On a per-biome basis just like every other instance of spawning bo2's with saplings,
red and brown mushrooms both spawn bo2 objects instead of growing Notch's standard 'hugemushrooms.'

Hmm Interesting, I generally turn that feature off after world is generated but would be nice to fix that.

Wouldn't it be great if you could assign to a bo2 from what sapling types or mushroom types it could grow? This way you could have mushroom bo2s growing from shrooms :)

I think its possible to do now too with the new features just included recently giving custom biome ID and mob group spawning control per biome.

I think since ANVIL it is now possible to actually assign Bo2 per biome ID now and sapling growth per ID, would need looking into that is for sure.