Birch polypore

Description

The birch polypore is a hemispherical or cushion-shaped mushroom 10–30 cm across, growing sideways out of a birch trunk, narrowed at the point of attachment. The top is greyish-white to brownish, covered with a thin, cracking skin; the underside is cream-white, with very fine pores. The flesh is white, compact and bast-like, corky when dried.

It grows exclusively on birches, living and dead, in forests, parks and by roads. Young fruiting bodies grow from August to October, and dead specimens can be seen all year round. It is common in Poland.

It is an inedible fungus, with an astringent, slightly bitter taste. It is known for traditional uses, including stopping bleeding.

Identification features

Cap

Fruiting body annual (may survive more than a year), diameter 10–30 cm, attached sideways to the trunk and narrowed at the point of attachment; shape hemispherical or cushion-like. Top greyish-white to brownish or grey-brown, covered with a matt, cracking and peeling skin. Texture rubbery, corky with age.

Hymenophore (underside of the cap)

Tubes 1.5–5 (exceptionally up to 8) mm long, white in young ones, straw-yellow in older ones; pores round, underside cream-white. The tubes of mature fruiting bodies separate easily from the flesh of the cap.

Stem

No stem; the fruiting body is attached sideways.

Flesh

White, uniformly compact in section, not fibrous, bast-like (tough and stringy); corky when dried. Taste astringent, slightly bitterish. Wood decayed by the fungus and its cultures smell of green apple. Never taste the flesh of unknown mushrooms.

Spore print and spores

Spore print white, non-amyloid. Spores cylindrical, sausage-shaped or ellipsoid, smooth, usually with two guttules, 3–7 × 1.5–2 µm (descriptions give different ranges).

Habitat and distribution

Ecology. A necrotrophic parasite of weakened birches and a saprotroph; grows exclusively on birches (including Betula pendula, B. pubescens, B. papyrifera), living and dead. Causes brown rot of wood, which eventually falls apart into powder. Infection through wounds; the fungus may remain dormant for a long time until the tree is weakened (drought, shading, fire). Hypocrea pulvinata is parasitic on the fruiting bodies. Many animal species are associated with the fruiting body: in Canada 257 species of arthropods were recorded.

Distribution. The whole Northern Hemisphere (North America, Europe, Asia). Common everywhere in Poland; in forests, parks, by roads, also on birch timber (posts, fences). No legal protection in Poland.

Fruiting. Young fruiting bodies from August to October; dead specimens visible all year round.

What can it be confused with?

• Other tree fungi — No confused species indicated; identification is made easier by occurrence exclusively on birches, the white, uniform, bast-like flesh and the skin peeling from the top of the fruiting body.

Never rely on a single feature. If you are not sure — do not pick it; show the mushroom to a qualified mushroom expert.

Interesting facts

• The velvety surface of the cut fruiting body was formerly used for sharpening razors (hence the English name razor strop) and as a backing in insect collections.
• The fruiting body was found with the remains of Ötzi, the Iceman from about 5300 years ago.
• The Polish name has changed: białoporek brzozowy (1999), since 2021 pniarek brzozowy, after the transfer to the genus Fomitopsis in 2016.
• In a study in Canada, 257 species of arthropods, including 172 species of insects, were isolated from 2600 fruiting bodies.