Mountain is actually impossible to pass, even on Easy. I don't know if I'm just logically illiterate, but there's either not enough trees or currency to spread Fynbos or to contain the bushfire after setting part of it on fire.
Yeah, this level frustrated me. I got to being *almost* there, with 42% completion, everything done but a little more fynbos. But now I'm stuck and have to start over because I basically can't ever undo forest growth and can't put it where I want it. Also, fires don't seem to burn forests, contrary to what the burner's tooltip says.
You can rebuild toxin scrubbers on the ruins after a fire, it makes things much easier, you can limit forest growth and have farmland just next to the trees for fynbos.
(And you can use the trees you planted for beehives too, not just the natural ones, seems obvious but took me a while to figure out).