Snape as a tree

[starduchess]

Someone on hpnewbie was asking for this fic: It went something in the lines of Harry taking a break in the Forbidden Forest, and he climbs up a tree. That tree turns out to be Snape (for one reason or another..). I think that Snape at some point gets stuck in that tree form. Snape's dark mark manifests on the tree like a wound that bleeds sap.

I know I've read it, but I don't remember its name. I don't remember exactly why Severus is a tree, so I put possibilities in the tagging (animagus, creature, cursed, dark magic, etc).

Thanks in advance.