lol (for the footnote: although it may not seem too rare, this is how rare it is. fyi, a dark oak forest has a chance of having oak trees in it. also, an oak forest has a small chance of having birch trees in it. this means that the oak tree and dark oak tree forest wasn't counted as one forest, like usual. it counted as a oak forest and a dark oak forest. [2 forests] because they can spawn together. this creates the very low chance that the oak forest, can spawn with birch. the chances of an oak and dark oak forest trying to spawn together, is already pretty low [if I had to guess, 5%] then, birch trees had to spawn. [if I had to guess, the overall percentage is 1%. if I add the chance that the biome is a forest, probably 0.2%. that's pretty low! all I know for sure, is that it is the rarest forest in Minecraft.])
