How hard can it be? A monster like a pie crust on an old beech or goat willow must be easy to track down, mustn’t it? But no, its a surprise to find out how many brackets there are, from several different families, and only having a picture taken using a mobile phone LED doesn’t help.
Browsing round the web for similar pictures and comparing the underside (see below) with pore pattern descriptions, maybe its a Lumpy Bracket – Trametes gibbosa. This picture from ‘One with Nature’ looks pretty close but we obviously have to go and have another look in daylight. The Lumpy Bracket is renowned for going green – older specimens seem to collect algal growth, as shown here – so that’s something to check.
Whether its the Lumpy or not, it seems certain to be one of the saprophytic polypores. Sapropyhtic means it usually lives on dead wood (many mushrooms fruit on the ground as mycorrhizal fungi, having beneficial relationship with the roots of trees and other plants. Polyphore means it has round or elongated pores rather than gills underneath the fruiting body which increase the surface area for bearing spores.
The renowned Tom Volk has published a Polypore primer. He explains that most polypores are wood decay fungi. Wood is composed mostly of two substances: cellulose (white) and lignin (brown). Cellulose forms the primary wall of all plant cells. Many plants add a second wall of lignin inside the primary wall, especially in wood. Brown rot fungi can degrade only the white cellulose and leave the brown lignin behind. In their simplest form, white rot fungi degrade the lignin and leave the white cellulose behind. The Lumpy Bracket is a white rot fungus.