Implausible Science: How can this be true?

[Klaatu B. Nikto]

What about the shrooms being so massive that the caps are able to support traditional plants? Or there are trees so massive like baobabs of Yggdrasil proportions so fungi take over most of the ground cover with traditional photosynthetics snatching up the sunlight filtering through the baobab branches.

Massive disaster with lots of biological detris and fungi being the first to benefit but for whatever reason (radiation! magic! magical radiation!) keep growing. Eventually dirt and debris form atop and begat a photosynthetic forest. As more traditional plants grow skyward, they create their own niche and choke out most competition from the actual floor since its rather shady and humid at the surface.

Kinda like an above ground Underdark but with room for more Underdark.

Maybe the fungi and plants have a mutual symbiosis like mentioned elsewhere. The fungi provides temporary height advantage for the plants, which has results in a root like system growing downwards providing a "cage" support for the mushrooms. Maybe even share a circulatory system. Both the root system and the mushroom provide water and nutrients but the photosynthesis likewise provides nutrients but also protection.

I guess I'm kind of imagining something like the trees in a swamp but close to the water's surface, fungi flourish. They provide mostly flat surfaces for other things to grow on like I mentioned earlier. Maybe like a mangrove? In which case, maybe the fungi is the only thing that lives in the brackish water.