Branch Strength of Bradford Pear ( Pyrus calleryanavar. ‘Bradford’)

Previously planted extensively as a street tree, Bradford pear (Pyrus calleryana var. ‘Bradford’) has fallen out of favor because of its reputation for branch breakage. Despite this reputation, Bradford pear branch strength has never been tested. Prior studies on branch breaking have discounted the influence of branch attachment angle, suggesting that the ratio of branch to trunk diameter, or aspect ratio, is a better predictor of branch attachment strength. Twenty-six Bradford pear branches from 10 trees were broken by pulling them with a winch. To assess the effect of branch cross-sectional dimensions, breaking stress was calculated considering the branch cross-section as either an ellipse or a circle. Breaking stress was normalized by dividing it by the modulus of rupture measured on wood samples from each broken branch. The location of failure, either in the branch itself or at the branch/trunk attachment, did not affect breaking stress. Aspect ratio was a better predictor of branch attachment strength than branch attachment angle. Breaking stress calculated considering the branch cross-section as an ellipse was greater than when stress was calculated assuming the branch cross-section was a circle. Results are compared with previous studies and the importance of measuring branch cross-sectional dimensions is discussed.

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