You can't really defend the tree. You have to survive as long as you can until your oxygen gone. Try it out!
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Took me for ever to find that reply so sorry in advance(I also replayed the game with the comment in mind). I still need to say that the game did tell you to defend the tree even tho you can't, also I have health even tho I loose health only when I get in touch with the smoke so I am not sure when do I actually loose my oxygen. But don't worry about it, just need to explain the task of the game more accurately in the future games.