I think tree spirits may be of two things. It could be the life force of the tree itself, the trees living essence or maybe a tree guardian. Possibly an angel or other spirit. I believe trees are the voices of Dana, our Earth Mother, so they have to have a spirit or life force and be spiritual to speak to us, and we have to learn to be quiet and listen. As far as a literal form that spirit may take, I don't know. If it is an angel guardian or messenger it may take human form but I feel that tree spirits are generally without form. They are something you feel and do not see. Like people, trees have a soul and may have a guardian ( Angel or Spirit Guide ) as well.
There are more to trees than meet the eye. I feel it every time I enter the forest.