I've read all sorts of things that are supposed to repel deer, but the only thing I've found that really works is to put up a chicken wire or hardware cloth cage that stays in place until your trees have grown big enough that the deer can't hurt them. The cage doesn't have to be anything elaborate, just a circle around them, staked well enough that it can't blow away. If the trees are already fairly tall, the cages will need to be a little bit wider so that the deer can't easily stretch their heads over to grab a bite. If it's too much work, they'll go elsewhere.