It's great to misuse clouds for something they weren't initially meant for.
I used a lot of localised clouds for this.
Four layers with different density and noise settings for the ground, three stacked layers on top of each other with the same cloud fractal for the trunk (is "trunk" the right expression?), one for the mushroom, two for the ring (localised clouds blended by a distance shader), and one for:
clouds.
From next sunday on I'll be on vacation for two weeks. When I'm back, I'll try to animate this. I hope it will work.
