This genus is readily recognizable, but a challenge to identify to species without a microscope or chemical tests. Stereocaulon is very unusual in structure, even for a lichen. It is generally considered a fruticose lichen, but “it’s complicated.” They have been given the common name “foam lichens,” which certainly seems fitting.

These fluffy, feathery fruticose-looking lichens actually consist of a crustose primary thallus that develops a secondary thallus of branched stalks, sometimes called pseudopodetia. In most species, the crustose part of the thallus disappears, leaving no clue that this critter is anything but fruticose.

To identify species, you also need to master a specialized vocabulary. Growing on the branches of Stereocaulon are granules or squamules called phyllocladia. The phyllocladia, whether flat or coral-shaped, contain Trebouxia, a green alga commonly found as a lichen photobiont. Nestled among them we may often find structures called cephalodia (you may recall we saw these in Peltigera as well). The cephalodia contain cyanobacteria, often Nostoc.

In this closer view, no apothecia are seen. I wouldn’t call this “mat-forming, without main stems,” so that kinda rules out S. rivulorum. Options (in LoNA key) for phyllocladia are “granular to squamulose, rarely coralloid” versus “warty, sometimes lobed.” What do you think? If I had to make a wild guess, I’d say Stereocaulon glareosum, which has been reported from the area.

I’d also say we need a dissecting microscope. This is why we like to talk about genera more than species! (And why we’ll maybe tackle an easier species next time!)

According to my search of collections at Lichen Portal (CNALH), in Colorado we have 15 taxa in 11 species of Stereocaulon, with some specimens not identified to species. Most of these collections are at elevations above 2400 m (8,000 ft). Our species are:

• Stereocaulon albicans
• Stereocaulon alpinum
• Stereocaulon dactylophyllum
• Stereocaulon glareosum (retains primary thallus; cephalodia brown)
• Stereocaulon glareosum var. brachyphylloides
• Stereocaulon incrustatum
• Stereocaulon microscopicum
• Stereocaulon myriocarpum
• Stereocaulon myriocarpum var. orizabae
• Stereocaulon paschale
• Stereocaulon paschale var. alpinum
• Stereocaulon rivulorum (arctic-alpine, no apothecia)
• Stereocaulon subalbicans
• Stereocaulon tomentosum (thallus erect, not matted; tomentose; apothecia common, brown and terminal; cephalodia blue-black)
• Stereocaulon tomentosum var. compactum

Of the 11 species (357 specimens identified to species), most commonly collected here are S. tomentosum (129), S. glareosum (85), S. rivulorum (69), and S. alpinum (45). Chances are this specimen is one of these four. If just going by these photos, I’d love to say it’s S. rivulorum!

Stereocaulon alpinum photo at Sharnoff Photos
Stereocaulon rivulorum photo at Sharnoff Photos
Stereocaulon tomentosum photo at Sharnoff Photos

Data summarized from:
Consortium of North American Lichen Herbaria (CNALH). 2014. http//:lichenportal.org/portal/index.php. Accessed on December 27.