Merle - If you have access to limestone "fines" - also known as screenings - pile those on to a depth of about 4" all around your trees. Screenings are sharp, jagged little chips of limestone about 1/8" to 3/16" in size. I slide the aluminum window screen down into the ground about an inch first. Then, like J-bird said in post #6, put landscape fabric down next and then pile on the screenings. We cut landscape cloth 3' x 3' to 4' x 4' and cut a slit to the middle so we can slip it in close to the trunk. I like to use landscape staples to hold the cloth together at the slit. Then we pile screenings to 4" deep to cover the entire piece of landscape cloth. This buries the window screen 4" below the top of the limestone screenings and makes it very, very difficult for mice and voles to tunnel in to the trunk to chew. Besides being sharp and jagged, the limestone chips collapse in on any tunneling activity, ruining the little b-tards' efforts. We have both mice and voles and haven't had a problem with this method. The sharpness of the limestone chips seems to be a big deterrent. They'll tear up their feet trying to dig in them.
To make this go easy & quicker, we have everything ready in advance of planting. Limestone in the back of a pick-up truck, window screens cut to size and ready, landscape staples, regular stapler to close the window screen, landscape cloth cut to size, etc. Then it goes pretty quick when planting. This system seems to keep weeds down to a minimum too, giving young trees a boost with no weed competition.
