October 19, 2021
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There are over 6 million trees along Britain's railway, some of which are very old and full of life. In particular ‘Veteran’ trees that show signs of wounds or decay. Such features offer a specialised deadwood habitat for a variety of wood-boring invertebrates, nesting and roosting locations for small birds and bats, as well as ideal conditions for lichen and saprophytic fungi.
These features are usually only found in trees that have taken hundreds of years to mature, but it is possible to mimic the characteristics of these ancient and veteran trees on younger specimens, which, in turn, enhances the local habitat in order to support an environment rich in biodiversity.
Veteranisation is the act of intentionally causing damage to young trees that would take years to occur naturally; whether it is woodpecker holes, broken branches, stripped bark, cavities caused by fungi, hollowing by rot, or even lightning strikes. This naturally occurring damage helps to create areas of deadwood within a living tree, a critical habitat for a wide range of wildlife.
Veteranisation has been used in forests and woodlands for centuries to bridge the generational gap between ancient/veteran trees and younger trees, while still retaining the ability to support a diverse habitat. Normally, we have to wait for trees to develop the dead and decaying wood habitat associated with old trees and the biodiversity that they support. Veteranisation helps to speed up this process.
A quick note on ancient and veteran trees; Ancient trees are mature trees with unique characteristics that provide their ecosystem with saproxylic habitat (dead or decaying wood). However, a veteran tree is not necessarily old, but it has some traits of an ancient tree. Therefore, an ancient tree is also a veteran tree, but not the other way around.
Veteranisation on the Network Rail estate
Although safety and time constraints can limit the use of some veteranisation techniques, there are some that can be used as an effective means of delivering habitat gains along the lineside estate when appropriate.
Natural Fractures
As the vast majority of arboricultural works along the rail involves the removal of hazards in the form of encroaching trees and vegetation or the removal of dead, dying and dangerous trees, coronet cuts can be used to complement conventional pruning. By using coronet cuts we mimic natural fractures that would characteristically be seen on broken branches following storm damage.
After reduction pruning or felling, the otherwise flat cut is shaped to be left with a ragged finish, similar to how a branch or tree would naturally break. This is performed by shortening the limb to a stub of about five times the diameter of the branch where it meets the parent member or a minimum of 30 cm. By carefully using the chainsaw, the flat stub face is turned into a coronet shape. The branch is repeatedly cut at slanting angles, which results in a jagged, spiky finish.
Artificial damage
The physical signs that a tree has been injured or damaged are known as its veteran characteristics, as mentioned earlier it is these features that we’re looking to replicate to provide deadwood habitat.
However, with safety being of prime importance, intentionally causing damage to healthy, living trees along the lineside isn’t recommended for habitat creation. We can however take advantage of trees that are marked for felling by leaving them as standing deadwood, which creates excellent aerial habitat for insects, bats and birds.
This process involves a fully qualified arborist who will use a chainsaw to ‘monolith’ the tree, which involves removing the entire crown (all the main branches). After being reduced to a suitably safe height a chainsaw is used to carve out the tree bark in order to mimic damage onto the main stem, which is then left as a standing deadwood structure, providing an excellent supply of decaying wood and cavities.
Retrenchment Pruning
Retrenchment is a natural part of the ageing process in an old tree. When nutrient and water supply from the root to the crown start to reduce, the tree will shed its smaller branches and begin to develop a new lower crown. This can happen multiple times over the course of a tree's life cycle.
Retrenchment pruning is a phased form of crown reduction, which aims to emulate this natural process, whilst retaining and creating habitat features.
Trees should be, and are treated as a valuable asset and work is only carried out if they pose a risk to the safety of the railway if they were to fail. In order to identify these potentially dangerous trees, ADAS carry out large-scale hazard tree surveys on behalf of Network Rail.
By visually inspecting every tree that has a trunk diameter greater than 15 centimetres, they can determine if it is healthy or poses a risk to the railway. If a potential hazard is identified, a scoring system is used to assess the level of risk, which also dictates the time period in which the potential hazard needs to be managed.
This scoring system takes into account the species, diameter and general health of a tree, as well as a defects hazard assessment, which covers everything from fractured limbs all the way to lapsed pollards or dieback. Proximity to infrastructure such as location boxes and track, as well as third party vehicles, and footpaths are also factored into the score. Based on the result of the assessment individual trees will be categorised from 1-7.
Generally speaking, category 1 to 4 trees require remedial work, such as lopping, topping, pruning, or pollarding, and category 5, 6 or 7 trees almost always require felling. Felling is treated as a last resort, but unfortunately, it is sometimes unavoidable, but as mentioned, wherever possible will look to leave behind standing deadwood.
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