Bracket fungi always emerge from tree branches and trunks. Many look very similar, however, and it can be difficult identifying them. One of the first steps to idenfication in this case is to look at what sort of tree it is on. You can actually find the alder bracket (Inonotus radiatus) on various dead broadleaved tree trunks whilst the trunks are still standing; however, as the common name would suggest, you are especially likely to find it on alder trees.
The fungus not only produces these bracket fruiting bodies but also appears on the surface as a white flakey rot. Alder bracket can be active in living trees but its presence will surely kill the tree eventually at which point it starts to rot down and decay the timber and the white rot is a sign this is happeneing.
This is a common and widespread species but not edible unless you like old boots!