Deer have eaten the leaves of my trees. I think they are holly. I am trying to protect them with deer repellant spray and eventually a fence. If I can keep the deer away, will my trees recover and fill back in? How can I help them recover?
Your trees may not be permanently damaged this year, but once deer find a plant they like, they continue to eat it. When I lived in a rural area, a joke we had was "Deer first eat your favorite plants, then when those are gone, they go for your most expensive plants. When those are gone, they eat any plants that have landscape value (like hiding a disagreeable view or enhancing an agreeable one), and then anything that's somewhat exotic. Finally, they'll get around to eating native plants. You'll know they're done with your property when you're looking out your windows and seeing only buckthorn, honeysuckle, and grass."
My experience living in "deer country" has led me to conclude that it's far easier to live with them than to try to prevent them from doing something that you don't like, such as eating your plants. In your case, that means limbing your trees up to about 6-7 feet or so from the ground and then installing hardware cloth cylinders around their trunks until they (the trunks) reach 8" caliper (diameter four feet from the ground) to deter the bucks from rubbing the trunks. This will actually benefit you, too, in that it will be easier to mow around/under the limbs. It won't hurt the trees, either.
As for perennials, we planted a TON of poisonous plants, which did indeed seem to deter the deer from eating them.
Fence around each tree. deer will eat most leaves they can reach. You need no action for those trees to recover. However, the deer will nibble every year until all remaining leaves are out of reach. Those do not look like Holly from the photo. I have a lot of holly and a lot of deer; they cause minimum damage to holly. Most of my holly damage is to the bark from antler rubs. Years ago I tried any spray I could find with little success. Some helped for awhile until rain or dew washed it off.
I'd suspect that those lower branches are simply done.
I had a similar story some years ago, in an unusually snowy and cold winter here in Minnesota (U.S.), where the deer ate all the greenery off several large arbor vitae trees. Up to about 5 feet. They surely were desperate, because there was no other episodes in 30+ years where they ate the arbor vitae. (Some nice young white pine saplings, and aspen saplings, sure, any time!)
Before trimming the denuded branches, I waited a year or two, but nothing was happening, so I did trim off the apparently-dead branches.
In particular, if the deer ate those leaves in the summertime, you've got an intense, on-going problem. Fences. Looks like that's your specific situation. My sympathies. On another hand, if they did or do eat something only in an unusual winter, it's a one-off, and not worth the trouble to deal with.