Polyporus umbellatus

Scientific name:   Polyporus umbellatus (Pers.) Fr.
Derivation of name:   Umbellatus means "with
umbrellas."
Synonymy:   Grifola umbellata (Pers.) Pilat;
Dendropolyporus umbellatus (Pers.) Julich
Common names:   Umbrella polypore.
Phylum:   Basidiomycota
Order:   Polyporales
Family:   Polyporaceae
Occurrence on wood substrate:  Parasitic and saprobic
on roots of hardwoods, buried wood; appearing as
compound clusters of numerous, circular caps - fruiting on
the ground, near hardwood stumps, and trees;
May through October.  
Dimensions:  Overall size up to 50 cm wide; individual
caps 1-4 cm wide; basal stalk 2.5-7.5 cm long and
2-3 cm thick.   
Upper surface: Caps are whitish to ochre to pale brown;
depressed in the center; dry, fibrous to smooth.
Pore surface: Whitish, becoming yellowish; decurrent;
pores 1-3 per mm.
Edibility: Edible.
Comments: The thick basal stalk arises from a blackish,
buried sclerotium (tuber-like structure). The stalk becomes
richly branched above ground such that each cap is
centrally stalked. Fruitbodies may appear over many years
in the same location.

More information at MushroomExpert.com:

Figure 1. Polyporus umbellatus as it might appear on the
forest floor. Photo © Steve Nelsen.    

Figure 2. A young specimen of umbrella polypore. The
white stalks are easily visible between the caps which are
not fully expanded. Photo © John Plischke III.

Figure 3. A specimen with caps lighter in color than the
specimens in Figures 1 and 2. Photo © Chris Snyder.

Figure 4. Although Polyporus umbellatus appears
terrestrial, it is associated with wood - usually attached to
the roots of a tree. Photo © John Plischke III.

Figure 5. Each cap is centrally stipitate. Photo © Steve
Nelsen. 

Figure 6. Caps are depressed in the center. The white
pores are decurrent on the stalks. Photo © John Plischke III.