R. Fraley

Physcia stellaris

Physcia stellaris

Family: Physciaceae

Star Rosette Lichen (Physcia stellaris) is a compact, distinctive foliose lichen commonly forming gray rosettes marked by characteristic white-rimmed apothecia. These pale, elevated margins sharply contrast with darker disc centers, creating a subtle starburst pattern that makes P. stellaris one of the most easily recognized bark-dwelling Physcia species. It thrives in areas with clean to moderately clean air and frequently co-occurs with related foliose lichens.

For representative images and visual context, see the Gallery page: Star Rosette Lichen.

Description

Physcia stellaris is recognized by:

  • Growth form: Foliose, forming compact, evenly rounded rosettes several centimeters across
  • Thallus: Pale gray to blue-gray; sometimes with a faint frost-like bloom
  • Lobes: Small (0.5–1.5 mm), rounded, overlapping, slightly pruinose; edges smooth or gently upturned
  • Surface features: Lobes often appear dusted or slightly powdered; lecanorine apothecia abundant and highly conspicuous
  • Apothecia: White-margined (pale rims), highly abundant; disc centers gray to dark brown; the most diagnostic feature
  • Underside: Pale to light brown, with short simple rhizines
  • Attachment: Firmly appressed via rhizines

The species is characterized by evenly distributed small lobes arranged in overlapping rosette patterns and the abundance of lecanorine apothecia with distinctive pale rims. These white-margined discs are often the first field mark noticed, appearing as tiny white rings before the rosette structure itself becomes apparent.

Substrate and Habitat

Typical substrates and settings include:

  • Hardwood bark (elm, maple, ash, oak)
  • Twigs and small branches
  • Weathered fence posts and rails
  • Occasionally on rock with bark lichen communities

Physcia stellaris prefers open or partially shaded habitats with good air circulation. It is commonly associated with moderately clean atmospheric conditions and often co-occurs with:

Identification

Key features for field diagnosis:

  • Color: Pale gray to blue-gray, often with faint frost-like surface bloom
  • Form: Tidy, rounded foliose rosettes; lobes regular and evenly overlapping
  • Surface: Slightly pruinose (frosted appearance); lacking abundant soredia
  • Apothecia: Abundant, lecanorine, with pale or white rims—the diagnostic feature
  • Underside: Pale to light brown; short, simple rhizines visible at attachment points
  • Lobe size: Small (0.5–1.5 mm) and regular
  • Chemistry: Typically K–, C–, KC–, P–

Distinguishing from Similar Species

vs. Physcia aipolia (Hooded Rosette)

  • P. aipolia has more contrasting dark spotting and dark-rimmed apothecia (not pale-rimmed)
  • P. aipolia lobes often appear more irregular with finer maculae
  • Star Rosette: White-rimmed apothecia, more regular rosette form, lighter overall appearance

vs. Physcia millegrana (Powdery Shield)

  • P. millegrana has much smaller lobes (0.2–1.0 mm) and uniformly powder-dusted surface
  • P. millegrana lacks crisp apothecial architecture; apothecia are rare
  • Star Rosette: Larger lobes, crisp white-rimmed apothecia, less uniformly dusted surface

vs. Phaeophyscia orbicularis (Mealy Shadow)

  • Phaeophyscia is notably darker with granular, more roughened margins
  • Phaeophyscia lacks white-rimmed apothecia; apothecia have dark rims
  • Phaeophyscia thallus appears soot-toned rather than pale gray
  • Star Rosette: Distinctly paler, with characteristic white-rimmed apothecia

vs. Physcia caesia

  • P. caesia is more uniformly blue-gray with frosted apothecia when present
  • P. caesia rosettes appear cleaner and more coherent
  • Both species can have pale rims, but P. stellaris rims are typically whiter and more prominent

Ecology and Notes

Indicator Value:
Physcia stellaris favors clean to moderately clean air and is sensitive to pollution. It indicates relatively low levels of nitrogen and sulfur deposition, making it useful as an air-quality indicator. Its presence suggests habitats with good atmospheric circulation.

Distribution in Minnesota:
Widely distributed throughout Minnesota hardwood regions. Commonly encountered on open-grown hardwoods and in lightly disturbed settings. Often forms extensive colonies on elm and maple where conditions favor growth.

Reproductive Strategy:
Physcia stellaris reproduces primarily via lecanorine apothecia, which produce ascospores. Vegetative reproduction may occur through fragmentation, but sexual reproduction is the dominant strategy, distinguishing it from soredia-dependent relatives like P. millegrana.

Community Context:
Star Rosette’s distinctive white-rimmed apothecia often stand out in mixed lichen communities, making it a conspicuous and readily identified species. It frequently appears alongside brighter species like Candelaria concolor (Candleflame), whose yellow patches provide warm visual contrast to the cool gray rosettes. This pairing creates characteristic multi-colored bark mosaics.

Seasonal Observations:
The apothecia are visible year-round and become especially prominent during periods of good moisture, when the white rims can be seen clearly even from a distance.

References