What would be the best way to set up a belay for a short - say, 20 ft climb on a rock crag when:

1) Belaying someone climbing up from below

2) Closest anchor (stout tree) is 25 ft from the rock edge

3) No cracks or horn near the edge to set any nuts

4) Unable to top rope this spot w/o a 50 ft length of static rope - which I don't own

Any suggestions? I was always taught that the belay anchor should be within arms length for a safe belay - but in this case that wouldn't be possible.

Is there a way to accomplish this safely?

I think manguts would concur.