City Trees

14 CityTREES will prove successful and junction failure will be averted. This effect of natural bracing not only explains why bark-in- cluded junctions are common features of many of our trees (as most species of tree have not evolved out of their branches interfering physically with each other), but it can help to identify forms of tree that are more prone to this problem and informs how they may be formatively pruned to avoid these two associated defects from occurring. In the UK, one of our commonest urban trees is a rap- idly-growing hybrid conifer, the Leylandii (X Cuprocyparis leylandii). It is often used to produce a quick evergreen screen at the end of a garden or as a windbreak. Unfortunately, at a large size, this hybrid has a reputation for the failure of its limbs in winter storms due to the formation of weak, bark-included junctions. If one can envisage the small branches that radiate out from a small conifer stem, one can also envisage that arrangement Figure 4 Recent tree pruning to a beech tree (Fagus sylvatica) in Lancashire, England. A: Upper white arrow shows location of the recent pruning cut, lower white arrow identifies an associated bark-included union; B: Close-up of the "welt" caused by the natural brace before it has been pruned out. This pruning has opened up to movement a very weak branch junction that has not been exercised for many years due to the presence of a strong natural brace. Either the junction will bulge and repair itself at the location of the lower white arrow, or it will fail in one of the next major wind events. when two competing leaders arise in such a tree, running parallel and near vertically to the top of the conifer. In that scenario, one can anticipate that many twig and branch touches will be very likely to occur across these stems. These twig-on-twig, branch-on-branch, and branch-on- stem contacts, when they occur, can cause the co-domi- nant junction to fail to form a good branch bark ridge, and a bark-included junction will form instead (Fig. 5). As the conifer grows larger, most of these internal twigs and branches get shaded out, die, then decay away. Even if they do not get shaded out, they are unlikely to keep up with the growth rate of the two vertical stems, because they are only lateral branches. It is the form of this tree, then, and its growth habit, that makes it more likely to form a bark-included junction within its struc- ture, and for that junction to subsequently fail, due to the loss or later ineffectiveness of these bracing minor branches. In a recent industry survey of 348 arborists A B

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