Is it a coniferous - type tree? And now you must show it in colour too!
I'm thinking it has edible berries that are consumed by rock lizards so that the seeds would be deposited (with a bit of fertilizer) in some other crevice or burrow. And insect pollination. I'm imagining these trees as being pretty few and far between, so throwing pollen into the air en masse just wouldn't do.
Too cool! If the trunk grows around rocks it could grow into really imaginative shapes with hollows and furroughs.
Oh yeah; and you could also make a mold around it with a shape you wanted filled, then water the tree excessively until the tree bulged to fill the shape of the mold. Then you could remove the mold and a nice tree trunk sculpture would be left.
It makes we wonder how much animal and even plant life might also cling to this tree, which presents itself as the perfect water and food source for herbivores.
I think the tree would be growing in fairly inaccessible rocky areas, although I bet that some small animals snack off of it. The water would be stored inside the trunk, so it would take some gnawing through bark and inner wood to get at it. You'd have to be a beaver to get in I think. By the way, I think the ERC Age needs giant beavers to build natural log dams over streams.