Birch Polypore Identification Guide

First impression

This is one of the easiest brackets to spot at distance because the host does the work. You are looking at a dead or dying birch, either standing as a bleached snag or lying across the woodland floor, and projecting from the white bark are one to several smooth, pale, rounded shelves that look almost artificial, like knobs or hooves stuck onto the trunk. They can be at any height, and a single snag may carry a dozen.

The silhouette is characteristic: a thick lens or kidney shape, 5 to 25 cm across and 2 to 6 cm deep, with a blunt, rounded, distinctly inrolled margin that curls under and overhangs the underside. Unlike most brackets, it does not taper to a thin edge. That fat, rolled rim is visible from several metres away and is worth training your eye on.

Reading the upper surface

Walk up and look at the top. It is smooth, dry and matt, without hairs, zones or lacquer, covered by a thin, tough skin that you can catch with a fingernail and peel away in a papery sheet from a fresh specimen. That separable pellicle is a good confirmatory test.

Colour is muted: whitish or pale grey-buff when young, ageing to pale ochre, tan or greyish brown, often darkest over the older centre. As brackets age and dry, the skin shrinks and develops a network of fine cracks, particularly near the attachment, and long-dead specimens bleach to a dirty grey-white. There is no bruising reaction on the upper surface.

The pore surface and the sterile rim

Crouch and look at the underside. It is white to cream when fresh, dulling to pale buff or dingy straw with age and browning slightly where handled. The pores are minute, roughly 3 to 5 per millimetre, round and even, and to the naked eye the whole surface reads as a smooth white sheet. Only with a hand lens do the individual mouths resolve.

Critically, the pore surface does not extend to the edge. The thick rolled margin is sterile, so there is a distinct pale band, several millimetres wide, between the outer rim and the start of the pores. Look for that gap deliberately: it separates this species from most flat-edged brackets at a glance.

Now test the tubes. Slice vertically and you will find the tube layer, 1.5 to 5 mm deep, forming a discrete stratum that peels away from the flesh above it as a single mat with very little effort. That clean separability is unusual and diagnostically handy.

Attachment, stipe and base

Most specimens attach by a short, thick, off-centre lateral stub, sometimes barely 1 cm long, which makes them look pinched at the back. Brackets forming on the underside of a fallen log can develop a stubby lateral stalk up to 3 or 4 cm. Trace the attachment back to the wood: it should be birch, and the bark is usually still present and still white.

Flesh and colour change

Cut a bracket in half. The flesh is pure white, thick, and has a texture unlike any other common bracket: soft-corky and slightly rubbery when fresh, so a knife passes through with a faint squeak and the cut face feels like dense foam rather than wood. It compresses under a thumb and slowly springs back.

There is no colour change worth recording. Cut and bruised surfaces stay white or dull very slowly to cream. The smell is pleasant and faintly resinous or mushroomy. With age and drying the flesh becomes light, hard and chalky, and the whole bracket loses most of its weight while keeping its shape, which is why old ones persist on snags for years.

Taking and reading a spore print

Detach a young, actively growing bracket, one whose pore surface is still bright white, and set it pore-side down on black paper under a cover overnight. The print is white, so black card is essential to see it.

Mature brackets often release very little, and specimens that have dried once may release nothing at all, so pick your material carefully. Under a microscope the spores are the giveaway: small, smooth, colourless and allantoid, meaning sausage-shaped with a curved back, roughly 5 to 6 by 1.5 micrometres. Cylindrical or ellipsoid spores mean you are looking at something else.

Substrate, season and ecology

The host association is close to absolute. This species fruits on birch and, for practical field purposes, on nothing else. It appears on standing dead stems, recently killed trees, and fallen trunks, and it will fruit on a tree that is still partly alive but failing.

Fruit bodies are annual, initiating in summer and expanding through autumn, but they are tough enough to persist through one or more winters, so you can find soft white new brackets and hard grey old ones on the same snag in the same hour.

Ecologically it causes a brown rot. It removes cellulose and leaves the lignin behind, so decayed birch wood turns reddish brown, shrinks, and cracks into cubes that crumble under a boot. Because brown rot destroys the fibres that give wood its tensile strength, heavily colonised birches typically snap partway up the trunk rather than uprooting, and a broken-off birch stump with cubed reddish wood inside is a strong prompt to look for brackets nearby.

Separating it from look-alikes on morphology alone

Fomes fomentarius often shares the same birch snag and is the main confusion. It is a hard, hoof-shaped, grey to grey-brown perennial with a thick crust, broadly attached over a wide base, with stacked annual tube layers visible in section and brown, not white, flesh. It has no rolled sterile rim and no peelable skin.

Fomitopsis pinicola is similarly thick but has a lacquered crust with a bright red to orange marginal band and grows on conifers and occasionally hardwoods, with a resinous surface that melts to a bead under a flame.

Ganoderma applanatum is flatter, with a hard brown crusty top and a white pore surface that bruises instantly dark brown when scratched, plus a rusty brown spore deposit that dusts everything below it.

Trametes gibbosa is thin and flexible with a hairy, often algae-greened top and radially elongated slot-like pores.

Laetiporus is bright orange and yellow, and Trametes versicolor is thin, zoned and velvety. None of these has the combination of a smooth peelable skin, a fat sterile inrolled rim, minute white pores and rubbery white flesh.

Field checklist

On birch, kidney or hoof-shaped, smooth pale top with peelable skin, thick rolled sterile margin, minute white pores in a separable tube layer, rubbery white unchanging flesh, white print of sausage-shaped spores. That is Fomitopsis betulina.