Star Rosette Lichen

Star Rosette Lichen makes tidy, tightly appressed grey rosettes on twigs and trunks, its narrow lobes radiating outward like the rays of a star. Mature colonies are typically studded across the centre with dark brown to black button-like fruiting bodies, often with a pale frosted bloom, giving the lichen a distinctly speckled look from a metre away.

The genus Physcia, in the family Physciaceae, contains many small grey rosette-formers that make up a large part of the lichen flora on well-lit bark. Species in the group are separated largely by whether they produce soredia, whether the upper surface is white-spotted, whether there are marginal cilia, and by spot-test chemistry of the medulla.

P. stellaris sits at the fertile, non-sorediate end of that range: it reproduces mainly by spores from abundant apothecia rather than by powdery propagules, so its surface stays smooth and unbroken.

Thallus

A flat, closely attached rosette 1–4 cm across, sometimes coalescing into larger patches. Lobes are narrow (about 0.5–1.5 mm wide), elongated, radiating and often slightly separated from one another so the star outline is clear. Lobe tips are rounded to slightly notched.

Upper surface

Pale to medium grey, sometimes with a bluish or greenish cast, and normally smooth and even. It lacks soredia and isidia entirely — an unbroken surface is an important character. Distinct white spots or flecks (maculae) are absent or very faint.

Lower surface

White to pale grey with sparse pale to darkening rhizines that hold the lobes tight to the bark.

Apothecia and chemistry

Usually abundant in the centre of the rosette: sessile discs 0.5–2 mm across, dark brown to black, frequently coated in a grey-white pruina, and ringed by a thin grey thalline rim. The medulla is white and does not turn yellow with potassium hydroxide, an important separation from the otherwise similar Physcia aipolia.

This is a bark lichen of open, well-lit situations, most often on the twigs, branches and small trunks of deciduous trees and shrubs — maple, ash, elm, poplar, willow, apple, hawthorn and other broadleaves. It also occurs on planted conifer bark, fence rails and occasionally on nutrient-enriched stone or concrete.

It favours nutrient-rich bark in hedgerows, orchards, gardens, parks, roadsides and open woodland edges, and it is reasonably tolerant of moderate air pollution and dust. The species is widespread across temperate North America, Europe and much of the northern hemisphere, and being perennial it can be recorded in any month.

The fungus forms the rosette and shelters a green algal photobiont in a layer beneath the upper cortex. As an epiphyte it takes nothing from the tree, using bark only as an anchorage while drawing water and mineral nutrients directly from rainfall, mist and dust settling on the branch.

Reproduction is chiefly sexual: the abundant apothecia release two-celled brown ascospores that must meet a compatible alga to establish. This spore-based strategy is common among lichens of twigs and small branches, where short-lived, frequently renewed substrate rewards rapid colonisation from the air. Populations respond strongly to nutrient enrichment, and the species is often abundant along roads and in farmland.

Physcia aipolia is the closest match and shares the grey rosette with black apothecia, but its upper surface is peppered with distinct white spots (maculae) and its medulla turns yellow when potassium hydroxide is applied, whereas P. stellaris is unmarked and K-negative in the medulla.

Physcia adscendens and P. tenella differ by having inflated, hood-shaped or helmet-like lobe tips that split open into soredia, plus fine dark cilia projecting from the lobe margins; both rarely fruit. Physcia millegrana has small, crowded, wrinkled lobes with soredia breaking out of the surface and margins. Phaeophyscia species look similar in outline but are darker olive- to grey-brown and have a black lower surface with dark rhizines rather than a white underside.