What kind of growth it is on this old Maple tree in Nelson, BC? Apparently it doesn't do the tree any harm.
Burrs. As you say, it does the tree no harm, except in that it is at risk of chainsaw-related death, as the wood in burrs is very valuable because of its curly wood grain.
Fairly rare. Commoner in older trees than young ones (or at least if present, is less obvious on young trees). Maybe something like one in 500 or 1000 trees?
Perrhaps that's abit over-estimated ;-) Trees (and not only maples) whose branches are carlessly cut off often develop this kind of cankers, or whatever you call them. It may be a rarity on trees that grow freely in the best conditions, but (I think) it is very frequent on trees that are grown and trimmed by the hand of man. And yes, for a cabinet-maker, planks from such trees are very valuable ...
On a walk through my local park in Vancouver, I noted that several of the old trees had large burls on them--maples, cherries, willows. They seem common on old trees here. I wonder if the local climate encourages them somehow? I recall seeing them on large weeping willows often. I guess, when I think about it, I don't see them in the wild as much, except in the coastal forest. Perhaps trees on the coast get more injuries from storms that allow these types of infections.
My guess would be the more wounds a tree has to encapsulate the more likely one is. They look all to be pruning cut height afterall.
