Could be rabbits. Voles prefer to chew roots underground, not girdle bark.

Then they're mice I guess. I caught a bunch with traps last fall.
Any ideas?

It could be voles, was this part under the snow?

Yes, there was nearly 3' of snow. We had an early melt off but snow is forecast, I went out and triple layered more tinfoil and mothballs.
But hoping for a better plan, losing trees like this hurts.

I have had the same problem, not as severe as you. Hardware cloth is about the best protection you can use. I have even put several inches down into the ground to protect the roots when I set out young trees. The nice thing about using hardware cloth is you can bring it up the trunk as high as you want. Most lower limbs should be cut off trees anyway. The hardware cloth comes in rolls and the wire can be very firm to less firm. Of course, you would want the finer mess to prevent a vole from getting through. I also leave the pointed sharp cut ends protruding as this acts as a deterrent for those that like to crawl up and over the wire! I use wire cutters similar to a pair of pliers to cut it and just fine wire to hold the cage cyclinder together. Actually, I have used twist-ties from the kitchen to hold them together. I believe you will find that the hardware cloth will take care of your problem. I use it for just about everything, protect seedling from voles and rabbits, lettuce and cabbage transplants in the garden, my wife's lily bulbs. I have lost too much over the years not to have hardware cloth!

Voles are scavengers and will eat about anything vegetarian. I agree with hoseman about the best solution.

Hoseman, thanks for that information. We have a new property that is so studded with gopher and vole holes, we don't know where a tunnel starts and ends. We heard about planting in gopher baskets, but they are very expensive, and we have several trees and an entire vineyard to plant, so I googled hardware cloth and found it has many other uses in the home as well. Don't know how we ever lived without it!

Here is a link that might be useful: Improve your Home with Hardware Cloth

I'm glad we don't have that problem where I'm at. The biggest problem we have a moles and the barn cats take pretty good care of them.