I had this same though recently after thinking about how much shrub/tree/grass growth I have pruned out of my yard over the last 4 years ;-). Keeping a compost pile of all trimmings keeps this in perspective. Conversely, the amount of mass left after you burn wood is quite minimal. There was a nice scene about that topic in the move "Smoke". Screenplay dialog here: http://www.script-o-rama.com/movie_scripts/s/smoke-script-tr...