"Vole damage should be visible at the base of the plants. They can chew the bark off at ground line, causing death of the tree. If they continue to be a problem, you may need to wrap the trunks with 1/4-inch-mesh hardware cloth to prevent the voles from chewing the bark.
If the entire tree is turning brown, it is most likely damage to the roots or main trunk. Have you dug up the dying ones to see if you can detect damage in the root zone or on the main trunk? Until the cause of the problem is deciphered, it will be difficult to come up with a remedy. You may need to take a sample of the damage (not already dead) to your local Cooperative Extension office to have them help identify the problem."