Strictly speaking, trees and shrubs do not 'heal' when damaged, in the sense that they can repair and regenerate new tissue to replace old tissue in the way that animals do.
But, plants can seal off or compartmentalize damaged areas using specialized 'scar' or callus cells.
The tree or shrub then sends out shoots and grows new healthy wood around the damaged area.
The area that is damaged is never repaired, but it needs to be sealed off keep any infection or moisture from spreading to the rest of the tree, or causing rot in the main trunk.
These 'band-aid' cells are called callus tissue or wound-wood.
However, these cells are not universally distributed all over the tree or shrub. Understanding where these cells are concentrated, and are most effective in sealing off damage, is the key to understanding the best way to prune trees and shrubs. You want to make the cuts and trimmings in areas that the plant can easily seal off and compartmentalize.
This article describes the sealing process, and where the callus tissue is concentrated. It provides a better tree pruning guide.The collar area in the branch junction is crucial for successful pruning as it contains cells that compartmentalize and seal off damaged branches from the rest of the tree. multiple cuts are recommended to prevent any damage to the collar ares.
The term for the way trees shield off a damaged or cut branch is 'Compartmentalization Of Decay In Trees' (CODIT). The collar area, which is the swollen ring where the branch meets the trunk or another branch, contains special cells, referred to as 'the branch defense zone', which triggers the growth of callus cells or woundwood. While trees can create woundwood anywhere the damage occurs on the tree, the process is particularly effective and fast in the collar area.
So, when making pruning cuts to remove excess branches, it is crucial that the collar area is not damaged in the process. This means making several cuts for large branches to stop splits and tears damaging the collar.
If the collar and the branch defense zone is damaged when pruning, this will stop the proper growth of woundwood and callus around the cut. This can mean that the compartmentalization and seal is incomplete. This can cause leakage, decay and rotting of the trunk or healthy branches and eventual death of the tree.
The compartmentalization and the branch defense zone is the way the tree can save the rest of the tree itself if a limb is badly damaged. The growth of the callus seals off the damaged limb from the remainder of the tree. It also cuts of the supply of nutrients to the isolated damaged limb, which will eventually die.
Sapwood is the outer region along a branch through which nutrients and water flow.
Sapwood is highly resistant to decay and heal themselves without the need for compartentalization and woundwood being needed.
Tree can cope easily with cuts to the bark and with shallow cuts into the sapwood.
Branches that are less than 2 inches (5 cm) in diameter are mostly sapwood. This means that the smaller branches can be pruned at will.
But, as the branch size double to about 4 inches (10 cm), there is a significant amount of heartwood in the branch, which makes it vulnerable to decay of the damage is not compartmentalized and sealed off.
Knowing this difference between small and large branches provides a way to decide where and how to prune based on branch size.
Small branches heal relatively easily, with low risk of infection and decay.
This means that small branches can be pruned almost anywhere.
It also means that the major pruning required to shape the tree should be done early when the trees is small and most branches are less than 2 inches (5 cm) in diameter.
You should shape your small trees to the final form in the first few years of its life, when the branches are still small.
When pruning branches larger than 4 inches you should prune above the collar, ensuring that it is not damaged.
After any pruning, you can help your trees to recover, by keeping your tree in good overall health and reducing stress.
► Pruning at the right time
► Fertilizing the tree properly
► Providing adequate water
► Aerating the soil around the tree if it is compacted
► Treating any pests and diseases that affect the vigor of the tree and its ability to respond.