Oh, don't cut or pull anything off yet! What you could do is get a shallow tray-type of "pot", just a few inches high, with good drain holes covered in plastic canvas (the kind from a needlework or craft store). Set the rock (with tree) on it, and fill in all around with bonsai grit and some organic component to hold some moisture. Then wait while the roots populate the whole thing. In future, of course, you'll have to get help dumping it all out (slowly!) to trim the roots and/or repot into a larger container. In other words, what you'd do is really just a backwards step from what you'd do if planting the tree on the rock for the first time, teasing the roots into growing down into the mix and around the rock, only lucky for you that part's already been done. I think the trick will be getting the mix just right - gritty enough for fast drainage but water-holding enough so it doesn't dry out so quickly that it succumbs. And of course from then on, you'd water from above as with any other tree. If you have a local club, this would be a perfect project to bring in to get an opinion consensus on and the help you could always use!

Have been watching this a little and don't really want to have anyone stepping off the cliff here. We don't know what kind of tree it is anyway...
It's been growing on the rock, inside, for 8 years with little or no problems (at least as far as the poster has said)
There was a change in its environment a few months ago
It's been getting water from the bottom all of those years with no problem

If the only thing that changed was the growing environment, why not give it time to adapt as most will?

It may be a case of more light or that and some fert., there was probably a change in the content of the water as well.

Just my thoughts on this.

Bob

Bob is right it could be just a change in environment and he has a lot of experience growing trees in the way that you are. If you are going to repot the tree, do take Larke's advice on that though. Your tree=your choice.

What type of tree is this?