City Trees

Figure 1. Here you see the "self-repair" of a stem wound caused by a previous failure of a bark-included union in a tulip tree (Liriodendron tulipifera) over a period of twelve years. I took the initial image of this tree, which is situated in the Myerscough College grounds, in 2006—note that the failure was probably several years before that time, given the amount of woundwood that can be seen in the first image. For the students, faced with the tree as it looks in 2018, this image helps them to appreciate that those markings on the outer bark relate to an old (and relatively large) wound. This process of wound occlusion is fascinating and my TOT images show that it is closely related to the age and growth rate of a tree (i.e. a young tree of high vigour quickly covers over such a wound, while a mature tree of low vigour may never fully cover over such a wound). When I have previously shared this image, a few arborists have said it is "a great demonstration of CODIT*." I have to disagree, as the photo shows only wound occlusion from the outside—it shows nothing of the processes that have gone on within the wood of the tree. For me, it is important to get these scientific principles correct; otherwise, I would not be of much use as a teacher of students. *Alex Shigo's Compartmentalization of Decay in Trees (CODIT) Figure 2. The occlusion of a pruning cut on a semi-mature ash (Fraxinus excelsior) over ten years. When I first started teaching at the College, I taught a set of students gaining a Level 2 forestry qualification, and they had to learn how to prune forestry crop trees to minimise damage to them and also to minimise the formation of knots in the stems of forest trees. It's essentially the same technique as an early crown-lift to an amenity tree— using a natural target pruning (NTP) approach to select the final cut. From this image, students can appreciate what the outcome of a good pruning cut is, what the final occluded branch end will look like. Note that over the years it was occluding, there was a slight "scuff" that left a further mark on the woundwood here: when callous is initially developing over a wound, it is softer and more vulnerable to damage; I have a few TOT images that show that callous gets the occasional scrape before the occlusion of a wound. Note also that the woodland has changed over this time—the initial algae on the young tree has changed to moss growth, and the woodland floor is now covered in ivy. This marks a significant change in the level of light in this woodland, as these young trees have grown together to form a continuous upper canopy. >> www.urban-forestry.com 33

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