My horses are eating my spruce trees. Seriously, they smell like Christmas these days. There are only a couple within their reach, but they play giraffe, and eat the needles, which should not be edible to horses. Seriously, they'll eat anything. And it's not like they're starving - in fact, our older gelding is so chunky that the vet has told me he needs to lose weight. But they all do it.
From the photo, those are clearly deciduous trees. So they will lose their leaves, which will further prevent grass from growing under them. Also, without leaves, they don't make much of a shelter. I'd aim to start cutting them down. Meantime, you'll want to get in there and limb them as much as possible. Other than eating things that are supposed to be inedible, horses have a very strong desire to poke themselves with sticks in places that hurt, and leave large mystery wounds.
Finally, I wouldn't blanket them in there. I can't imagine you being able to limb all those trees quickly enough to let them in safely anytime soon, so they will rub against those trees, and they will shred those blankets.
Among species to avoid, you know that several maples are supposed to be toxic right? We have some on our property, but none in or near the pastures/paddock. That said, I actually witnessed my mare eating a red maple leaf as I gasped in horror, unable to rip it from her mouth in time, and I was convinced she would fall over and die. She didn't. I don't recommend that approach, however.
So again, if I were you, I'd start chopping down those trees. Aim to get most of them cut, leaving just a few here and there as ornaments, and chew toys. I think birches are pretty safe (if not, my horses are once again proving that they have a death wish). White pine too. But do your homework, and go get yourself a chain saw. If any of them make for good firewood, you might be able to get someone to come in and clear the area for the free wood.