Per that bit of code a few posts above (suspect you might've been replying to the previous page like I did when I interjected that thing about Loyalty) they're not currently spawning at all (in 0.16) - the new forests can't grow because growth only happens when there's a like feature in an adjacent tile and new forests don't tend to be around much. It's been returned to vanilla behaviour (or close to it) for Fire. I'd imagine it's entirely possible to allow features to spawn a different kind of feature with a chunk of SDK changes and some new definitions in the python files, but whether the effort's worth it (and the bug cleaning for tiles which are bordered by multiple spawning features - although that must be present somewhere since forest and jungle are often next to the same tile in vanilla) would depend on whether the team needed or wanted any other features to spawn different ones - I can think of a few uses for it, but none of them are HUGELY exciting.
I *think* roads reduce the chance of feature spread (feature creep?
) but don't eliminate it altogether - could be wrong though.