Totally agree with this kind of ideas. I've also made a similar suggestion : viewtopic.php?f=6&t=63245

Thanks for all your replies :)

I like the idea of randoming stuff but not permanent damage. I think that if a tree has leaves it should always have a chance to regrow, but of course each tree will have its own strength value, which would be random. Also, if we want it to be more complicated ...

The problem with aliens don't just remain at you producing pollution. Even if you literally produce 0 pollution you still are not safe, not even close. Why? Aliens like to expand on their own so eventually will expand right into your base. A variety of mechanics need to be introduces for friendly ...

TL;DR
Trees lose leaves by pollution, trees should gain leaves by lack of pollution.

What ?
Currently the trees (with leaves) absorb a specific amount of pollution according to their leaves (stages).
As they absorb a curtain number of pollution they start to lose leaves (increase stage level ...