Man, that's so good! I really like the colors that you used, i hope i can get this good someday!
Just because you asked for it, i will be really picky ehehehe. It's kinda of what HANDRO said.
Maybe it was by choice, but for me it's not entirely obvious if the stump is rooted there or if he is just on top of the rocks... i mean, from the context of the piece i believe that it is rooted because of the mushroons and what a stump is, but maybe it was a eathquake that removed it from the ground, i don't know hahaha. Maybe it would be cool to see some parts actually entering the ground, or something else that you may think.
It's hard to give critique to someone so higher my level, i don't want to look cocky or delusional hehe.
I hope i could help somehow.