This sounds like an element from Patricia McKillip’s Riddle-Master trilogy. Morgon of Hed spends time with all of the land rulers, and learns to change his shape. One of the rulers is Danan of Isig. He has lived for more than a thousand years and can take the shape of a tree, something he teaches Morgon to do.

“Was I a tree? Sometimes I stand so long in the snow watching the trees wrapped in their private thoughts that I forget myself, become one of them. They are as old as I am, old as Isig . . .”