By Andrew Noakes, Qualified Arborist and TRAQ Certified Assessor.
Though their bodies can be large, their trunks wide, and branches far reaching, the most important living woody layer of a tree lies just beneath the bark: it is called the cambium. When the cambium is damaged, even in the slightest way (i.e., nails, ropes, scuffing from animals and machinery, pruning…) trees NEVER heal from that damage to their living system. Instead, quite incredibly, they undergo a process of shutting off the damaged part, to protect itself, and re-routing the living vascular system around it.
In this way a damaged tree doesn’t heal but can learn to live with the wounds it suffers. The effectiveness of which, of course, is dependent on the conditions and extent of the wound; and is why respectful treatment of trees and proper pruning (in the right season, with the right technique) is key to the overall vitality of any tree.
So how does a tree protect itself from damage?
A tree's ability to compartmentalise influences can be observed throughout the tree structure from a cellular level (cell walls) to annual rings (with boundaries on each side), to the epidermal layer (bark). These boundaries aid the tree in isolating any threat and allow for it to send valuable resources to the injury site to keep it from spreading throughout the structure.
Compartmentalisation Of Decay In Trees (CODIT) is a means by which the tree attempts to isolate the injury or introduction of a pathogen by the building of four (4) “walls” around the injury site to ensure it does not compromise the vitality of the tree:
• Wall 1: Resistance to vertical spread of pathogen, plugs up vascular tissue above and below the wound.
• Wall 2: Stops the radial spread of pathogen toward the tree centre, formed by lignin (polymer) rich cells of the annual ring, making them rigid and woody.
• Wall 3: Plant defence mechanisms to resist lateral spread of pathogen, isolating pathogen through the rays.
• Wall 4: The strongest wall is the inner barrier, which closes the wound from the living cambium, with formation of strong new wood.
CODIT is triggered when living and dead tissue meet at the injury site. The term “woundwood" refers to the development of varied tissue in response to wounding at the injury site of a tree, this forms the walls.
The process of CODIT was a revolutionary discovery in the 1980’s when American Dr. Alex L. Shigo wrote about it in his book A New Tree Biology; it changed the thinking of accepted Arboriculture practices worldwide. However, knowledge gained in the years since has given rise to questioning a tree’s ability to compartmentalise all four walls as it requires significant energy that is often unavailable in a wounded tree. As ever, if in doubt, consult a Qualified Arborist.
Next Month… Tree biomechanics and correct pruning techniques.
Andrew Noakes, of NZ Arb Consultancy, is a local Qualified Arborist and TRAQ Certified Assessor with over 30 years experience caring for trees in a variety of situations. email@example.com .
Caption: CODIT discovered in an oak reduction.