Abstract
Key messageLarge 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.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.
Highlights
Many urban trees must undergo regular checks by arborists to ensure that they do not present unnecessarily high risks to persons or property in the location in which they are growingCommunicated by Speck.(Lonsdale 1999)
Many trees have been braced, felled or heavily pruned on that basis by European arborists. This theory contrasts with the advice given by the International Society of Arboriculture (ISA) that “a bulge is caused by new tissue formed as a response to movement
The mean bending moments of the branch junctions by junction type were similar in distribution to the mean bending stresses by junction type reported by Slater and Ennos (2015a), except that the cup unions were not statistically differentiated from the wide bark inclusions
Summary
Many urban trees must undergo regular checks by arborists to ensure that they do not present unnecessarily high risks to persons or property in the location in which they are growingCommunicated by Speck.(Lonsdale 1999). Mattheck’s hypothesis states that when internal growth pressures within a branch junction are high, secondary growth is “pushed out” to develop at the sides of the junction This “pushing” is considered by Mattheck (1998) as the cause of prominent bulges of xylem (often informally referred to as ‘ears’) at the edges of and in the plane of the included bark. Many trees have been braced, felled or heavily pruned on that basis by European arborists This theory contrasts with the advice given by the International Society of Arboriculture (ISA) that “a bulge is caused by new tissue formed as a response to movement. This raises the question as to whether this bulging is always a sign of a developing weakness at a junction—or whether the bulging should be interpreted as having the function of compensatory growth around a previously weaker branch junction
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
