| TAXON: | Parmelia saxatilis sens. lat. |
| RECENT SYNONYMS: | – |
| FAMILY: | Parmeliaceae |
| GROWTH FORM: | Foliose, heteromerous |
| SUBSTRATES: | Acid-barked trees and shrubs; siliceous rocks including worked siliceous stone, tile |
| PHOTOBIONT: | Chlorococcoid or trebouxioid alga |
| REPRODUCTION: | Isidia; apothecia, pycnidia occasional |
| ASCUS: | If present, cylindric-clavate; apex Lecanora-type; 8 spores |
| SPORES: | Ascospores if present ellipsoidal, aseptate, 16–18 × 9–11 µm; conidia if present cylindrical to bacilliform |
| NOTABLE FEATURES: | Thallus sometimes forming rosette; lobes grey to grey green, often brown at tips, laced with linear or networked pale pseudocyphellae; isidia sparse to crowded, typically cylindrical to coralloid, often brown at tips; lower surface black, often brown at tips; rhizines black, numerous, simple or occasionally forked |
| CHEMICAL TESTS: | Upper cortex K+ yellow, K/UV(dry)+ yellow (atranorin); K+ yellow turning red, Pd+ orange, UV± violet (salazinic acid, ±lobaric acid) |
| HABITAT: | Wide ecological and altitudinal range where suitable substrate exists; intolerant of eutrophicated or nitrogen-polluted environments |
| DISTRIBUTION: | Widespread and common in Britain, except in East Anglia |
| CONSERVATION STATUS: | Least Concern |
| LICHENICOLOUS FUNGI: | Abrothallus parmeliarum, Erythricium aurantiacum, Homostegia piggotii, Lichenoconium erodens, Lichenopuccinia poeltii, Nectriopsis rubefaciens, Nectriopsis sp., Nesolechia oxyspora |
| IDENTIFICATION DIFFICULTY: | Green 1: Field identification possible |
| CONFUSION SPECIES: | Parmelia omphalodes, P. sulcata |
FIELD NOTES
Parmelia saxatilis sens. lat. is a group of three cryptic species: P. ernstiae, P. serrana and P. saxatilis sens. str. In Britain at least, these species can’t be reliably distinguished from one another without advanced laboratory techniques, including DNA sequencing. Field lichenologists without access to the necessry techniques should always record their finds using the British Lichen Society identifier for the group as a whole (see BLS number above).
This group of lichens is extremely common and widespread across Britain, except in East Anglia. As its name suggests, you’ll find it on siliceous rock, worked siliceous stone, roof tile, and the like. But it’s also found on trees and shrubs with more acidic bark, such as young birch and oak.
P. saxatilis sens. lat. has a very distinctive appearance, and it’s one of the easiest lichens to learn to recognise. Its large and loosely attached lobes are distinctly angular, with pale, linear markings. These markings are pseudocyphellae: patches of thin cortex through which the white medulla is visible, and which help to regulate gas exchange for optimal photosynthesis. In mature specimens, the pseudocyphellae tend to lace together in a network, lending the lichen the look of metal that’s been roughly heated and hammered.
P. saxatilis sens. lat. could be confused with cousins P. omphalodes and P. sulcata, which also have the same angular, ‘hammered-metal’ lobes. But there’s a sure way to distinguish these common species:
- P. saxatilis sens. lat. is isidiate, with typically cylindrical or coral-like isidia, often brown-tipped, that develop from pseudocyphellae before spreading over the lobes
- P. sulcata is sorediate, with soredia developing in elongated, linear soralia tht tend to follow the lines of pseudocyphellae as they split open
- P. omphalodes is neither isidiate nor sorediate, and it’s brown rather than gray-green
Immature specimens of P. saxatilis sens. lat. and P. sulcata cannot be told apart, either by morphology or by chemistry. (Note that the list of chemical spot-tests for P. saxatilis sens. lat. in the LGBI3 entry linked above appears to contain typos.) If your specimen has no reproductive propagules, and it’s not P. omphalodes, try to find a nearby specimen that’s more mature.
Interestingly, while P. sulcata is tolerant of eutrophication and nitrogen-pollution, and somewhat intolerant of sulphur-pollution, P. saxatilis sens. lat. is tolerant of sulphur-pollution, and intolerant of eutrophication and nitrogen-pollution. Along with a dearth of suitable substrates, these preferences help to explain why P. saxatilis sens. lat. is uncommon in East Anglia, where intensive arable and livestock farming contribute to high ammonia levels. Indeed, the specimen from East Suffolk below arguably displays nitrogen-pollution stress, with its unusual flattened or spatulate isidia.
It’s to be hoped that a future increase in P. saxatilis sens. lat. in East Anglia, especially relative to P. sulcata, will serve as a sign of improving ecological health in the region.
LAB NOTES
Parmelia saxatilis sens. lat. can normally be identified without microscopy.
SPECIMENS
Parmelia saxatilis sens. lat. : England : VC25 East Suffolk : TM4087 : October 2024 : On Quercus twig
Parmelia saxatilis sens. lat. : Scotland : VC101 Kintyre : NR6974 : August 2024 : On siliceous outcrop
Parmelia saxatilis sens. lat. : Scotland : VC101 Kintyre : NR7785 : August 2024 : On siliceous gravestone
