Alder Bolete

The Alder Bolete is a European species with an exceptionally narrow habitat: wet, silty ground beneath alder, in carr woodland, along stream banks and around pond margins. Where alder is absent it does not occur, which makes habitat almost a diagnostic character in its own right.

Structurally it is one of the boletes that blurs the boundary with the gilled fungi. Its tube layer is very shallow and runs conspicuously down the stipe, and the pores are wide, angular and irregular, giving the underside a honeycombed or partly veined look rather than the fine sponge of a typical bolete.

It is the type species of Gyrodon, a small genus related to Boletinellus in the Boletinellaceae. Its softness, pallor and dramatic blue-black bruising reaction make it memorable once seen.

Cap

The cap is 4-12 cm across, convex then flattening and often becoming shallowly depressed, with an irregular, wavy and sometimes lobed outline. The surface is smooth to slightly velvety, faintly tacky when wet, and coloured pale buff, ochre-yellow, olive-buff or dull yellow-brown, sometimes with rusty patches in age.

Pores

The pore layer is very shallow, only 1-3 mm deep, and is strongly decurrent, running well down the stipe. Pores are wide, angular and irregular, and coloured pale yellow to olive-yellow. The whole surface bruises blue-black almost immediately on the lightest touch, quickly followed by brownish discolouration.

Stipe

The stipe is short and often eccentric, 3-6 cm long and 0.5-2 cm thick, tapering downward and frequently curved where fruitbodies grow in tight clusters. It is yellowish to brownish, sometimes reddish-brown at the base, smooth and without a ring or a network.

Flesh & spore print

The flesh is soft, pale yellow, and turns blue then brownish where cut. Compared with most boletes the fruitbody is notably flabby rather than firm. The spore print is olive to yellowish-brown.

Gyrodon lividus is confined to damp and seasonally flooded ground beneath alder: alder carr, stream and ditch banks, wet flushes, pond and lake margins, and damp scrub. It grows on wet mineral or silty soil, commonly among nettles and marsh vegetation, and typically appears in clustered troops or arcs, often with several fruitbodies fused at the base.

The season runs from late summer into autumn, from August to October in most of its range. It is widespread but local in Europe, becoming scarce where alder carr has been drained, and is reported from parts of temperate Asia.

This fungus is ectomycorrhizal with alder, forming root sheaths on Alnus glutinosa and related species. Alder is itself a triple partner, hosting nitrogen-fixing Frankia bacteria in root nodules as well as mycorrhizal fungi, so the Alder Bolete operates inside an unusually nutrient-active root zone and is thought to help supply phosphorus in the waterlogged, often anoxic soils where alder thrives.

Its strict host specificity means it functions as a good indicator of intact alder carr habitat. Where wetland drainage removes alder from a site, the fungus disappears with it, and the loose, quickly decaying fruitbodies recycle rapidly into the wet litter.

The Ash-tree Bolete (Boletinellus merulioides) has a comparably shallow, decurrent, blue-bruising pore layer, but grows only near ash, has a darker yellow-brown to reddish-brown kidney-shaped cap, and a more distinctly merulioid, vein-like underside. Suillus species can look superficially similar in colour and can have decurrent angular pores, but they associate with conifers, usually have sticky or slimy caps, and many carry a veil or ring.

The Brown Roll-rim (Paxillus involutus) shares alder and birch habitat and a strongly inrolled margin with brown-staining flesh, but has true crowded decurrent gills that peel readily from the cap, not pores. Small brown Xerocomellus species have firmer flesh, deeper tubes and adnate rather than strongly decurrent pore layers.