Shelf fungi (polypores) are visible signs of extensive living activity in tree trunks. They are mostly found on standing or fallen dead trees. Shelf fungi help decompose dead woody material and their presence is an indication of the beginning of the end of a tree.
The undersides of shelf fungi contain microscopic structures called Basidia, which are spore producing cells through which millions of spores are released to grow filament mats. These filament mats are capable of covering entire forest floors.
In Washington, the fungal mat of honey mushroom covers an impressive 1,500 acres. Another in Oregon covers 2,200 acres; if you measure size by the total area over which it grows, this fungal mat is the largest living organism in the world.