In Feb 2013 I bought a Beni Hime Japanese maple at a local nursery. I bare rooted it and put it into a moderately deep pot of Turface MVP just as the buds cracked in March 2013 (first photo). It subsequently leafed out but wilted as though I had been too aggressive with root pruning. So, I kept it shaded for several weeks until it seemed to have recovered. Nevertheless, its growth last season seemed weak. I expected it to rebound this spring, but to my dismay it died over the winter - the very few leaf buds on the tree were dry, brittle, and never colored or swelled this spring. I finally realized the tree was really dead when I noticed that the trunk bark was 'blackish' - a sure sign of dead cambium (second photo).

I am at a loss to figure out what I did wrong in my treatment of the tree or what I overlooked that led to its death.

I performed an 'autopsy' of my victim by sectioning the trunk with a large pruner. In brief, I found that the cambium and parenchyma in the vicinity of the graft union died.

At section 1, well above the union (third photo), I found that a quarter section of the xylem was still alive. Throughout the region of section 2, the cambium and xylem parenchyma were all dead. I made more sections near the nebari with a Dremmel wheel and discovered a live root at section 5 (forth photo), which explains why, even at the time of this 'autopsy', I had apparently healthy/live roots on one side of the (dead) tree.

What did I do wrong in my treatment of the tree or what did I overlook that led to its death?