204 Slater and Ennos: Included Bark Affects the Strength of Bifurcations in Hazel just above bifurcations in hazel, the frequency and extent of oscillations separating a smaller diameter branch and a larger diameter branch, where their bases are conjoined at a bifurcation, will both be greater than when two branches of equal diameter are bent in a wind of the same force. As a conse- quence of experiencing higher strain levels more regularly at its apex through this different bend- ing behavior, lower diameter ratio bifurcations are likely to develop a higher level of modification of their tissues to adequately resist those forces (Metzger 1893; Jaffe and Forbes 1993; Telewski 1995). In contrast, the bifurcation with included bark is a structure where little to no strain is reg- ularly experienced at its apex, so no substantial resources are committed by the tree to reinforcing it. On average, bifurcations with bark inclu- sions were only three-quarters the strength of the normally formed specimens, but there was a wide range of peak stress values, with some bark- included samples experiencing branch failure rather than splitting at the bifurcation itself and other bark-included bifurcations having less than 40% of the bending strength of the smaller branch. A simple analysis of the strength of the bifur- cations with included bark and their morphology provided two useful insights. Firstly, it can be concluded that small areas of embedded bark do not give rise to a significant difference in bifurca- tion strength. Secondly, cup-shaped bifurcations in hazel were significantly stronger than those that had bark at their apex. The conclusion from these findings is that the main reason why the strength of bifurcations with included bark was found to be so variable in the tested specimens was that the areas of included bark in the samples were at different stages of occlusion at the bifurca- tion apex; a higher level of occlusion of the bark inclusion resulted in an increase in the bifurca- tion’s strength. Thus, the cup-shaped bifurcations tested in this study represented different stages of repair of the structural flaw that was caused by the initial inclusion of bark into those junctions. From this experiment, the authors can provide an interpretation of the mechanical perfor- mance of bifurcations with included bark in trees; however, it is very important to recognize the limitation of this study in that young bifur- cations of only one species (Corylus avellana L.) ©2015 International Society of Arboriculture that contained solely juvenile wood were tested, and the mechanical behavior of mature bifurca- tions in different woody species may well vary from what was found in this study’s samples. Wide-angled bifurcations, which are U-shaped at their apex and without bark inclusions, and bifurcations with embedded bark should both be considered adequate structures, as there should be interlocking wood grain present at the bifurca- tion apex. Where a significant width of included bark is found at the apex of the bifurcation, this indicates a significantly weaker bifurcation, and a tree assessor should evaluate the proportional width of this bark in relation to the overall width of the join perpendicular to the plane of the bifurcation. They should also take into account the extent of adaptive growth at each side of the bifurcation, account for the extent of occlu- sion of the bark inclusion by the formation of a cup-shaped bifurcation, and most critically, take into account whether the level of wind exposure of the bifurcation has been heightened by recent site changes or pruning work. The rapid forma- tion of additional xylem that lies at either side of a bifurcation (often indicated by a change in bark texture) may be an indication of instability of that bifurcation (Mattheck and Breloer 1994). Features that tree assessors should survey in bark-included bifurcations, based on this study using hazel specimens, are identified in Figure 11. It would seem that a bark-included bifurcation’s notoriety as a defect in trees comes from the risk of this structure being exposed to a wind event or other loading event that causes the two arising branches to oscillate or move apart in a way that has not frequently occurred during the bifurcation’s prior development. This problem can be compounded by arboricultural practices, like crown thinning, the felling of adjacent trees, or the transplanting of trees into new locations, where these practices would lead to abrupt changes in the level of exposure to which the bifurcation is not sufficiently adapted (Wood 1995). Studies of the strength of bifurcations with included bark in trees should be taken further. As in this study, researchers tested juvenile wood in only one species (Corylus avellana L.); a similar study using mature bifurcations in a range of spe- cies would assist in determining their mechanical behavior. In addition, a better understanding of
July 2015
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
