There are several options when looking at the treatment of weak structures within a tree and to reduce the risk of limb or branch failure in areas with high volumes of foot and vehicular traffic, or where the tree has great amenity value. The tree bracing process usually involves installing hardware or reinforcements to a tree in the form of invasive cable bracing, rod bracing and or non-invasive dynamic bracing where trees have defects such as heavy, over extended limbs, weak unions, included unions and cracks or splits in the trunk or limb(s).
Specifying the type and location of any bracing or hardware installation can be carried out once a thorough assessment of the tree in question has taken place. There are several methodologies for the arboriculturist to follow when inspecting trees to determine what, if any works are required, especially when deciding whether or not to carry out invasive or non-invasive bracing. Whichever process is used, it must be a logical, systematic, and diagnostic in approach. The inspection should consider the species of tree(s), it’s likely life span and it’s response to either invasive tree bracing or non-invasive tree bracing types. It must take into account the surrounding environment in which the trees are growing and any potential targets, i.e persons, property or infrastructure at risk should the tree or parts of the tree fail.
The static bracing of a tree involves drilling through the trunk or limb and inserting a wire through the hole which is then held firm with a nut and cone locked in place at each end of the wires termination. The wire is tensioned using a hand held winch and the final termination set in place with the winch being released for the wire to take the loading forces. This type of invasive, static bracing is usually used in scenarios where a greater degree of protection is required and where larger forces are likely to be encountered with the branches and limbs being on the larger side. The species of tree will need to be able to compartmentalise the wound where the hole has been made by drilling and this is a major part of the consideration when specifying this type of bracing. These species are usually long lived, broadleaves such as oak, London plane, sweet chestnut etc as they have a better ability to seal off or barrier off the wound. This process is called CODIT, short for Compartmentalisation Of Decay In Trees where the tree forms four walls within its cellular structure to resist the spread of decay and pathogens throughout its structure following wounding / drilling.
A major plus side of using a stainless steel cable is that the strength of the cable is easily quantifiable in the different sizes it comes in and the strength does not degrade readily over time. This type of bracing has the potential to last a very long time and so can be more cost effective in the long run and can last the life span of the tree. As the tree compartmentalises the ‘nut and cone’ termination point the bracing becomes a permanent fixture within the tree unless removed in future by an arborist.
Dynamic bracing such as Cobra or Boa bracing is a non-invasive method of bracing and is held in place by friction. The strand of hollow braid rope is installed around the back of the limb or branch and is spliced back into itself, usually just above a fork or branch to stop it slipping down the tree. The plus side to the dynamic bracing is that it does not require drilling / wounding of the tree at the point of installation. It also allows the tree to move more naturally rather than rigidly holding the limbs so the tree continues to put on reaction growth to help strengthen itself to outside forces such as wind loading. Where the tree is subject to excessive forces such as wind loading the bracing acts as a damper and prevents these forces being transmitted to the weak union or defect and helps reduce the risk of catastrophic failure. There are a few downsides to this type of bracing in that it is sometimes not installed at the correct point of 2/3rds above the weak union because it cannot be held in place correctly. The hollow braid rope also degrades in strength with exposure to UV light and so does need more frequent inspection and also replacing after a period of time. This type of bracing is usually used on more short lived species of trees as they tend not to be able to perform CODIT and compartmentalise the wounding / drilling all the way through a large branch or limb.