Admirable Bolete

Aureoboletus mirabilis, described originally as Boletus mirabilis, is one of the signature fungi of the temperate rainforests of the Pacific Northwest. It is admired for its colour combination: a dark maroon to reddish-brown velvety cap above a pore surface of clear bright yellow, carried on a robust reddish-brown stipe that often flares outward at the base.

What makes it unusual among boletes is where it grows. Most members of the family fruit from soil in association with tree roots, but A. mirabilis appears directly on well-rotted conifer logs and stumps, especially western hemlock, or on the ground immediately beside them where buried wood lies below. This habit long puzzled mycologists, since the genus is otherwise root-associated.

It has been transferred to Aureoboletus, a genus of mostly golden-pored boletes, on molecular evidence, and it remains one of the most distinctive and easily recognised boletes on the west coast of North America.

Cap

The cap is 5-15 cm across, convex to broadly convex, and coloured dark reddish-brown, maroon, wine-brown or chocolate. The surface is dry and distinctly velvety or tomentose, and often develops a scurfy, scaly or slightly shaggy texture, particularly near the margin, where fine flaps of tissue may overhang. The cap margin is frequently sterile and slightly extended.

Pores

The pore surface is bright yellow when young, deepening to golden and finally to olive-yellow or brownish-yellow in age. The pores are relatively large and angular, and the tube layer is thick. The surface may stain slightly brownish where handled but does not turn blue.

Stipe

The stipe is 6-15 cm tall and 1-3 cm thick, solid, and characteristically enlarged downward into a swollen or flaring base, sometimes markedly so. It is reddish-brown to maroon, usually matching or slightly paler than the cap, often paler and yellowish at the extreme apex, and may show fine reticulation on the upper part in some specimens.

Flesh & spore print

The flesh is whitish to pale yellow, sometimes pinkish in the stipe base, thick and firm, and does not turn blue when cut, at most darkening slightly. The spore print is olive-brown; the spores are notably long and spindle-shaped.

This bolete is a Pacific Northwest species, ranging from northern California through Oregon and Washington into British Columbia and southern Alaska, with records from parts of the northern Rockies and reports of related material in eastern Asia. It is a fungus of moist coastal and montane conifer forest.

It fruits on and around decaying conifer wood, above all western hemlock, and also on Sitka spruce and Douglas fir debris. Fruit bodies emerge directly from the surface of well-rotted mossy logs and stumps, from woody debris buried in the forest floor, and from soil close beside such material. The season runs from late summer into late autumn, peaking with the first sustained rains of the wet season.

The ecological role of Aureoboletus mirabilis is unusual and has been much discussed. Although it fruits from rotting wood in a manner that suggests decay, the species is understood to be ectomycorrhizal with western hemlock, whose roots commonly grow up into and through decaying logs in these forests. The fungus therefore associates with living roots that happen to be located inside a rotting log, giving the appearance of a wood-rotting habit.

This makes it a good illustration of how nurse logs function in Pacific Northwest rainforest. Fallen trunks become the germination and rooting medium for the next generation of hemlocks, and the mycorrhizal fungi of those seedlings fruit from the same decaying wood. The long spindle-shaped spores are released from the wide angular pores and dispersed by air currents through the humid understorey.

Aureoboletus flaviporus shares the bright yellow pores but has a sticky, slimy cap surface rather than a dry velvety one, and lacks the flaring stipe base. Boletus edulis in western forests has a paler tan to chestnut cap, whitish pores that only slowly yellow, and a pale reticulate stipe, and it fruits from soil rather than from wood.

Xerocomellus species have smaller, cracking caps with reddish flesh in the fissures and flesh that often blues. Leccinum species in the same forests are separated by scabrous dark tufts on a pale stipe and by flesh that darkens to grey or black rather than staying pale.