Star Rosette Lichen | Physcia stellaris Identification Guide

First impression: grey stars on bark

You are standing under a roadside ash, an old orchard apple, a hedgerow maple or a streetside poplar, looking at a well-lit branch rather than a shaded trunk. On the bark are neat, flattened, pale grey rosettes 1 to 3 cm across, radiating outward from a centre like a starburst, sometimes separate and tidy, sometimes merging into a patchwork of grey where several have collided. In the middle of the older rosettes are scattered black discs. That combination is the whole first impression, and it is enough to place you in the genus. The remaining work is separating this species from three or four extremely similar grey rosettes.

Step 1: growth form and lobes

This is a foliose lichen, meaning it has a leaf-like thallus with a distinct upper and lower surface, but it lies closely appressed to the bark rather than standing up. Put a lens on the outer edge. The lobes are narrow, roughly 0.5 to 1.5 mm wide, flat or very slightly convex, elongate, and they radiate outward while staying discrete and separate rather than piling over one another into a jumble. Lobe tips are rounded, often shallowly notched, and lie flat on the bark. The upper surface is smooth, pale grey to bluish or whitish grey, dull or only slightly shiny, and generally without conspicuous white spots, though very faint mottling can occur.

Step 2: the decisive absence

Run the lens slowly along the lobe tips and across the upper surface. What you are checking for is the absence of vegetative propagules. Physcia stellaris produces no soralia, no powdery or granular soredia and no cylindrical isidia anywhere on the thallus. Lobe tips stay entire; they do not become hooded, do not turn up at the ends and do not burst open into a powdery mass. Because several very similar grey Physcia species are defined by exactly those structures, confirming their absence removes most of the genus in one observation.

Step 3: the apothecia

Fertile material is common and often abundant, especially in the centre of the rosette. Each apothecium is a disc 0.5 to 2 mm across, black to very dark brown, flat to slightly concave, sometimes lightly dusted with a grey-white pruina. Around it runs a raised thalline margin the same grey as the thallus, and that margin stays smooth and entire rather than becoming crenulate or breaking into soredia. Under a microscope the ascospores are brown, two-celled, thick-walled with a characteristic thickening at the septum, roughly 16 to 23 by 7 to 10 micrometres, and eight per ascus.

Step 4: the underside and rhizines

Slide a knife tip under a lobe and lift it. The lower surface is white to pale buff, never black, and carries scattered simple rhizines that are pale to greyish and do not project beyond the lobe margins as visible hairs. A dark lower surface with dark rhizines takes you into a different genus entirely, so this single flip is worth doing on every collection.

Step 5: chemistry instead of a spore print

A lichen offers no spore print in the mushroom sense, and there is no practical field equivalent. Its place in the workflow is taken by spot tests, which are quick and decisive here. Apply a pinpoint of potassium hydroxide solution to the upper surface: the cortex goes yellow, reflecting atranorin. Then make a tiny window by scraping the cortex and algal layer away with a blade to expose the white medulla, and apply KOH there. In Physcia stellaris the medulla does not react. That single negative test is the cleanest separation from its nearest twin, which has a medulla that turns yellow. Bleach and para-phenylenediamine give no useful positive reaction on this species.

Step 6: substrate and setting

The habitat preference is specific enough to be evidence. Look on bark of deciduous trees with a slightly raised pH or some dust and nutrient enrichment: ash, maple, elm, poplar, willow, hedgerow shrubs and orchard trees, particularly along roads, field boundaries and in open parkland. Twigs and small branches in good light are more productive than shaded trunks, because this is a light-demanding species of airy, exposed situations and it tolerates a moderate degree of nitrogen enrichment. It occurs occasionally on nutrient-enriched stone and concrete. Being a lichen, it is present all year; the practical advice is to examine it damp, when the thallus swells, colours deepen slightly and the lobe structure is easiest to read.

Separating it from look-alikes by structure and chemistry

Physcia aipolia. The closest match and the reason the medulla test exists. It is typically more strongly white-maculate, so the upper surface looks finely speckled with white dots under a lens, and its medulla reacts yellow with KOH where P. stellaris does not.

Physcia tenella and P. adscendens. Both have narrower lobes that ascend at the tips and produce soralia, hood-shaped in one and lip-shaped in the other, and both carry long marginal cilia along the lobe edges. Apothecia are uncommon. The presence of soredia and cilia settles it.

Physcia caesia. More blue-grey, with rounded soralia scattered across the upper surface, and it is characteristically a lichen of nutrient-rich rock and concrete rather than twigs.

Phaeophyscia orbicularis. Darker olive-grey to brownish thallus, a dark lower surface with dark rhizines, and no yellow cortex reaction with KOH.

Physconia grisea. Brownish-grey with a pruinose, dusty upper surface, sorediate margins, and squarrose (branched) rhizines beneath.

Hyperphyscia adglutinata. Much smaller and extremely tightly glued to the bark, with tiny lobes and essentially no rhizines.

Xanthoria species. Superficially similar rosette form but bright yellow to orange, with a cortex that turns purple with KOH.

Field sequence

1. Grey appressed rosette on a well-lit deciduous twig or branch.
2. Lobes narrow, flat, discrete, tips rounded and entire.
3. No soredia, no isidia, no cilia anywhere.
4. Black apothecia with a smooth grey thalline rim, often abundant.
5. Lower surface pale with pale simple rhizines.
6. Cortex KOH yellow, medulla KOH negative.