"Things You Wouldn't Know If We Didn't Blog Intermittently."
That's a work of art for sure.
Not sure if your term "work of art" implies (incorrectly) that this is man-made, or whether you are using the phrase to praise the beauty of the natural world. A human has done some cutting to reveal the process, but what you see is a normal natural phenomenon.
Some of the garden beds in my yard are edged with short, vertical sections of old telephone poles (two still have identifying tags attached). I see a similar pattern of decay in those (hollow center but inner portions of limbs not rotted). As I was treating the fresh-cut ends of some new round fence posts with copper naphthenate, I had a realization: I think that the outer layer of wood (in my garden and in your photo) did not decay because the posts were treated with (probably) creosote, and the creosote is only absorbed into the outer layer of the wood.
Well, that does explain the "knots" in wood, doesn't it? Makes sense to me, even though I never remotely imagined this as such. It also reminds me of a pioneer wagon wheel--I wonder if that has the slightest bearing on the matter?