The game looks good, but it needs some fixes.
1) When all the trees have been cut, you get the "game over" screen, but the game behind it goes on. Enemies keep coming, you kill them, and after you kill them you carry on to the next level, still with the "game over" onscreen;
2) You should add an explanation of the upgrades you can buy;
3) You should mention that to shoot you must keep the mouse button pressed: at first, I simply clicked every time I wanted to shoot, and I didn't understand why the hippie didn't shoot;
4) The game is off-centered in the NG game frame: it's on the top-left corner.
Nice game, though! Maybe adding more enemies would be great!