Barometer Earthstar

The Barometer Earthstar is one of the most recognisable gasteroid fungi of dry, sandy ground. It begins as a rounded, half-buried body that splits from the top into a star of thick, leathery rays, exposing a papery grey spore sac at the centre. The common name refers to the strongly hygroscopic rays: they unfold and flatten when wet and roll back tightly over the spore sac as the air dries, a cycle the same fruiting body may repeat many times over months.

Despite its earthstar appearance, Astraeus is not closely related to the true earthstars in Geastrum. Molecular work places it in the order Boletales, alongside boletes and their allies, in the small family Diplocystaceae. The resemblance is a case of convergent form: both groups solved the problem of protecting and then exposing a spore mass with a splitting outer wall.

What makes it distinctive in the field is the combination of toughness and texture. Old specimens dry to hard, woody stars that blow about the ground, and the upper surface of the rays typically breaks into a mosaic of small cracked plates, quite unlike the smooth or fleshy rays of most Geastrum species.

Fruiting body

Starts as a rounded to slightly flattened ball 1–3 cm across, often half-buried in sand or soil with grit and debris adhering to the surface. At maturity the outer wall splits into roughly 6–15 thick, tapering rays that expand to give a star 3–8 cm across.

Rays

The rays are leathery to almost woody rather than fleshy. Their upper (inner) surface is pale buff to greyish and characteristically breaks up into a network of small cracked scales or plates, giving a crazed, mosaic look. The lower surface is darker brown and often felted with sand. Movement in response to moisture is diagnostic: wet rays lie flat or curl backwards, dry rays fold up over the spore sac.

Spore sac

The central sac is sessile (no stalk or collar beneath it), 1–2 cm wide, greyish to greyish brown, with a finely felted or scurfy surface. It opens by an irregular tear or ragged slit at the apex rather than by a neat, well-defined mouth. There is no distinct raised, grooved or fringed peristome such as many true earthstars show.

Flesh & spores

When young the interior is firm and whitish, becoming a powdery cocoa-brown spore mass threaded with fine capillitium as it matures. Spores are large, globose and conspicuously warted or spiny under the microscope, and the mass released is dark brown.

Astraeus hygrometricus favours dry, nutrient-poor, well-drained ground: sandy banks, gravel, heath edges, roadside cuttings, thin soil over rock and open sandy woodland. It is very often found with oaks and pines, and also occurs with other broadleaf and coniferous trees on impoverished soils. Fruiting bodies frequently sit partly buried, so the star may appear to emerge directly from bare sand.

The species, or a complex of closely related species now recognised under several names, is widely distributed across temperate and subtropical regions of Europe, Asia, Africa, Australia and the Americas. Fresh specimens appear mainly from late summer through autumn, but because the old fruiting bodies are so durable, opened stars can be found at any time of year and often persist through the following spring.

Astraeus is ectomycorrhizal, forming sheathing root partnerships with trees, most commonly oaks and pines. The fungus wraps fine root tips in a mantle of hyphae and grows between root cells, drawing sugars from the tree and in return greatly extending the root system's reach for water and mineral nutrients — an important service on the dry, impoverished sands where this species thrives.

Spore release relies on weather rather than wind alone. Once the sac wall thins and tears, falling raindrops striking the papery membrane puff out clouds of dark spores, while the hygroscopic rays repeatedly reposition and re-expose the sac as conditions change. Because the tissues are so tough, one fruiting body can keep releasing spores over many months.

True earthstars in the genus Geastrum are the main confusion. Most Geastrum species have thinner, fleshier rays that do not open and close with humidity, a smooth (not crazed and cracked) inner ray surface, and — critically — a well-defined mouth on the spore sac, often raised into a cone, ringed by a pale disc, or finely grooved and fringed. Geastrum triplex, for example, has a fleshy collar of tissue around the base of the spore sac that Astraeus never shows.

Geastrum fornicatum arches its rays to lift the sac clear of the ground on tips, a posture unlike the flat star of Astraeus. Unopened specimens can also be mistaken for small earthballs such as Scleroderma species; those stay closed and break open irregularly, having a thick, warty, yellowish rind and, when cut young, a solid purple-black interior rather than a wall that splits into rays.