Okay, I've been sitting here very late at night/early morn reading what people have posted and the various positions put forward, I have very carefully reread what I posted as well as everyone else, I have reread my favourite sections of A Shigo "A New Tree Biology" and hope I can offer something of a solution to what seems at face value to be an impass re the presence or not of wall 4 across the face of wounds that never (or have not yet) occluded.
As has been said by a number of others it comes down definitions and getting tied up in semantics......Let me give you some quotes...
"The CODIT model has 2 parts: I; walls, 1, 2 and wall 3; II; wall 4. Wall is a model term and
should not be identified too closely with chemical and anatomical terms....walls 1, 2 and 3 are model representations of the reaction zone. The reaction zone is a continuous boundary about the developing column of infected wood. The walls help to identify the 3D nature of the column. the
reaction zone is the chemical strengthening of existing boundaries
Part II is represented by wall 4, which is the
barrier zone. The barrier zone forms after wounding.
the cambium forms cells that differentiate to form the barrier zone" (p46 A New Tree Biology)
The italics are Shigo's emphasis the bold mine.
So it would seem by this little quote Eric has the points...except for the fact the Shigo is adamant that we should not be thinking of this model as a rigid blue print to be overlaid ever tree we encounter, its a model of the structures and processes not the physical reality...but we're not done yet...oh no not by a looooong mark!
Just remember that in all the following quotes Shigo uses the term barrier zone and wall 4 as interchangable terms.
"What happens to the wood inside the boundaries will be determined by many factors. Trying to assign one term or name for the process is like trying to give one colour for the rainbow. We need terms like rainbow or terms that will imply changes over time. Living systems do not stand still." (p311 A New Tree Biology)
Again cautioning against the religeous application of a model onto the emmense variations in nature.
"If I had to name one, and only one factor that has caused more confusion throughout the research history of trees than any other factor, I should name
the study of the cross section alone." (p323 ANTB)
We all need to be aware that the radial cross section is what Shigo often referred to as an "artifact"
In a diagram that Eric used earlier in the thread he highlighted the orange line in the new growth as wall 4, but the diagram clearly states that the orange depiction (not reality, a model) is both wall 4 and barrier zone.
The diagram depicts the barrier zone being present..formed in the tissues of the open wound...this is not very helpful, somewhat confusing perhaps unless you recognise that even though wood tissues may be critically compromised functionally they are still capable of alteration as part of the defensive response by the tree....from my post #50
Quote:
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let me first say that the wood tissues and cells exposed have been altered chemically and physically, the effectiveness of those changes (in terms of preventing or retarding increasing decay and disfunction) is dependant on a host of inter-related environmental and species specific factors.
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Eric does recognise this and has said stated this in his own posts in this thread. I think many of us are guilty of skimming much of the literature (I know I am!!!
) and this can result in confusion in our own minds when coming to grips with specific applications of a model to a very complex physical reality in the tree.
"I am getting concerned that tree people with spial interests may begin to skip over material they feel is not germane to their needs. I hope you will not do this. I caution you against this, because the points I keep trying to make all focus on the tree and how it manages to survive against an unending number of agents and pathogens that try to outwit the tree." (p387 ANTB)
"As cells are injured and die, their cell contents come into contact with the air. Chemicals in the cell react with the oxygen and these new compounds are usually antimicrobial-inhibitors-and they cannot (for the most part) be broken down by the inading organisms. Yet some pioneers can grow into the tree against the antimicrobial compounds. Some pioneers have developed unique ways to digest the defense compounds. Other organisms fight fire with fire they alter the trees compounds in such a way that make the compounds not harmful" (p121 ANTB)
What we end up seeing in the dissected tree is often the manifestation of this complex interaction between pioneers and tree, and can be very hard to definitavely decipher.
"A new growth ring has yet to form so there is no barrier zone" (p120 ANTB)
For those that disagree I'm sorry but for Shigo the barrier zone he describes yes has a relationship to the wood tissues at the time of wounding (of course) but is not formed in those tissues...
other kinds of barriers are......the barrier zone/wall 4 is formed in the new growth after wounding, ther is little confusion here.
I leave the last words from me on wall 4 and the barrier zone to Alex Shigo
"When the cambial zone about the wound resumes growth, the newly formed cells differentiate to form a boundary that seperates the wood present at the time of injury from new wood that will form after the boundary is completed. That seperating boundary is called the barrier zone." (p599 ANTB)
