This is an Acer ginnala, which is native to northeastern Asia and north to southeastern Siberia. Apparently its core is fairly delicate and is susceptible to rotting(not surprising in this climate). And this particular specimen has gone and rotted all the way through, leaving what looks almost like pure bark in a ring, topped by branches and foliage. You could imagine wearing it as a vegetative bracelet, if you had a wrist big enough, that is.
I saw this tree for the first time last year, before it was out in full leaf, and it was quite spectacular then. I must say though that I prefer the proportions when it is in leaf like this, as the foliage mass balances out the trunk - or what there is of the trunk.
'Cause this isn't a little tree, folks. Check out the size of the trees in relation to John in this post. No, that's not the same John as in this post, that's the other John - have I confused you yet?