Well, level 4 (Twisted Caverns) has this thing where an aboleth is killing off the fish in the river, and the "Aftermath" section says that if the PCs don't get rid of the aboleth, the fungus forests will die off too, "destroying the delicate ecosystem".
Maybe they want PCs to kill the aboleth, either kill, chase off, or make a deal with the drow, and start expanding Wyllowwood's forests into level 4? They could mix the real plants from Wyllowwood into the fungus forests... (Maybe some continual flame spells for light? It's already moist enough...)