The mechanical effects of bulges developed around bark-included branch junctions of hazel (Corylus avellana L.) and other trees

Key message Large bulges formed around bark-included branch junctions can be conceived of as ‘compensatory growth’. Despite Claus Mattheck’s hypothesis that ‘the larger the bulges, the more likely the branch junction is to fail’, this study identifies that the extent of the defect inside such bulges is key information in predicting the junction’s load-bearing capacity. Abstract A currently prevalent rule in European arboriculture is that if a bark-included branch junction in a tree is associated with a large bulge in-line with the plane of the included bark then it is more likely to fail than if there is a smaller bulge or the absence of bulging. This rule for arborists originates from an initial suggestion with no associated data and is not logically consistent with recent research into the effects of natural bracing in trees, nor guidance provided by the International Society of Arboriculture. This also raises the question of how to correctly interpret the function of these bulges formed at bark-included junctions: either as secondary growth that has been pushed to the side by internal growth pressures, or as compensatory growth developing around a weakened component. To test the veracity of this rule, 117 branch junctions of common hazel (Corylus avellana L.) were subjected to tensile tests, comprising of controls with no bark included within them and bark-included specimens exhibiting a range of bulge sizes. In addition, photographs from the failure of 110 bark-included junctions were categorized to assess the frequency of failed specimens with different degrees of bulging. The results of the mechanical testing identified three significant factors that affected the maximal bending moment of these branch junctions: their categorized morphology, the diameter ratio of the branch junction and the width of the included bark at the apex of the junction. Overall, and in each category of branch junction tested, the extent of bulging was not found to be a significant predictor of the junctions’ maximal bending moment. This finding was reinforced by the analysis of the images of bark inclusion failure where the most frequent bark inclusions to fail were those associated with little to no bulging. Both findings identify that the bulging would be better interpreted as compensatory growth. This study highlights the need for further research on the load-bearing capacity of bark-included branch junctions to better inform arborists and tree managers as substantial variations in their biomechanical performance have not yet been elucidated.