Ok to remove the ring, highly recommended not to let the grass grow in though.

citrus tree trunks exposed to sprinklers are at higher risk of Gummosis type problems. It may be dry enough out your way to be safe in sprinkler watering.

6b Steve

Sounds like he might have used the smaller stones as kind of a permanent mulch and to impede growth of weeds as well as help water drainage. Not a bad idea, really.

If you let the grass grow up under the tree then it will compete and steal nutrients that should go to your lime tree when you fertilize, grass is a heavy feeder and will win over the long haul. You can remove the rocks but you will have to keep removing any weeds or grass that tries to fill in. Personally I'd leave it to prevent having to do extra maintenance.

If it were my yard, I'd get rid of the lawn.... and get better looking rocks.!

Googles "suicidal citrus trees".

Thanks all, that's what I needed to know...I'm leaving it the way it is!