[riskable]

It actually wasn't the trees it was mostly their bark!

https://en.wikipedia.org/wiki/Carboniferous#Rocks_and_coal

So it wasn't mountains of wood that lay on the ground during the Carboniferous period, it was like, the best mulching job ever. The trees pwned all other species at that time. Can you imagine being a weed trying to grow up through all that bark? You'd have one hell of an up-hill battle!

[baq]

I'd evolve growing on the bark itself, given that the nearest soil was a few (dozen?) meters below. See also anything that grows on bare concrete :)