314 Kane et al.: Strength of Branch Attachments Figure 8. Frequency table of failure modes (ball and socket (BS), embedded bark (EB), and flat surface (FS)) for all species by diameter ratio. codominant failures of large maples exposed included bark. Only one codominant failure in their study did not have included bark, and it was much stronger than the others (Kane and Clouston 2008). This was not true of sawtooth oak, for which only one- third of attachments with included bark failed at the plane of included bark. Because the area of included bark found in at- tachments was always less than 40% of the area of the attach- ment (the mean was 18% and 20% for red maple and sawtooth oak, respectively) and, in most failures, it was confined to a narrow strip in the center of the attachment that did not extend to the most recent growth rings, it may be that a threshold amount of included bark is necessary to weaken the attachment. An additional factor is the smaller range of the percentage of area of included bark (Figure 7), which limited the ability to predict from this variable. Perhaps a more illustrative measurement would have been the thickness of sound wood between included bark and the adaxial surface of the attachment, because, for a single attachment in a Norway spruce (Picea abies [L.] Karst.), strain was greatest at the adaxial surface of the attachment (Mul- ler et al. 2006). It is also possible that the presence of included bark can weaken attachments but that the effect of diameter ratio supersedes it. Using the form of an attachment as a surrogate for diameter ratio appeared to be reasonable for red maple and sawtooth oak, as reflected in the strength differences between u- and v-shaped attachments for those species. It was unexpected, however, that diameter ratios differed between u- and v-shaped attachments for sawtooth oak, but not for red maple. It was quite unexpected to find only a single v-shaped attachment on callery pear, but this may be an artifact of the way the form of an attachment was characterized. The presence of a branch bark ridge on almost all callery pears was not associated with small-diameter ratios as it was for sawtooth oak, although the range of branch and attach- ©2008 International Society of Arboriculture ment angles was similar for both species. This observation, as well as the finding that the rank order of predictors of breaking stress was not exactly the same for all species, highlights the effect of species on which morphologic measure(s) should be used in the field to determine tree risk. Description of the three modes of failure is consistent with Figures 6, 7, and 9 in MacDaniels (1923), which show, as clas- sified in the current study, flat surface, embedded bark, and ball and socket failures, respectively. The modes of failure corre- sponded quite well with diameter ratios, which further supports the idea that relatively large branches have comparatively less overlapping of branch and trunk fibers in the attachment (Shigo 1985). This is illustrated clearly in Figure 4: ball and socket failures show a substantial amount of trunk fibers pulled from the trunk as the attachment failed. Embedded branch failures show fewer trunk fibers pulled away from the trunk, and flat surface failures reveal, essentially, two trunks because the fibers of each are intact and parallel after failure. Flat surface failures resembled codominant failures of large maples described by Kane and Clouston (2008), except that failures always revealed included bark if it was present. Kane and Clouston (2008) spec- ulated that failure of codominant stems was initiated in a com- plex stress state of shear and tension perpendicular to the grain at the attachment, which agrees with observations in the current study. Thus, the size of the attachment appears to be irrelevant in predicting the strength loss of the attachment. Breaking stress of callery pear was somewhat less than pre- vious findings for Bradford pear (Kane 2007), which may be the result of actual differences among the respective populations of trees. It is not possible to confirm this because of methodological and, possibly, cultivar differences. Although diameter ratio was similarly a reliable predictor of breaking stress for red maples tested by Gilman (2003), the breaking stress Gilman (2003)
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