Where can I find mitre angles for creating a circular border around
the base of trees? For example, what is the angle cut needed to
create a 7 foot circle, an 8 foot, or 9 foot circle?

Per previous poster, depends on how many "sides" the circle will have.
A method that seems like it should work is to remember that the number of
degrees, cut at an angel, in the "n" sided circle must total 180 degrees. In
other words, a four sided circle (grin) would have each cut made at 45
degrees (180 degrees divided by four). An eight sided circle would have 22.5
degree cuts, six sided circle 30 degrees.
I'm not much of a Gardner nor mathematician, but I believe that should
work.

Old Chief Lynn