Impact of branch reorientation on breaking stress in Liriodendron tulipifera L.

It has been observed that during ice, snow, and wind storms, branches oriented vertically tend to incur more damage than branches oriented horizontally. A study was conducted to determine breaking stress and breaking position of branches reoriented nearly horizontal and nearly vertical. Branches oriented 40-50° from horizontal with mean diameter 4.9 cm (SD ± 0.73) were removed from two trees and transferred to a custom branch pulling station. Branches were reoriented either nearly horizontal (76-89°) or vertical (6-29°) to a reinforced vertical post. Branches were pulled vertically downward from three equidistant positions along the branch until they broke. Failure stress for horizontal oriented branches (64 MPa) was double the stress required to pull vertical oriented branches to failure (32 MPa). Nine of ten horizontal branches failed between the branch base and the pull point closest to the base (proximal pull point); whereas seven of ten vertical branches failed farther from the base, between the proximal and middle pull points. Average length from branch base to failure point for horizontal branches was 12.8 cm, and 74.6 cm for vertical branches. Despite requiring less stress to break, branch angle change at the distal and middle pull points from the original position to the position at failure for vertical branches was greater than for horizontal branches; whereas angle change at the proximal pull point was greater for horizontal branches. Branch taper was not different between reorientation treatments. Implications on pruning strategies are discussed.