burning trees, if possible, would = OH NO MY FRAMES!
Seriously; if you just shoot the trees, your fps suffers horribly.
Go on, try getting a mech with a few mg's/uac's/rac's and shred a decently sized patch of trees; watch your fps DIE.
Now imagine not only that, but then each of those trees catching on fire and creating smoke, possibly spreading to other trees.
It's a really neat visual and tactical idea, but it'd probably kill the vast majority of players who don't have really really good computers.