JimR said:

To help keep
moisture at an acceptable level, you also need about 2-3" of good mulch over
the rootball, but not touching the tree trunk.

The last part of your statement is one of the best tidbits of advice I've
seen in this group, yet. Too many people just mulch right on up the bark of
a tree. After "topping" a tree, that's about the worst possible thing you
can do to one. Hopefully, someone will heed that advice. =)

And, mulching two feet out from a trunk does no good, really. If you're
going to mulch a tree, might as well go all the way out to the dripline.
Most people won't do that, though. They'd have no yard left, heh.

--
Eggs

-I'd rather have a free bottle in front of me than a pre-frontal lobotomy.