Hi! Actually, there are a few things you can do if the ground is frozen to protect your Apple tree from rodents. One is a corrugated plastic wrap thing you wrap around the trunk. Here's a link to a photo to show you: http://www.amleo.com/4-foot-3-inch-diameter-corrugated-tree-guard-bundle-of-10/p/NP348/?kc=amlshop16n,&mkwid=sb6ALpbaq|dm&pcrid=51228669712&pkw=&pmt=&plc=&gclid=Cj0KEQiA7K7CBRCrwt26v5uHs98BEiQA0JzsZ1jc0lm7mQX0tYPLU05p0smA6wDklNYu07Nq91sVzP0aAiot8P8HAQ 

Another thing you can do is a wrap around plastic tree wrap, which is probably best as your tree looks to be very young. Here's a link to a photo: http://www.amleo.com/leonard-tree-guard-vinyl-spiral-36in/p/36TP/?kc=amlshop16b,&mkwid=sb6ALpbaq|dm&pcrid=51228669712&pkw=&pmt=&plc=&gclid=Cj0KEQiA7K7CBRCrwt26v5uHs98BEiQA0JzsZwvvKQ0qkztd4y8m4-oYfK-bXCCdq8_6JcGLiFOS0u0aArZ78P8HAQ

Another thing, probably the best quality, would be mesh or metal hardware cloth and wrap it around the tree. You actually don't need to sink it into the dirt very far, you can make it just enough to be sturdy and place large rocks around the base of the mesh and that should work the best to keep pests out.

What I do for my cherry is just make a triangle with connecting grids and twist ties and sink it into the dirt a bit.

Good luck! :)

Thanks for the tips! Do you think the tree will be able to recover from this?

Unfortunately, it is probably doomed. It looks like it went all the way around and is quite extensive in height, making it tough for the tree to grow over it. The roots may put up some suckers, but it looks like the girdling is all the way down to the graft, so the suckers will be from the root stock and not the grafted scion. If it does sucker, you could let it grow and eventually graft a desired scion on to it, but that could take a couple of years.

>>I read they are supposed to be buried a few inches below the surface<<

A few inches of the graft I assume... if the graft is buried a few inches below the soil then dig the soil away from trunk in spring to the graft union and it might push new growth...otherwise the damage is too extensive to have around...especially when this young.

Hm but I thought the graft union was supposed to be above ground

I'm not positive if you can save the tree. If the mouse chewed deep enough, it will block nutrients from the roots transferring to the trunk and leaves. I've never had a problem like this with my fruit trees, so I'm not sure how deep that is exactly. Some research on that may help you out further.

It had a lot going against it already - I really wanted to espalier a tree against my chimney but it is west-facing and doesn't get any sun until about 13.00 and then only will get about 5-5.5 hours until it's taller. Any recommendations for a next tree to try next year? Maybe I should focus on flowers and foliage rather than fruit.

In your zone, I'd say cherries, apples, nuts (like walnuts), a hardy peach, pears, a hardy persimmon, plum, maybe a paw paw tree are good options. I personally prefer fruit trees over ornamental trees or flowers, as you get the best of both worlds- flowers and edible fruit later. :)