selectively removing branches to decrease the height or spread of a tree) may be used to obtain a favorable ratio of height to diameter. Of course, canopy reduction should be employed within the context of other industry standards, such as removing no more than 25 percent of the crown in a growing season. For example, if our 50-foot tree with a 1-foot diameter begins branching 15 feet above the ground, reducing the height of the crown by 20 percent would achieve a total tree height of 43 feet. This treatment would result in a more favorable height-to-diameter ratio and reduce the risk of trees being thrown by the wind.

Some trees may not lend themselves to this type of treatment because of their natural shapes; that is, some species normally develop crowns that are high on the trunk and not well distributed. Dr. Kim Coder at the University of Georgia, a leader in tree biomechanics research, offers similar recommendations regarding the height and diameter of trees. He also advises that wind force on tree crowns can be lessened by reducing crown volume by either crown reduction, crown raising (removing lower branches, resulting in a smaller overall crown height), or crown thinning (selectively removing branches to reduce the density of the tree crown). Again, always apply industry standards, which hold that exceeding thresholds for the removal of live leaves and branches has negative effects. Although reduction is the preferred treatment, other options exist if a tree’s shape makes reduction unwise.

A Maryland Licensed Tree Expert can provide these treatments. DNR licenses commercial tree-care practitioners, and all tree-care professionals practicing in Maryland must obtain a license to practice or to advertise services. A full list of licensed tree experts can be found at the DNR Web site, http://dnrweb.dnr.state.md.us/forests/oflists/lte/treeexpert.html.

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