never used a button_target, but i know you can get the effect you want with a func_button and an env_shooter. set up the shooter with the model or sprite you want to use and get the the other settings the way you want them. cover the "tree area" with the func_button and make it invisible (render mode solid, fx amount 0). target the shooter with it, give it a very small delay before reset (maybe .1 or less), give it .1 health, and click the "dont move"flag.
if the tree is large, you may want to separate it into different areas covered by different func_buttons which trigger separate shooters.
gl