Great responses, though tomc's was a bit too brief to be clear -- but I think imafan's last comment was very well thought out.
I was re-reading the OP and realized the tree is 20 ft
But it's probably a good time to note that some trees need the protection of the mulch ring more than others -- imafan pointed out that they create the buffer zone to protect the base of the trees from getting weed whacked. Some trees are very susceptible to diseases, and injuries like that could become a vector. Also nasty burrowing type pests could find their way in.
I think crabapple is probably such a tree. Maple once grown will drop its own mulch of leaves and the shade and the shallow roots hogging all the surface moisture tend to kill off the grass.
In fact, especially with young trees, if you have rabbits and deer, mice, etc., you should protect the base of the trunk with a collar of hardware cloth or plastic tree wrap for the winter so they won't get girdled from them eating the bark. Actually if you have deer browsing through, you may need to fence the new trees, though I suppose the 20 footer is safe enough....