Parmelia sulcata

Family: Parmeliaceae

Hammered Shield (Parmelia sulcata) is one of the most widespread and easily recognized foliose lichens, forming pale gray rosettes on bark and wood. Its subtly ridged, “hammered” surface and powdery reproductive structures give it a textured, frosted appearance. It is commonly encountered on trees in both natural and developed landscapes and often occurs as part of mixed lichen communities.

For representative images and visual context, see the page: Hammered Shield in the Gallery section.

Description

Parmelia sulcata is recognized by:

Growth form: foliose
Thallus: pale gray to bluish-gray, moderately thick, with a dull to slightly frosted surface
Lobes: broad (typically 3–6 mm), irregularly branching, rounded tips
Surface features: distinctive network of ridges with linear soralia producing powdery soredia
Underside: black with abundant black rhizines
Attachment: firmly attached by rhizines

The thallus forms loosely circular rosettes that may coalesce into larger patches. It is most often found on bark, especially on mature trees, and occasionally on weathered wood.

Substrate and Habitat

Typical substrates and settings include:

Bark of hardwood trees (commonly oak, maple, and ash)
Occasionally on decaying wood or fence posts

Notes may include:

Tolerant of a wide range of light conditions, from partial shade to open exposure
Frequently found in suburban and urban environments
Commonly grows in mixed lichen communities alongside Candelaria concolor, Physcia, Phaeophyscia, and crustose lichens

Identification

Key features for field diagnosis:

Color: pale gray, sometimes with a slightly bluish cast
Form: foliose rosettes with broad, rounded lobes
Surface: “hammered” or reticulate ridges with linear, powdery soralia
Underside: black with clearly visible black rhizines
Reproductive structures: soredia produced along ridges; apothecia uncommon
Chemistry: cortex K+ yellow; medulla K+ yellow to orange (salazinic acid), C–

This species is most reliably identified by the combination of:

Reticulate, ridged surface
Linear soralia aligned with those ridges
Black underside with rhizines

Common lookalikes:

Hypogymnia physodes — lacks rhizines and has hollow, inflated lobes
Platismatia glauca — broader lobes, smoother surface, lacks the “hammered” texture
Punctelia species — have distinct pseudocyphellae (white dots) rather than ridged soralia

Ecology and Notes

Parmelia sulcata is a highly adaptable species and is often one of the first foliose lichens to establish on bark in disturbed or urban environments. Its tolerance of air pollution makes it a common and reliable presence in areas where more sensitive species are absent.

It plays a role in bark communities by contributing to surface complexity and providing microhabitats for small invertebrates and microorganisms.

In Minnesota, it is widespread and frequently observed at Research Grade on platforms such as iNaturalist, reflecting both its abundance and ease of recognition.

References

Consortium of Lichen Herbaria
https://lichenportal.org
Lichens of North America (Brodo et al.)
(print reference)
iNaturalist taxon page
https://www.inaturalist.org/taxa/54452-Parmelia-sulcata