Posted: Wed Oct 04, 2017 9:51 am
by Rand
Didn't know this:
Trees start growing wood from the buds down, so a very harsh growing season may prevent the tree from adding wood cells all the way to the base of the trunk. Each place where growth stopped is where two or more rings become locally “wedged” into one ring.

I guess that explains why you don't see the bark crack and crack on bigger trees until later in the summer.