Barometer Earthstar

Description

The barometer earthstar is a small fungus resembling a star lying on the ground. The young fruiting body is round and partly hidden in the soil, then its thick, leathery casing splits into several to a dozen or so rays. In damp weather the rays spread out to the sides (the whole fungus is then usually 3–6 cm across) and reveal a grey-brown, smooth sac with a slit at the top, from which brown spore dust escapes. In dry weather the rays curl up and cover the sac, hence the name "barometer" (in Polish, wilgociomierz, literally "hygrometer").

It grows in dry, sunny places: on sandy verges, at the edges of deciduous forests, in open pine forests and oak woods, singly or in groups, usually from July to September. Dried fruiting bodies can last as long as a year. In Poland it is very rare and under strict protection.

It can be told apart from earthstars by its thick, leathery exoperidium cracked into small areas, and by the lack of a distinct round rim around the opening. It has no stem or cap, so it should not be confused with mushrooms. As a protected species, it should not be collected.

Identification features

Cap

No cap: the fruiting body is gasteroid, globose or ovoid when young, partly sunk into the substrate and covered with a whitish layer of mycelium. The exoperidium splits in a star shape into 4–20 irregular rays; once spread open, the fruiting body is 1–8 cm across (usually 3–6 cm). The rays are thick and leathery, cracked into small areas on the upper side, grey-brown to dark grey or black; the inner layer is beige to light brown. In moist weather the rays spread open, and in dry weather they curl up and cover the spore sac (hygroscopic behaviour).

Hymenophore (underside of the cap)

No gills or tubes. The spore sac (endoperidium) is sessile, globose, 1–3 cm in diameter, light grey to brown, reticulate-fibrous in young fruiting bodies and smooth and papery in older ones; it opens by an irregular slit or a pore at the top with a ragged edge. The interior (gleba) is white and compact when young, becoming chocolate-brown and powdery with maturity.

Stem

No stem; dark, brittle rhizomorphs extend from the base of the fruiting body and often break off after maturity.

Flesh

The young gleba is white and compact, divided into oval chambers; in mature fruiting bodies it is a brown powder of spores mixed with capillitium. Colour change when damaged, smell and taste: no data. The flesh should not be tasted.

Spore print and spores

Spores brown to reddish-brown, globose or nearly globose, thick-walled, 7–11 µm in diameter (descriptions give different ranges; usually 8–10 µm), densely covered with warts and spines about 1 µm long. Non-amyloid.

Habitat and distribution

Ecology. A terrestrial ectomycorrhizal fungus living in association with many tree species, including oaks and pines. It prefers sunny, dry sites, nutrient-poor sandy or clay soils and acidic substrates; it avoids calcium-rich soils. The fungus Gelatinipulvinella astraeicola develops on the dead rays of the fruiting bodies.

Distribution. Very rare in Poland, an endangered species (category E on the Polish Red List of plants and fungi); by 2020, 12 historical, 12 contemporary and 15 doubtful sites had been reported. It grows on sandy roadsides, at the edges of deciduous forests, in light pine forests, oak woods and thermophilous scrub. Under strict protection in Poland (Dz.U. 2014 item 1408; partial protection in 1995–2004). It is also on red lists in, among others, the Czech Republic, Germany, England and the Netherlands. In the broad sense the genus occurs on all continents except Antarctica; under the narrowed species concept, its range is limited to Europe, from southern France to Turkey.

Fruiting. Usually July–September; dried fruiting bodies are durable and can persist for a year, or longer according to some descriptions.

What can it be confused with?

• Geastrum floriforme (Polish: gwiazdosz kwiatuszkowaty) — Also hygroscopic, but the exoperidium is thinner, without the thick, leathery layer cracked into small areas; the spores are much smaller.
• Geastrum corollinum (Polish: gwiazdosz brodawkowy) — Also hygroscopic; the spore sac has a distinct round peristome, the rays are thinner and the spores smaller.
• Geastrum mammosum — Has thick, brittle, moderately hygroscopic rays, but the spores are smaller (about 4 µm).
• Other earthstars (Geastrum) — In earthstars the opening of the sac is usually surrounded by a peristome or disc, and the fruiting body has a columella; Astraeus has a single, ragged slit and no columella. Older specimens can be difficult to tell apart in the field.

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 rays of the fruiting body open and close in response to air humidity, because their inner, fibrous layer absorbs and releases water. This lets the fungus disperse its spores under favourable conditions and limits evaporation in drought.
• Despite its similarity to earthstars, Astraeus is not closely related to them and belongs to the order Boletales, the same as the boletes.
• The German Mycological Society chose it as Fungus of the Year 2005.
• A specialised fungus, Gelatinipulvinella astraeicola, grows on the dead fruiting bodies and is known only from the inner surface of the rays of species in the genus Astraeus.