don't do your kicking a tree idea. Aside from the fact that Mr Miagi taught us all in Karate Kid III that trees don't attack us... You're gonna hurt yourself.
If you cant afford a bag, check for a used one on kijiji or craigs list. If you can't afford a used bag... i dont know, maybe something filled with sand? Just don't go kicking an immovable object like a tree unless you've got a hankering for some serious shin injury. it takes time to build up your trebeculi -- don't rush it and break the thing making it weaker.
striking a tree with your shin, with force = terrible, terrible, terrible idea.