Bending branches and trunks – what happens to the cellular structure?
A recent question asked by a student is what happens to the tree or plant when it is wired into position. This may seem a relatively simple question to answer but all trees are different, much depends on the climate zone where they are found. Meaning some species are more malleable and easier to shape, whilst other are more difficult.
For example, Lignum vitae also called ‘guayacan – guaiacum’ or ‘pockholz’, from the genus Guaiacum has a structure so dense that wiring is virtually impossible unless undertaken before lignification has taken place. Alternatively, the Cecropia genus are mainly hollow in their structure and thus are extremely difficult to shape. Some species are prolific in growth (Acer palmatum amoenum) are easily scarred by wiring, whilst others Larix kaempferi (Weeping larch) are slow-growing and more manageable. Arguably, researching a particular species gives rise to a more understanding of its characteristics.
The cellular structure of a tree’s trunk and branches are referred to as the xylem. The inner most part of the trunk consists of the heartwood, which although inactive gives strength to the tree. The sapwood, carries moisture from the tree’s roots to its leaves. The cambium layer is where the tree increases in girth or diameter. The phloem commonly known as the inner bark, carries nutrients manufactured by the leaves down to the roots allowing them to develop. The cortex area gives support and elasticity enabling the tree to withstand bending. The outer bark is the trees protection from injury. The following two images which are NOT to scale, will give some idea of how a tree’s cellular structure and its components work.
As we can see there is much activity within a tree’s structural system and remaining undisturbed will function normally. However, when bending or shaping a trunk or branch the cellular structure of the xylem becomes disrupted. Meaning the outer radius of the bend will be in tension whilst the inner radius is held in compression.
How does the tree cope with this disruption? – More than 90% of a tree’s cells are long thin tubes closely packed together, arranged along the direction of the trunk or branch. Their function is to transport the nutrients and water from the leaves (Phloem) and roots (Sapwood) respectively. They also provide support because, no matter which way a trunk or branch is bent, the internal forces always act parallel to the cells and are able to adapt to tension and compression. In addition, because the cells are hollow, the tree’s trunk and branches can be thicker as opposed to it being a solid mass.
Bending or pre-stressing
A tree does pre-stress its trunk and branches quite naturally as it has to combat the forces of nature. Nonetheless, the long thin-walled tubes in a tree’s cellular structure, have a tendency to collapse when severe bends are applied and this can cause problems for example. Many trees are susceptible to a condition known as ‘brittle-heart’, because as heartwood ages it can be attacked and broken down by fungi. Eventually the tree becomes so weak it dies.
Another problem with bending trunks and branches is that if a bend is too severe, the cortex may splinter or break resulting in damage to the phloem and a damaged phloem is not just open to attack from Lepidoptera and Fungi, it can weaken the movement of moisture and nutrients. It can be agreed that all potential bonsai will at some point in their existence, be subjected to some form of design. In order to produce a miniature copy of what is found in nature and this is achieved via the use of wire and applies to all species both coniferous and deciduous.
Regardless of whether the specimen has been grown from seed or collected from other sources, shaping a tree’s design can often call for some drastic measures. With deciduous species much of the work can be achieved in a shorter space of time than with coniferous varieties. But any severe bending should be done in stages to eradicate the problems mentioned above. Once the initial design has been accomplished, further shaping is usually achieved by pruning, but if additional wiring is needed it should be checked on a regular basis to ensure it is not causing damage. And of course, knowledge of the tree’s anatomy and development in natural circumstances is an advantage in determining this.
Another technique used for deciduous varieties is Uro, (Discussed in a previous article) where a small indentation is left after the removal of a branch. As new wood forms a protective callous around the wound, it forms a small hollow. This is later enlarged going deeper into the trunk and in some cases through to the other side leaving a hole, presumably to create character. But regardless of how deep one bores into the xylem, the tree is susceptible to attack from disease and many varieties are unable to survive. To combat this many bonsai enthusiasts use fire, sealants and pastes to protect the wound, others leave the tree to fend for itself – ‘a natural course of events found in nature?’
Conifers regardless of origin (Seed or other sources) are subjected to the same shaping techniques although Uro is seldom one of them. Conifers have more elasticity when bending for example, long branches can be shaped and reduced in length to add character to a tree. In some cases the branch is cut at strategic points and wired down as shown in the next 2 images.
However, this practice does require some thought prior to undertaking because, the branch can be attacked by disease and restriction of nutrients and moisture to other areas. Once the cuts have been made and the branch is in position secured by guy wires to it’s container, the wound areas should be protected, but some disagree. It is said that a conifer’s natural defence system will heal the wounds via it’s resin or sap.
Other methods for severe bends can be used for example. Wet raffia is tightly wound around the area in question, then wrapped in a rubber tape then the wire is applied and the limb bent into position. In some cases a strand of wire the length of the bend is inserted between the raffia and tape. The ideology behind this approach, is that the inserted wire (Placed on the outer radius of the bend) will protect the bark, cortex and phloem from splitting although much depends of the severity of the bend. (Shown below)
But it should be known that conifers take some considerable time before a wired branch or trunk will stay in its given shape. Quite often more than one growing season is needed to achieve the required result, but much depends on the species and diameter of the trunk or branch in question.
Other techniques for conifers include Jin and Sharimiki which have been explained in a previous article. Such techniques of removing bark and creating deadwood effects are said to add characteristics to a particular specimen, giving a natural appearance that is highly prized in bonsai. Nonetheless, what ever decisions are taken when designing and shaping trunks and branches; care, patience and the health of the tree is the most important factor. Until next time, BW, N.