R. Fraley

Physcia millegrana

Physcia millegrana

Family: Physciaceae

Powdery Shield Lichen (Physcia millegrana) is one of Minnesota’s most common small foliose lichens, recognized by its soft, powder-dusted surface and compact growth form. Its fine soredia give the thallus a matte, frosted look that distinguishes it from darker, more textured Physcia relatives. The species thrives in areas with clean, well-circulated air and often grows in mixed lichen communities on hardwood bark.

For representative images and visual context, see the Gallery page: Powdery Shield Lichen.

Description

Physcia millegrana is recognized by:

  • Growth form: Foliose, forming small, loosely organized rosettes or diffuse patches
  • Thallus: Pale gray, blue-gray, or slightly olive; overall matte appearance
  • Lobes: Extremely small (0.2–1.0 mm), rounded, irregularly branched, tightly overlapping; form dense, overlapping mats
  • Surface features: Abundant powdery soredia across the upper surface and margins, creating a frosted or dusted appearance visible at modest viewing distances
  • Underside: Pale, with short, simple rhizines; edges often slightly uplifted
  • Attachment: Tightly appressed to substrate via numerous short rhizines

The species is characterized by its fine, granular surface texture and the prevalence of soredia as the primary reproductive structure. Apothecia are rare to absent. The combination of tiny, densely overlapping lobes and uniform powder-coating distinguishes P. millegrana from related Physcia species and other small foliose bark lichens.

Substrate and Habitat

Typical substrates and settings include:

  • Hardwood bark (maple, elm, ash, oak)
  • Twigs and young branches
  • Old fence rails and posts
  • Occasionally weathered outdoor wood

Physcia millegrana prefers open or lightly shaded habitats where bark dries intermittently and receives scattered sunlight. It commonly occurs in mixed communities with:

Identification

Key features for field diagnosis:

  • Color: Pale gray, blue-gray, or slightly olive—distinctly lighter than Phaeophyscia relatives
  • Form: Small foliose rosettes or irregular patches; loosely organized, often intermixed with other bark lichens
  • Surface: Uniformly frosted or powder-dusted from abundant soredia; matte finish
  • Underside: Pale (lighter than Mealy Shadow), with short rhizines; occasional slight lobe uplift reveals underside
  • Reproductive structures: Soredia abundant and prominent; apothecia rare or absent
  • Lobe size: Extremely minute (0.2–1.0 mm), instantly distinguishing from larger Physcia species

Distinguishing from Similar Species

vs. Physcia dubia

  • P. dubia is less uniformly dusted and forms tighter, neater rosettes with a slightly shinier upper surface
  • Soredia in P. dubia tend to cluster in patches rather than forming overall powdery coating
  • Powdery Shield: More evenly frosted throughout, softer and more diffuse texture

vs. Physcia tenella

  • P. tenella has slightly longer lobes and less irregular branching
  • Soredia in P. tenella often appear at lobe tips rather than across the whole thallus
  • P. tenella may develop rimmed apothecia
  • Powdery Shield: Lacks apothecia; appears uniformly dusty across all lobes

vs. Physcia adscendens

  • P. adscendens shows distinctive “hooded” lobe tips (“helmets”) that curl upward
  • Soredia appear in localized clusters
  • Powdery Shield: Never forms hooded tips; consistently powder-dusted

vs. Physcia caesia

  • P. caesia is more blue-gray overall with frosted apothecia when present
  • Rosettes appear cleaner and more coherent
  • Powdery Shield: Paler gray-white with softer, more granular surface

vs. Phaeophyscia species (P. orbicularis, P. hirsuta, P. sciastra)

  • Phaeophyscia species are notably darker and more matte, with granular or roughened surfaces
  • Lobes thicker and more shadowed
  • Powdery Shield: Noticeably lighter, finer, and more uniformly powder-coated

Ecology and Notes

Indicator Value:
Physcia millegrana is a sensitive air-quality indicator, thriving in areas with clean, well-circulated air. Its presence suggests relatively low levels of sulfur and nitrogen pollution. It is commonly observed on open, sun-exposed bark in minimally disturbed settings.

Distribution in Minnesota:
One of the most common small foliose bark lichens throughout Minnesota hardwood regions, though often overlooked due to its minute lobes and subdued coloration. Frequently encountered in mixed communities on maple and elm.

Reproductive Strategy:
The species reproduces primarily by soredia, which are dispersed readily by wind and water. This strategy allows rapid colonization of new substrate. Apothecia are rarely observed in field conditions, though sexual reproduction may occur.

Community Context:
Physcia millegrana frequently occurs alongside brighter species like Candelaria concolor (Candleflame), which provides vivid visual contrast. Its subtle, frost-gray appearance often serves as a cool counterbalance to warmer, more vibrant neighboring lichens.

References