THE CASEBOOK OF DR. D. R. SLATER - No.4 - Occlusion of foreign objects

MANY TREES WHICH SHOULD BE ENHANCING URBAN LANDSCAPES ARE SADLY NEGLECTED; DR DUNCAN SLATER IS CALLING FOR CHANGE

One of my interests is how trees can grow and ‘swallow up’ other objects: this process is known as ‘occlusion’ and our urban trees have been found occluding all sorts of odd objects – park benches, bicycles and golf balls. I have quite a few images?of trees that have occluded?metal railings or part of a wire fence, which happens quite frequently on low-maintenance?or neglected sites.

Much at stake...?

This example of extreme neglect is a car-park birch tree (Betula pendula Roth.) It must have been in contact with its supporting stake for several years for this level of occlusion to occur. This has malformed the stem of the tree – not a good start for a tree that could potentially be in place for several decades. Our urban forests consist of a wide range of trees and shrubs in various planting locations – gardens, parks, street trees and landscaped areas. In my experience, this problem of neglect is most often seen on commercial and privately-owned sites. Typically, some money is put into the initial landscaping scheme to fulfil the conditions set in the planning permission for the development, but little money is invested into the future care of these?the planted trees and shrubs in the future.

If we are serious about the value of our urban forests and if we intend to enhance them for future generations to enjoy, then things need to change.?We need to ensure retail parks and other commercial landscaped areas receive the level of maintenance needed to produce good quality tree stock on these sites.?There is much at stake here – the future ‘greenness’ of our towns and cities has, to some extent, been handed over to commercial companies as?they develop within our towns and cities.?We should insist tree planting and maintenance is part of any such development, so we end up with well-greened urban areas, not just slabs of concrete, patches of tarmac and empty pits where the trees used to be.

Sitting on the fence...

Occlusion of objects can result in some interesting biomechanical changes in trees. Pictured here is a sycamore (Acer pseudoplatanus L.) that has occluded an old Victorian park railing, with the sequence showing a ‘time lapse’ over 10 years. The growth rate of the tree has slowed, as it is mature, so not that much of the railing has come to be occluded over the last decade.?

However, if you look at the diameter of the trunk of the tree above the railing and compare it with the trunk’s diameter below the railing, it is clear 10 years on that there has been a change. By ‘sitting on the fence’, the upper part of the trunk has grown more, and the highly-supported lower side of the trunk has not grown half as much. This is due to a lack of mechanical stimulation of the trunk below the occlusion of the metal rail – this additional support has ‘switched off’ the process known as ‘Thigmomorphogenesis’ – Thigmo for short - in the tree’s lower trunk.?Thigmo is a complicated word for the way in which a tree acclimates to the forces and loading that it senses. By resting on this metal rail for decades, the lower part of this tree does

not match up with the strength needed to hold up the upper part of the tree, becoming more and more reliant on the railing for support, which could lead to problems if the railing rusts away or is damaged. This is a good example of Thigmo in action but oddly, although scientifically validated and taught to plant scientists on a regular basis, the scientific work in this area has not properly filtered through to arboricultural textbooks and articles. I believe it should be part of the curriculum of all advanced arboricultural educational courses. In particular, a good understanding of Thigmo can really help an arborist interpret what is happening to the structure of a tree and what can be done to prevent an unwanted failure.

Plane gets a grilling...

This image shows a London plane tree (Platanus x acerifolia (Aiton) Willd.) that has, for many years, been absorbing a metal grill that acted as a small ornamental barrier at the base of the planting, which probably had flowering plants in at some point, before the tree’s trunk grew wider and occluded this metalwork. Many people find images like this intriguing as it looks like the tree growth has ‘flowed’ over?the object.?The early stages of secondary growth in trees does produce soft tissues, which are made rigid later in the process, which helps to explain this appearance of a ‘gloopy’ or flowing material. Wood, when mature, is relatively stiff – but the cambium that produces it is a set of very soft-walled cells. I enjoyed sharing this image at conference talks in New York State and Ontario, Canada last month. In particular, because I told them that this is how British arboriculturists make sure that a tree can never be felled: allow a big metal grill to be absorbed into the base of a tree like this, and it is permanently chainsaw-proof.?For a moment, the audience believed me!

You're Choking...

When I saw this dead limb in a sycamore (Acer pseudoplatanus L.) growing in Alexandra Park, Manchester, there was something about the interface between the dead limb and the rest of the tree that made me look more closely: the shape around the attachment was wrong.?On closer inspection, I could see the ‘choke chain’, as pictured. A chain had been wrapped around the base of the limb some considerable time before and the tree had tried to occlude it but had failed. The limb has essentially been throttled now. Why anyone would put a chain up there? - I don’t know, but it caused the death of a good third of this tree. The girdling of a tree by wire, string, washing-line or chain can sometimes just result in the occlusion of the girdling object – this process may leave some marks on the bark, or you can see a bit of the washing line sticking out of the stem, but it has little consequence to the ongoing life and structure of the tree.?In other instances, it’s quite the opposite – the stem or branch of the tree fails to occlude around the object and the part above the obstruction dies off because it gets throttled.

To my mind, there are two key factors involved in whether trees get throttled in this way, or not.?In young and vigorous trees, secondary growth is rapid, so young trees are much more likely to fully occlude a length of wire or string than a mature tree that is growing very slowly – so the age and growth-rate of the tree is the first key factor.?The second factor is that it matters how broad and wide the girdling object is: the thinner it is, the more likely it is that the tree will occlude it with only minor harm – the wider and thicker it is, the more likely it is to cause major damage or ‘throttle’ that part of the tree.

To prove the influence of that second factor, I recently set up a rather cruel experiment in a set of young birch trees, putting different diameter cable ties around their main stem at eye level, to see what thickness of cable tie they will readily occlude and which cable tie is too broad and ends up strangling the tops of the trees.?One year into this experiment and parts of some of the smallest diameter cable ties are already occluded into the stems of some of these birch trees. Why be so cruel to these trees? What could such an experiment hope to show?

Well, it’s no joke: there is a modern trend to strap Christmas lights and other ornamental lights to trees in the UK (and elsewhere) and they are often strapped to the trees by cable ties.?I have seen quite a lot of damage done by this practice, so this experiment will hopefully highlight how damaging this is and what can happen to cable?ties around the branches and stems of amenity trees. This casebook is full of examples where trees have been neglected and thus bad things happen to them. Young tree plantings on private commercial sites in particular can receive low levels of maintenance or none at all, unfortunately.?If you are a bit of an ‘urban tree watcher’, you will notice the dead and dying trees in supermarket car parks and around retail and commercial sites in your local area on a regular basis.

Duncan Slater is a senior?lecturer in arboriculture?at Myerscough College, Lancashire, England.

*This article first appeared in the November edition of Pro Arb Magazine*