My entire place is oaks and dry ravines and the horses have a section in the middle that is 95% shaded year round thanks to the oaks. No grass - just dirt floor. One of my horses has metabolic issues so I perfer the dry lot method - but they get free choice hay year round. Yes, they chew the bark off the trees and sometimes the younger trees die, but I don't mind because they just get used in the winter as firewood. If you LOVE your trees, you won't want them chewing them up (and they will) but if you don't mind because you have so many like me, then the positives outweigh the negatives. My horses don't get sunburned or bleached out too badly and there is always shade. They have enough room to run around and it never gets too boggy.
Here's a pic of how mine live: