As described in Monday’s release, there are four walls plus the branch collar that the tree uses to block the spread of decay.
Below are the descriptions of these walls as listed in the order of weakest to strongest response:
Wall 1 limits the vertical spread of decay up the tree. It is the weakest wall and requires the tree to plug the vascular “Xylem” vessels bringing water up the tree from the roots to the leaves. This vascular tissue is constructed with soda straw like cells connected end to end. To function, these cells die after growing and open at their junction to allow water to pass through. Since they are not living, they cannot respond to limit the movement of decay organisms introduced into them.
However, at the junction of each cell, there is an organism called a tylose. This organism, when it senses the presence of a decay organism, will grow like a balloon inside of the vessel and seal off the flow of water to the adjacent xylem cells. This process is slow and therefore (depending on the species) it is common that some decay is spread vertically prior to successful sealing of the xylem vascular tissue.
When limbs are removed from a tree (whatever the reason) there is always a possibility for introducing decay pathogens. If done correctly the tree’s life is far more likely to be extended and its removal postponed.
Wall 2 in Dr. Shigo’s model is created by the living cells in the interior of the trunk. This is in the area generally referred to as sapwood. Sapwood loses living cells annually. Therefore, further into the trunk you get to heartwood, which is essentially the non-living structural wood in the tree from which construction timber is milled. Because of the limited amount of living cells inside the trunk to react to decay, movement of infection internally into the tree is common and, in some cases, decay will completely destroy the entire trunk wood existing at the time of injury.
Wall 3 of Dr. Shigo’s model is much more reactive as it consists almost entirely of living vertical planes of “Ray” cells. These cells are arranged like vertical walls radiating out from the center of the tree like bicycle spokes. This is why many trees exhibit columns of vertical decay in a pie shape tapering back to the center of the tree. The remaining wood beyond, on either side of the walled in decay, is thus protected from “Lateral” spread of the decay throughout the entire trunk by these ray cells.
Wall 4 of Dr. Shigo’s model is the wound wood grown by the cambium upon injury. The cambium alters itself completely around the existing tree trunk at the time of injury. This alteration provides a protective barrier to all future lateral growth of the trunk from any infection that may develop in the injured tree. Thus, even if walls 1, 2 and 3 fail, wall 4 gives the tree a chance to grow a new tree around the existing tree during the time it takes the old tree to decay. Given enough time, the new tree can develop enough structural wood tissue to continue supporting the tree for decades. The general rule for minimum structural support needed is a ratio of 1/3 healthy solid wood growing completely around and enclosing the decayed void, which should have a diameter equal to no more than 2/3 that of the tree’s trunk diameter.
And finally, there is the “Branch Collar.”
The junction between the trunk and the base of the limb is called the branch collar. This area is identified by the “Branch Bark Ridge” at the top of the junction and the slightly swelled area located around and at the base of this junction.
When removing selected limb(s), it is essential that the branch color be retained intact. The final cut should be located at the face of the branch collar. This final cut should be preceded by two additional cuts to avoid pealing bark away under the removed limb. Pealing bark will make a much larger wound, much of which will be outside the protection zone of the branch collar.
Properly done, the remaining branch collar will have a much better chance to seal the remaining tree from potential infectious organisms that otherwise might enter if the complete collar is removed with what we call a “Flush Cut.”
In conclusion, trees have within their genetic programing the potential to survive injury. If we choose to do our part to ensure our best effort to work with, and not against this instinctive ability, it will benefit not only the trees but will add significantly to their ability to continue to contribute to our quality of life.
As compensation, they provide oxygen for us, reduce urban runoff, provide shade in the summer, reduce air and water pollution and provide many other benefits to our urban environment.
It is not a hard choice to make when you think about it.
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