OK, this game really stinks
Actually I think you have a pretty good game design, however I think there are two things you can do to improve this:
1) Zombies should be immune to bear flatulence. I mean most zombies stink already, what with the rotten flesh and everything. It was way to easy to keep the zombies from reaching the lumberjacks.
2) Get rid of the endless trees. Introduce levels with a finite number of trees. This give the game a more achievable goal and a real sense of purpose. There is a true feeling of accomplishment to laying waste a whole forest whilst simultaneously increasing greenhouse gases.