Mushrooms are not plants. They do not have photosynthesis but are saprophytes, which means they get their energy from dead organic matter and not from sunlight. Photosynthetic organisms fix carbon from the air, heterotroph organisms like mushrooms need carbon already fixed by other organisms. Like plants they have cell walls, but those contain chitine instead of cellulose. Mushrooms are part of the large kingdom of Fungi, together with yeast and moulds and other, mostly small and monocellular organisms.
You can find the Grey Knight (Tricholoma terreum) until the first frost around pine and fir trees, and they are edible (actually, they taste good!).
Here is part of a fairy ring of Grey Knights. A tree is in the centre of the ring, and around it underground the mycelium spreads, with the spore-carrying mushrooms, the flowers of the mycelium, above ground. In the circle a lot of plants cannot grow, because the mycelium spreads poisonous substances or covers the roots of other plants. At the outside of the circle, out of reach of the mycelium, grasses grow normally. When the mushrooms disappear, the ring will still be visible.a-600-8.jpg){kind=link}