Following this thread, or trying to fit somewhere inside it, makes me think of a centipede trying to put on its shoes. It's easy to be distracted when one comes across a new or different pair, and if the activity goes on long enough, it's difficult to remember what the first pair looked like.
We started with the question of finding proof that deadwood removal had a health benefit to a tree. That seemed a reasonable question and yet here we are, a surprising 125 posts later, on tangents and obliques of angels dancing on the heads of pins.
I will confess to being somewhat befuddled myself and trying to figure out which pair of shoes to lace up.
Deadwood has exited trees for a few hundred million years without any of our interventions. We didn't have the tools until we acquired opposable thumbs and saws. And actually, we likely didn't need to think about deadwood at all until we invented fire.
Waddling back to what I think is still the original point, I'm not sure that anyone goes out into a "natural" forest to cut off deadwood, either as a charitable act to trees, or as an anticipatory public safety measure for those wandering in that forest without looking overhead.
That leaves us urban forests, which have always seemed to me to be rather oxymoronic, but I don't need to pick a fight here and read posts that I'm antisemantic.
Dead wood is the expected residue of live wood. Live wood is the ongoing product of the cambium; a single layer of meristematic cells twixt the wood and the bark. Once live wood is created, it is "dedicated" or modified to its particular position and purpose--and apparently never divides again.
Depending upon the species and conditions, the live cells on either side of the cambium serve their functions until their appointed, expectable demise, when they fill the definition of dead wood. They can also die before that time by circumstances, drought, pathogens, etc--in any case, dead is dead.
(Using a space between the words dead and wood in one context and the words drawn together for the debris of woody cylinders left on a tree, may be a part of the solution in our discussions.)
The larger the woody cylinder, the greater amount of dead wood inside it. We simplify, and use the laws of fuzziness, to mess up our own understanding of the biology by calling one "sapwood" and the other "heartwood". This is the "angels dancing on the head of a pin" part; it is mostly, visually subjective as determined by discoloration with many subsets of factoids and conjecture.
http://upload.wikimedia.org/wikipedia/commons/0/0b/Taxus_wood.jpg
At some point, the older cells are indeed dead parrots. They are structural and meaningful in many ways, but they do not contribute by serving living functions. They have contents and chemicals that can retard decay significantly, but they cannot be called "live" because of those effects.
When the cambium of a woody cylinder disappears for reasons of starvation, pathogens, or interferences with its continuity and growth, no more live wood can be produced and the cylinder itself eventually becomes "deadwood".
A limb or cylinder without living cambium has severely diminished protections against decay. If there is no living, active and productive, cambium there simply can't be any Wall 4. The most important and effective defense available in CODIT is the cambium's modification of daughter cells against an interior progression of decay.
No cambium; no Wall 4. No cambium; the end of life in that woody cylinder.
http://www.forestryimages.org/images/768x512/1409010.jpg
I'm also surprised by the range of explanations of how trees work. I'll try to get back in comment.
Wulkowicz