Thanks all,
I have no problem with Dirk's (and others) use of pliable rubber to keep water out. For it's effectiveness, you should talk to someone who uses it.
As for spray foam, It might have a role to play as a cosmetic treatment for the tree or a psychological treatment for the arborist or homeowner. I have no problem with it, but be aware that the products I've seen do break down with exposure to UV from daylight. I've seen some applications with gnawing or chewing marks that I've figured to be caused by rodents.
My entry into tree wound response with Dr. Shigo was with wound treatments after branch removal. The collective view, from Alex and from his European colleagues (including Dirk) was that if the position and manner of the cut was correct, no wound treatment provided a benefit. By "manner" I mean seasonal or vector timing among other things.
Healthy trees carry dying, dead, and decaying branches all the time. During normal development, branches are being shaded and shed. Usually, the network of living tree cells (symplast) is withdrawn from the fading branch and physiologically shed well before it is physically shed. That physical shedding is usually mediated/facilitated by wood decay fungi.
Sorry for that exposition, but it goes to Tennarbor's question about decay encroaching on the column. What does that look like in the case of interest? In the natural branch shedding process in response to canopy closure described above, decay will go slam bang up to the visible collar and stop. Yes, an attenuated cone of embedded branch may rot out, but be surrounded by healthy stem. So no treatment is necessary there. In storm breakage that leaves a branch stump, the symplast will die back due to aeration before a compartmentalization boundary will form in the living stub...or the formerly living stub just won't have the juice and the decay will proceed, but again be restricted to the branch base.
Where the neat process fails is with canker fungi, but that is a story for another thread!
