Failure potential of limbs receiving reduction or heading cuts and of resulting sprouts, in directional, structural or restoration pruning

2011 | Dr. Edward Gilman, University of Florida 

Structural pruning, directional pruning, restoration pruning, and crown reduction are recommended methods for arborists to use when performing tree care operations. All of these methods use reduction cuts to meet their objectives. There is no data, however, on the long-term impacts of reduction cuts on branch breaking strength. Two studies will be performed to investigate this.

The first seeks to determine the influence of directional pruning, crown reduction, structural pruning, and restoration pruning on failure potential of retained limbs under load, and decay of large limbs. Load will simulate that from ice, snow, or wind downbursts. Oaks pruned in 2004 with reduction cuts ranging in size from 4 to 9 inches in diameter will be pulled with a winch to failure. Influence of reduction cuts on structural strength will be measured. Primary objectives are 1) determine if limbs fail at the site of the 7 to 8 year-old pruning cuts or at other places, and 2) compare failure patterns on pruned limbs with failure patterns on limbs not pruned. Discoloration, cracks, barrier zones, decay, wound wood formation, limb taper, mechanical stress at failure, and other attributes will be quantified and related to size of cut and point of failure.

The second study evaluates strength of attachment and failure patterns of 2- to 5-inch diameter upright-oriented sprouts growing on trees pruned to simulate storm damage (i.e. essentially they were topped in 2006). Some sprouts are growing from vertical stems originating just behind the 2- to 4-inch diameter heading cuts, whereas others emerge from horizontal lateral limbs. Sprouts will be pulled parallel to gravity using a single attachment point to the limb to simulate load from ice, snow, or wind.

Study Results

Through our interdisciplinary collaborative efforts, a novel system was devised to safely break 12 inch diameter branches using a controlled and repeatable method. A system was also devised to test attachment strength of 2 to 5 inch diameter sprouting branches resulting from simulated storm damage (topping cuts). Breaking stress calculations, sprout attachment strength, and the implications of these results will be included in the manuscripts which will be submitted to Arboriculture and Urban Forestry for publication by spring 2015.

For more information on this project, contact the researcher via TREE Fund at treefund@treefund.org.