Factfiles

Defence Systems to Parasitic Decay

Trees cannot mend what is dead. Take humans for example, we cut ourselves in the finger and within a few days it has healed. This is not the same for trees. Instead, they have a defence system to counteract infections and wounding. It is called Compartmentalisation. This is what blocks off the decay using four separate walls depending on how far the decay has reached. I will explain further later.

Compartmentalisation is most effective in the summer because the tree’s energy levels will be up due to the leaves photosynthesising with the sun’s ray s. Photosynthesising is when the tree is making starch, a protein which is stored for energy. This is stored in the xylem which will eventually be used when it comes to autumn, when the leaves drop to stop parasitic decay penetrating the leaf scars. Once the leaves will have fallen, the tree’s energy levels will be low and this would be a bad time to prune the tree because it would leave the tree even more susceptible to disease. It would be wiser to prune in the summer, when the energy levels of the tree are higher.

Tree Defence System (Codit)

Wall 1

Codit is a model of Compartmentalisation. Wall one is the reaction zone which stops the decay spreading vertically. Xylem and flowem cells flow up and down the tree, and these are called vascular bundles. When wall one is put into action (Tylosis), the vascular bundles produce a sticky substance that blocks the way of the infection. This results in Tylosis blocking vascular tissue.

Wall 2

Wall two resists inward spread. It reduces the radial spread using the annual rings. This wall is stronger that one and once the infection brakes down this wall, the infection will reach the pith.

Wall 3

For wall three to come into action, one and two must have failed. To prevent decay in the centre of the tree, wall three comes in. The medullar rays stop circumferential spread and are far stronger that half were. It takes many years for a disease or a fungus to punch through this wall, though that is of course if an Arborist doesn’t drill through it first.

Wall 4

Wall four is the barrier zone and this limits decay to wood present at time of wounding. This wall is very strong and continues to form wood and grow outwards, eventually covering up any holes that were ever there, making it difficult to tell whether the tree is hollow or not.

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