Mmmm. Thanks Creativetides for your comment.

Right now I am busy packing stuff because I am moving in 5 days to another city for studies so I will post sporadicaly.

I agree with you that moisture is important. But what makes mushroom grow is a good question. In order to give some value to this thread, I will try to do some reading and some small research in order to open as many doors as possible to explore
the idea of a mushroom biome.

Generally speaking mushrooms are not big. at least not to the point of being a bush size or small tree. But what would they require to grow bigger? What ecosystem would be helping them? I know that in fall, we often have some mushrooms in our yards where leaves tend to fall. Now, we can't have a biome that appear in one season and dies afterwards, wouldn't make sense. I know that we often grow edible mushrooms with horse manure, but unless we plan to have some Godzillas around, there is no way any normal animal would be sufficient enough. Then again, thats FARMING, not natural growth. So certainly some reading on how mushrooms naturally grow will be quite a bonus here. Will do some research on it later.

But what I was thinking would be to have some disk shaped mushrooms, you know those you find on tree trunks, but instead growing out of some rocks or dirt in a denivelation. I like the idea near a mountain, because water does come and streams tend to merge in rivers or pounds. Water tend also to dig underground galeries and half open galeries (those that are not entirely cut off from the surface). So maybe where long ago an icecap stood and scrapped the mountains to the rock itself, leaving down below lots of humus soil could also be a boon for mushrooms. Just speculating here.

Maybe thinking also how we often see those biomes in videogames may be an insight in the matter, though they are not to be taken as authorities in the matter. Terraria tends to have them underground, those big mushrooms, and they have some small fluorescent light in them.
Its not rare to have mushrooms side by sidec with pale coloured light, especially blue and green. But that sounds unlikely if we are talking of a biome much closer to the surface if not on the surface itself.

Minecraft has them in two areas, either where you have lots of big trees, or in the fungus mystical land where you have shroomcows.

Mario tends to have them in much diversity but in the same shape. May it be undeground or on the surface. They tend to be tall, skinny, but the head to be flat and disk-like.

Now, I never really played D&D, but I am quite sure some people here could take some time to describe mushroom biomes in D&D?