Birch Mazegill Identification Guide

First impression

The usual encounter is on a fallen birch log or a cut stump, at any time of year, in almost any kind of woodland where birch grows. A row of thin, semicircular brackets runs along the upper side of the log in overlapping tiers, each one 3 to 10 cm across but only a few millimetres thick. From above they look grey-white and softly hairy, with clear concentric bands, and older ones are stained green by a film of algae.

At this distance the bracket says Trametes to most people, and that is the trap. The whole identification turns on flipping one over. Choose a specimen you can detach cleanly, or crouch and use a mirror or your phone camera, because the underside of Lenzites betulinus is not made of pores at all.

Reading the upper surface

The cap is broadly semicircular to kidney-shaped, attached along a narrow line to the wood, with a thin, often wavy margin. It is flat or slightly undulating rather than domed, and it is thin enough that a fresh bracket flexes when you bend it.

Run a finger over the surface. It is densely tomentose, a fine dry velvet or short shaggy felt, and in fresh material the hairs are obvious under a hand lens. Concentric zoning is present but subdued: alternating bands of cream, buff, pale grey and grey-brown, with the outermost zone usually palest. The colours are muted rather than the bold ochre, blue-grey and black rings of Trametes versicolor. With age the whole upper surface fades, the felt mats down, and a green algal or cyanobacterial crust develops over most of it, sometimes so heavily that the zones vanish.

The spore-bearing surface, and the key character

Turn the bracket over. Instead of tubes you find true gill-like plates: thick, tough, radiating blades running from the point of attachment out to the margin. They are 1 to 3 mm deep, whitish to cream when fresh, dulling to pale buff or greyish, and spaced widely enough to count individually with the naked eye.

Study how they behave along their length. Near the margin they are mostly simple and parallel. Toward the attachment they fork repeatedly and cross-connect, producing short bridges between neighbouring plates and, in places, an irregular maze of elongated compartments. This is where the common name comes from: the surface is gill-like at the edge and maze-like at the base.

Crucially, the plates are tough and leathery, not soft. Press a fingernail across them and they resist and spring back rather than crumbling like the fragile gills of an agaric. Nothing bruises, discolours or exudes anything.

Attachment, base and flesh

There is no stipe. The bracket is sessile, joined to the wood along a narrow contracted base, and it can usually be peeled off intact with a knife. Look at where it joins: the attachment is often slightly thickened but never forms the swollen hump seen in Trametes gibbosa.

Cut a bracket vertically through the middle and look at the section under a lens. The flesh is white, thin, and corky to leathery, typically less than 2 mm thick, sitting as a distinct pale band between the hairy upper layer and the plates. Fresh material is pliable and can be bent double without snapping; dried material becomes hard, light and rigid. There is no colour change on cutting, no latex, and no darkening line separating the flesh from the tomentum, which is a point of contrast with several look-alikes.

Taking and reading a spore print

Brackets on a log spore downwards, so the practical approach is to detach a fresh, actively growing specimen and rest it plate-side down on black paper, propped level, under a cover to hold moisture. Leave it overnight. The print is white, sometimes so faint against pale paper that black card is worth using instead.

Be aware that weathered brackets, and any bracket that has already been through a dry spell, may drop nothing at all. If you get no deposit, look instead at the plate edges with a lens for a fine white bloom of released spores. Microscopically the spores are small, smooth, cylindrical to slightly curved and colourless, roughly 5 to 6 by 2 to 3 micrometres, and the flesh is built from thick-walled skeletal and binding hyphae as well as generative hyphae, which is what gives such a thin bracket its leathery toughness.

Substrate, season and ecology

The strong association is with birch: fallen trunks, standing dead stems, cut stumps and thick branches, especially in the middle and later stages of decay once the bark is loosening. It also occurs on beech, oak, alder, willow and other broadleaves, so the host narrows the field but does not settle it on its own.

Fruit bodies are annual but tough enough to persist through a winter, so you can find fresh cream-margined brackets and bleached, algae-covered old ones on the same log. There is no meaningful season; new growth flushes in warm damp spells from spring to late autumn.

Ecologically it is a white-rot fungus. It degrades lignin as well as cellulose, bleaching the birch wood to a pale, soft, fibrous mass. On a well-colonised log you can often crumble the surface wood between finger and thumb into white strings.

Separating it from look-alikes on morphology alone

Trametes versicolor shares the thin zoned bracket habit and often the same log, but its underside is finely pored, with tiny round pores you need a lens to resolve, and its upper surface is boldly multicoloured and often silky rather than uniformly velvety.

Trametes gibbosa is thicker, with a distinct hump above the attachment point, a white to greenish upper surface, and pores that are elongated radially into slots. Those slots can look maze-like, but they are pore mouths stretched sideways, not free-standing plates, and the bracket is far more substantial.

Daedaleopsis confragosa is thicker and reddish brown, with a smooth to wrinkled rather than velvety top, a maze-like to gill-like underside, and a pale surface that stains reddish brown where you scratch it. That bruising reaction alone separates it.

Daedalea quercina is a thick, corky, cream to tan bracket of oak, with a deep, coarse, genuinely labyrinthine underside and flesh many millimetres thick.

Gloeophyllum sepiarium has true gill-like plates too, but it grows on conifer wood, and both the plates and the flesh are rusty orange-brown rather than white.

Schizophyllum commune is smaller, white, densely woolly, and its gill-like folds are split lengthwise along their edges, curling back in dry weather.

Field checklist

Thin, flexible, velvety, faintly zoned grey-white bracket on birch; underside of tough whitish radiating plates that fork and anastomose into a maze near the base; white flesh under 2 mm; no bruising; white print. That is Lenzites betulinus.