Contact information:

Dr. Brian KaneUniversity of Massachusettes- AmherstArboriculture and Community Forestry

bkane@forwild.umass.edu

Dr. Roger HarrisVirginia TechHorticulture

rharris@vt.edu

Dr. Susan DayVirginia Tech

Urban Forestrysdd@vt.edu

Reducing Storm Damagein Urban Forests: A Pruning Guide

� is project was supported in part by the United States Department of Agriculture Forest Service Urban & Community Forestry Program on the recommendation of the National Urban & Community Forestry Advisory Council

The Mall, Elm Walk in Central Park, New York CityPhoto and brochure design by Sarah Dickinson

For more information, please visit:www.cnr.vt.edu/UrbanForestry

Pruning and Storm Preparation

Historically, techniques to reduce the risk of storm damage included topping and “lion’s-tailing” (stripping out interior foliage and branches and leaving only “tufts” foliage and branches and leaving only “tufts” at the end of large branches)—practices that are harmful to trees and no longer recommended. Instead, the ANSI A300 standard for tree pruning describes seven pruning types and requires arborists to develop objectives before undertaking pruning.

Pruning reduces the risk of storm damage two ways:

1- by reducing drag (wind resistance on the crown)—this is true of all pruning types the crown)—this is true of all pruning types 2- by lowering the height of the pressure center of the crown—this is only true of reduction pruning.

Pruning can thus reduce “bending moment”, Pruning can thus reduce “bending moment”, the product of drag and height of the pressure center.

The Experiment We measured the effects of crown reduction, thinning, and raising on Freeman maple (Acer x freemanii), swamp white oak (Quercus bicolor), and shingle oak (Quercus imbricaria), all at 3-3.5 inch caliper. We cut trees just above the root fl are and fastened the tree in a steel sled mounted in a pickup truck (see back photo). The truck was driven from 0 to 55 mph before and after pruning treatments. Gauges attached to the tree recorded how different pruning methods reduced drag and bending moment.

What We LearnedTo reduce the risk of storm damage, we

focus on bending moment. Bending moment takes into account drag and height of the pressure center. In our tests, pruning reduced bending moment largely because it reduced tree mass. Species played an indirect role. Crown architecture is different for different species, which in turn affects how much of the crown is removed by each pruning type. For Freeman maple, crown reduction and thinning reduced bending moment better than raising, but for the amount of biomass removed, reduction pruning was more effective than thinning. For swamp white oak, crown reduction pruning most effectively reduced bending moment, but required more biomass removal. For shingle oak, all treatments reduced bending moment, but per unit mass removed, crown reduction and thinning were more effective than raising.

Crown Reduction pruning is the most effective overall at reducing bending moment because it directly reduces the height of the pressure center as well as reducing crown mass. Indiscriminant reduction pruning, however, can cause excessive branch and trunk decay.

Crown Thinning can effectively reduce bending moment by reducing the drag on the crown. These reductions are similar to those achieved by crown reduction but more canopy must be removed. Removing mass from the crown clearly reduces bending moment, but removing too much foliage reduces photosynthesis and may cause undue physiological stress.

Crown Raising reduces bending moment, but more tree mass must be removed to attain the same effect as crown reduction and thinning. Removing foliage and twigs from the lower portions of young trees can reduce trunk taper development.

Forces of Nature: Drag, Pressure Center, and Bending Moment

Drag refers to the wind resistance created by the crown. The pressure center is the center of the

area experiencing drag, in this case the crown. These two forces determine the bending

moment, how the tree experiences the wind and thus how likely

tree failure is.