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Tree Pruning - Professional Practice at the Cutting Edge

Without Trees We Become History....

In natural environments trees grow perfectly well without intervention from people. Since the Paleozoic era, over a time period exceeding 350 millions years, trees have evolved in to highly complex organisms. Trees are compartmented, shedding, woody perennial plants with specialised differentiated cells containing vascular conducting tissues.

Trees have developed effective mechanisms for dealing with injury. In most normal circumstances they are readily able to counteract, mitigate or isolate any life-threatening pathogens or decay. Throughout their life cycle - from emerging seedling to senescence and eventual death - trees live as dominant yet delicate organisms within a finely tuned continuum of natural succession. In nature the largest organisms - trees - are continuously under seige from the smallest - fungal, bacterial or viral microbes.

However, the evolution of plant life did not especially account for human actions. When we prune, we wound. Incorrect pruning can have a severely adverse effect on trees. To avoid unnecessary injury and maintain healthy trees it is essential to adopt proper pruning practices. Additionally, understanding how trees respond to wounding allows us to undertake pruning operations with the least damage.

Without trees and associated green plants there would be no life on earth as we know it. We owe it to the earth, to trees and to people to do everything within our intellectual power to care for trees. Correct tree pruning is best achieved by competent specialists.

The Guiding Principles...
 
Natural Target Pruning (NTP)
Compartmentalisation of Decay in Trees (CODIT)
 
To prune trees and cause the least adverse affect we must firstly understand the correct methods of how to do so, then secondly why the methodology is necessary.

Fig.1 Pruning principles. The first cut (A) undercuts the limb. The second cut (B) removes the limb. The final cut (C) should be made just on the outside of the collar, to remove the branch stub.

1. Natural Target Pruning - NTP
The how to part is based upon a procedure known as Natural Target Pruning, or NTP. Every competent practicing arborist should prune trees in accordance with this procedure - if they do not then their knowledge and awareness is in doubt and they should not be employed. The actual difference between the "right" way and the "wrong" way to prune is a very fine line, but instantly obvious to anyone with an understanding of contemporary tree care concepts. In basic terms it's all about angles and attention to detail. The wrong angles or the wrong approach - the wrong outcome.

Proper pruning is all the more essential when one appreciates that once the wounding is inflicted there is almost no going back, no opportunity to make good what has been damaged. Pruning must be right first time, every time.

You can recognise the key indicators of good pruning yourself. Three things contribute to the desired result - the Branch Bark Ridge (BBR) retained, the Branch Collar (BC) intact, the final cut line (FCL) correctly aligned between them. See Figure 1. There are no set pruning angles applicable to every tree - only targets - the BBR and BC. Correctly using the targets as guides should ensure the right final cut every time.

The appearance and prominence of the Branch Bark Ridge and Branch Collar may vary considerably between species. In smooth-barked species for instance both features are usually readily visible, whereas in many rough-barked trees the area of trunk and branch union is often less well defined. Whatever the species a professional pruner will achieve the correct results through the application of knowledge and skill. Branch removal itself is always a 3-step process - thereby avoiding the weight of a branch tearing or ripping to below the collar. See Figure 2.

2.Compartmentalisation of Decay in Trees - CODIT
The simple act of applying the principles of correct pruning outlined in NTP will radically improve the tree's capacity to continue growth without harm from microbial infection or progressive decay. This is because wound responses in trees are critically linked to internal tissue structures and phytochemical reactions. Some decay or infection is inevitable and should be expected - a consequence of the constant competition between microbial organisms for nutrients and advantage in the never ending struggle of succession. The key to the tree's survival is its capacity to counteract, mitigate and isolate the threat.
Superficial infection of the wound surface by decay-causing fungi may occur - but with no significant or long-term adverse affect to the tree providing access to the internal vascular system is denied. Making the final cut at the site of the trunk/branch union exposes the inner vascular tissues to oxygen and light, instantly initiating a chemical reaction which creates an area generally known as the barrier zone. The barrier zone defines the extent, or boundary, to which any invasive pathogen may spread. By compartmentalizing the wounded area the tree restricts the vertical, radial or lateral spread of infection.

Fig.3 CODIT - The relative position of each "barrier" wall and how they collectively create an internal compartment which restricts the spread of pathogens or decay.

CODIT is the model used to visualize internal tissue structures and the changes which occur inside them when wounding takes place. Figure 3 is a highly simplified view of the vertical and radial arrangement in cells which make up annual growth rings, together with the four compartment "barrier" walls formed as a reaction to wounding. The "barriers" are not precise actual biological features - they merely represent an image which allows us to appreciate the wound response process. Shigo (1979, 1989 and 1991) provides the most fascinating insight in to tree decay and the CODIT concept.

Briefly, the compartments alluded to in the CODIT model comprise four inter-active barriers or walls. In Figure 3 Wall 1 limits the vertical spread of infection by blocking the vascular conductors with chemical deposits; Wall 2 is formed by the last cells within the current growth ring limiting inward spread; Wall 3 limits lateral spread because the tangential ray cells do not allow direct linkage to adjacent vascular tissues and Wall 4 (not shown in the picture) - the strongest barrier - prevents the spread of infection to new wood formed after the wounding has occured.
 
Trees can not heal, replace or repair damaged or infected tissue. Remember, trees have evolved as highly ordered, compartmented plants, which instead of healing compartmentalise injured and infected tissues. By containing the wounded area within a distinct compartment trees effectively limit and isolate damaged tissue. The damaged tissue remains within its self-contained compartment thereafter, gradually becoming overgrown by successive annual increments and having no detrimental affect upon the continuing development of the tree.

Pruning correctly, using the principles of NTP as a guide, ensures the chemical barrier zone remains intact - able to react and function as the first line of defense against injury. Poor pruning - leaving cuts which are flush or ripped - opens the vascular system to invasive pathogens and allows the processes of disease and decay to overcome the tree's natural protection system.

Copyright Bryan Gould