196 Slater and Ennos: Included Bark Affects the Strength of Bifurcations in Hazel which has been well researched with respect to its anatomy and mechanical behavior, justifies the single species choice in this study. Testing the strength of young Figure 2. Potential development pathways for a bark inclusion, showing the morphol- ogy of the xylem perpendicular to the plane of the bifurcation, leading to the formation of (a) embedded bark, b) a cup-shaped bifurcation, or c) a wide-mouthed bark inclusion. Despite these general observations by experi- enced researchers, there are no current means of quantifying the heightened risk of failure of bifur- cations with included bark in trees from observing their external morphology or the position and size of the bark inclusion present. In this study, there- fore, researchers investigated the strength of bifur- cations in relation to the presence or absence of bark inclusions, and, if present, the position, shape, and size of bark inclusions found. The authors sought to find a simple rule by which the relative weakness of a bifurcation with included bark could be predicted. The authors chose to model this mechanical behavior in one species, Corylus avellana L., as similar research on this species has been carried out by Pfisterer (2003), which allows for a compar- ison in findings, and the wood grain orientation and mechanical contributions of different com- ponents of such bifurcations in this species have recently been uncovered (Slater and Ennos 2013). This species was favored as an experimental subject because it provides a sustainable source of bifurca- tions, and because working with coppice-grown material of one species limits the effects of other factors (e.g., age differences, differences in levels of exposure) that could affect bifurcation strength. Having a more comprehensive picture of the bio- mechanics of bifurcations in one woody species, ©2015 International Society of Arboriculture tree bifurcations may provide use- ful insight for tree assessors as they inspect larger-growing tree species with bark-included junc- tions, although this approach will likely have its limitations in terms of the scale of the tree bifurcations tested. An important limitation to consider is that young tree bifurca- tions will consist mostly of juvenile wood, whose mechanical behavior is different from wood in mature tree boughs. It would therefore be errant to assume that findings from testing young bifurcations could be directly applied to the much larger bifurcations of mature trees. MATERIALS AND METHODS Between November 2010 and January 2012, 241 junctions of hazel were harvested from a hazel coppice situated at Prestwich Country Park, Man- chester, UK. All the junctions harvested had two emergent branches, making each one a ‘bifurca- tion’. Collecting from only one site was necessary to limit the number of factors affecting bark in- clusion formation and bifurcation strength. For example, if one collected from more exposed and more sheltered locations, then the strength of the individual bifurcations within the sample would vary much more widely. Collection of the samples was randomized throughout the coppice, avoiding obtaining more than two bifurcations from any one tree and not taking any bifurcations from trees growing along the edges of the coppice. This resulted in 96 samples being collected from the same tree as one other sample, and 145 samples each being the only one collected from a particular tree. Samples were cut to retain approximately 100 mm of the parent stem and 215 mm of each branch arising from the bifurcation. Samples were wrapped separately in plastic bags and put in cold storage at 2°C–3°C to reduce sap loss before testing. The hazel bifurcations had an average parent stem diameter
July 2015
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