The earthstar fungus has an outer layer that splits into rays and bends backward, lifting the spore sac off the ground. Some species sense humidity. The rays close when dry and open when wet, optimizing spore dispersal timing.
Richard Sullivan (enchplant), CC BY-SA 3.0, via Wikimedia Commons
A fungus that forecasts the weather
On the forest floor, among the leaf litter of deciduous and mixed woodlands, lives a peculiar organism that acts like a natural barometer. Known as the barometer earthstar (Astraeus hygrometricus), this fungus can predict weather changes, uncurling its star-like rays in high humidity and folding them back over its spore sac when the air is dry. This behavior is a curious trick; it is a strategy for survival.
The fungus's outer layer, or exoperidium, is composed of several tissues that absorb and release moisture at different rates. When humidity rises, typically before and during rain, the outer layers of the rays expand more than the inner layers. This differential expansion forces the 6 to 15 rays to bend backward and spread out, creating a star shape that can be 4 to 10 cm across. This action lifts the central spore sac, a papery sphere 1 to 3 cm in diameter, a few centimeters off the ground. Conversely, in dry conditions, the rays curl inward, protecting the delicate spore sac. The name Astraeus hygrometricus is a direct reference to this ability, combining Greek words for "star" (astron), "wet" (hygros), and "measure" (metron).
An engineered spore release
The hygroscopic movement of the barometer earthstar is an evolutionary solution for effective spore dispersal. By opening only when wet, the fungus ensures its spores are released during optimal conditions. Raindrops striking the exposed spore sac act like a bellows, puffing out clouds of microscopic brown spores through an irregular tear at the top. Each puff can release billions of spores.
Lifting the spore sac even a short distance above the ground debris significantly increases the chances of the spores catching air currents and traveling greater distances. This mechanism is a case of convergent evolution, as the barometer earthstar (Astraeus) is more closely related to bolete mushrooms, while the visually similar but non-hygroscopic "true earthstars" belong to the genus Geastrum, which are related to stinkhorns.
These fungi are ectomycorrhizal, forming a symbiotic relationship with the roots of trees like oak and pine. The fungus helps the tree absorb water and nutrients, particularly phosphorus, from the soil. In return, the tree provides the fungus with carbohydrates produced through photosynthesis. The persistent, leathery fruit bodies can last for several years on the forest floor, waiting for the right conditions to release their genetic legacy.
💡Fun Facts
The Blackfoot people of North America called earthstars "ka-ka-toos," meaning "fallen stars," and considered them indicators of supernatural events.
In traditional Chinese medicine, related earthstar species have been used to reduce inflammation and staunch bleeding.
Despite their star shape, barometer earthstars are not closely related to true earthstars (*Geastrum*); their similar appearance is an example of convergent evolution.
The tough, leathery texture of the barometer earthstar makes it inedible.
Accessible year-round. Fruiting bodies are most likely to appear after rain in late summer and autumn but can persist for years.
Admission
Free.
Accessibility
Found on the ground in mixed and deciduous forests, often in nutrient-poor or sandy soils. Terrain will be uneven and requires walking through woodland.
The Star-Shaped Weather-Sensing FungusLower Austria, Austria
