The Claus Mattheck Lectures: A Review by Mark Chester (Part l)
By Mark Chester
2018-07-16
Claus Mattheck is held in such high regard that it was no surprise that the dates for his final tour of the UK were sold out within days, and a fuller programme was soon being prepared. Monday had involved a field trip to the nearby Bradgate Park, with its fantastic collection of veteran trees. I was able to join the tour on Tuesday and Wednesday. Claus has a unique presentation style, one in which he works to ensure the audience remains attentive. He explained at an early stage that this was his final tour not because he is feeling his age, but he no longer wishes to encounter airports with their cancellations! Anyone remember volcanic ash? That loomed over his previous tour. This time, the challenge was the more routine cancellation of individual flights!
There have been critiques of some of the theories underpinning arboriculture, including Mattheck’s own work. He emphasised from the beginning that Visual Tree Assessment is an on-going project, with new aspects continually being appreciated. It was important, he explained, that our theories should be simple to explain, and justifiable from observations. One should not need to understand mathematics in order to appreciate the mechanics involved!
The Axiom of Uniform Stress is also only present in a tree which has experienced stress. Its presence indicates problems and the need to investigate further. Its absence only indicates the absence of stress! A tree does not like having a weak point!
He spoke about lines of stress, and the shear square. There is no failure under compression, only under tension! There was a resume of the different types of fungal decay and how they can affect a tree. He explained the difference between stiffness and strength, citing steel as strong and stiff but nylon as strong under tension yet flexible. A cracker is stiff, but not strong, whereas jelly lacks both strength and stiffness. This is a most memorable analogy.
Claus explored the impact on the tree when apical dominance is affected. If the leader is weak or missing, this affects the tree with lateral branches needing to take over. Geotropism causes the trip to grow upwards. If the leader is unable to lead, or light becomes an issue, phototropism occurs, with lateral branches becoming dominant. With phototropism, the branch structure focuses on the need for light, not strength, which means that branch develop is weak and failure potential becomes in-built.
Reflecting on his own journey, Claus shared how the technology enabling us to assess tree condition has advanced. We can now drill trees to establish the present and extent of decay and cavities. Indeed, he has concluded that the mallet has only a limited role in this, and that the decision on whether to fell a tree, or not, should not be taken solely on the results of using the mallet! He was full of praise for the work of IML, without which his own research would not have progressed as it has.
However, the basic principles remain: start at the bottom, look upwards and note what you observe. If, for example, there is Ustulina at the base, one does not need to undertake a detailed and time-consuming assessment of the crown! Having reminded us of the basic principles of VTA, Claus then reviewed some of the core principles of tree biology. He reminded us that compression wood exists in conifers, which is good because it strengthens. In broadleaved trees, the tension wood is on the other side to conifers. In broadleaved trees, supporting wood forms are a stop gap until the tension wood has formed. He reminded us that wood is four times harder under tension than compression. The tree does not want fibre buckling, he observed. Bracing should be avoided, because it generates tension, reducing the strength of the timber fourfold.
London Plane trees have a soft timber but one which it can support. The Poplar, on the other hand, has a weaker timber. For Claus, it is utterly unsuited to the urban setting.
Claus then explored differences between trees and soil. The tree can be thought of as a sailing board; it collects wind. Soil is not so good at this. If it is too wet, there is a risk of slippage; if too dry, it turns to dust. It cannot carry tension, only wood can.
After a much-needed break (yes, that was just the first morning session), Claus resumed what was rapidly becoming a many course feast. He shared about the value of pruning a tree, highlighting that the tree will prune itself, but not in such a good way, leaving it vulnerable to pathogens.
Claus has added to his terminology (of the repeatable nature) the Chinese moustache and the Cupboard door. We also now have the sheer square which shows areas of stress and tension. These are accompanied by a plethora of illustrations. We were treated to photos of trees whose roots had attached themselves to rocks and cliff tops. These are secure bases, we were assured. The rock isn’t going to move whereas soil can become decidedly unstable, especially if it is on a slope exceeding 45 degrees!
Trees matter, and our veteran trees are particularly important. Claus cautioned against applying the 30% rule too rigidly; it is designed for specific scenarios and should not be used to condemn veteran trees. At the start of the week, he had visited the nearby Bradgate Park, home to some great veteran trees. Some of these are little more than shells, yet without over extended branches causing the lever arch, the risk of failure is minimal.
He emphasised that we should not follow rules regardless of the situation. We should not do what we cannot justify. How hollow can a tree be? Claus has seen hollow trees with less than 30% of the wall remaining, and still standing. Branches have been shortened and the trees are safe. At Bradgate Park, there were trees with walls of as little as 10% thickness.
It is equally important that we do not seek to retain trees as habitat features when their condition makes them unsafe and they are need to roads and footpaths. Showing one such example, Claus suggested we provide a home for the beetles away from the footpath!
Looking at different forms of decay, Claus highlighted that with butt rot, which is longitudinal, the trunk will split before it breaks. Decay can easily attack root wood, especially late wood. Structural kinking can initiate failure.
A leaning tree is not inherently unsafe; branches can also lean, they just happen to be attached on the trunk and not the ground.
With this, and a flow of slides to progress through the extensive library Claus had brought with him, it was time for a break. Claus encouraged us to drink plenty of strong coffee but to eat only enough food to keep us going!
Our review, part 2, will continue next time!