It should be fairly simple to make up. Wear brown pants or a brown wrap around toga to show the trunk. Also wear a brown Tshirt but then wear a shorter cut off T-shirt that is gree till your chest, so that your neck, shoulders and chest are green. Get a green mask and green hat and make sure your arms and hands are green too.
An artist's impression of the "tree man" is shown. (Painstakingly created at 2am using GIMP)
Voila, you are a tree and all the tree huggers at the party can come and hug you! haha. just make sure there are no dogs in the costume party. If there are, then you better turn into a carnivorous tree. Of course, my friend Heba wants me to be a Christmas tree, with all ornaments and blinking lights - for her amusement. I am going to resist that.. haha.