The trouble with this, other than the very fast growth rate, is that trees will take over the world given the chance!

To have forests (as a natural steady state), there needs to be something stopping continual tree expansion. This could be fixed areas of bad ground, it could be "dynamic bad ground" (e.g. trees make the ground under them bad, eventually killing themselves, but the ground becomes "good" again a while later — this might instead lead eventually to thin forest everywhere depending on variables).

Or it could be animals: e.g. biters start killing trees when there are too many trees in some area (to high density). But to prevent a "thin forest everywhere", the biters would have to target the killing, e.g. one biter randomly starts killing trees, then others kill more nearby trees.

Hmm, maybe you should get together with Veden and make this work with Rampant?

The rate does slow down and stop when it reaches 800,000 trees (can be changed in the config file)
I'm planning to have it kill trees in high pollution areas, also thinking about having medium pollution area trees get disease that could spread to nearby trees, that might work similar to the "dynamic bad ground" you mentioned.

What about MEGA factory maps? The entire game engine and CPU would croak under the mass of self replicating trees. :(