City Trees

Assessing tree junctions is a key part of the role of a tree sur- veyor. As an ex-tree surveyor and urban forestry officer, I undertook my research to help others with this task of surveying and, more impor- tantly, understanding tree forks. Our research work at the Universities of Hull and Manchester, England, has identified a new model for branch attachment, based on using CT scanning and electron scanning microscopes to look at the arrange- ment of wood grain at branch junc- tions (Slater & Ennos, 2013; Slater et al., 2014). The basis of this new anatomical model is that, where a smaller branch is joined to a larg- er stem, the faster-growing stem occludes the base of the slow- er-growing branch, embedding it to form a branch knot over time. In addition, the dense twisted wood formed centrally under the branch bark ridge provides mechanical sup- port to the branch junction, and this is especially important for the strength of a branch junction where the two branches or stems that are conjoined are roughly equal in diam- eter, where a knot will be absent. Fortunately, you do not need to borrow a CT scanner or have X-ray eyes to see the wood grain patterns formed at some tree forks: in mature junctions of oak (Quercus spp.) and ash (Fraxinus spp.), the large vessels often allow the interlocking wood grain patterns to be visible upon debarking a specimen (Fig. 1). The presence of circular vessels at branch junctions has been reported by several authors (e.g. Hejnowicz & Kurcry´nska, 1987; Lev-Yadun & Aloni, 1990; André, 2000), but only recently has it been consid - ered that these beautiful grain patterns are associated with the mechanical properties of tree forks (Slater et al., 2014; Slater & Ennos, 2015b). As the wood formed under the branch bark ridge is a significant contributor to the strength of codominant branch junctions, it follows that bark-included junctions, where the branch bark ridge has not formed properly (or not at all) must be inherently weaker than a normally formed branch junction. In an experiment, we found gradations in the strength of bark-included junctions: these gradations were based on the position and extent of the bark included in the join (Slater & Ennos, 2015a). Unsurprisingly, wider areas of included 10 Dr. Duncan Slater is a Senior Lecturer in Arboriculture at Myerscough College, England. He recently graduated with a PhD in Plant Sciences from the University of Manchester, based on novel research into the anatomy and mechanical performance of tree forks. Figure 1 Debarked codomi- nant stem in Turkey oak (Quercus cerris), exhibiting whorled and dense wood grain at the top of the junction, which lay under the branch bark ridge. F O R K S A N D FA I L U R E S The Effects of Natural Bracing in Trees by Dr. Duncan Slater, Senior Lecturer in Arboriculture, Myerscough College, England

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