Hi,
I keep a small flock of shropshire,s in among xmas trees and young woodland, they won,t debark your trees, but will certainly eat the leaves (not xmas trees). Best bet is to wait to trees are big enough that they can't reach the top third, I found 5/6 ft ok. You could probably winter graze before this and take them out when leaves emerge.
The shropshire's have been easy enough worked with and produced good lambs (pure) of grass and have used the tip on x breeds again with good results. Just don't expect as many lambs as your llyens.
p.s. first post on forum (have found it very useful in past).