Stereocaulon, Easy to Spot

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.

Stereocaulon sp., by Jack R. Darnell. On soil, Pike National Forest near Staunton State Park.

Stereocaulon sp., by Jack R. Darnell. On soil, Pike National Forest near Staunton State Park.

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.

Closer view of Stereocaulon from previous photo by Jack R. Darnell.

Closer view of Stereocaulon from previous photo by Jack R. Darnell.

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.

Old Man’s Beard, the Genus Usnea

Long pendulous thallus of Usnea cavernosa, found on conifers in Rocky Mountain National Park, the San Juan Mountains, and other high elevations in Colorado. Not common here; this scan is of a specimen from Idaho.

Some fruticose lichens form long webs, dangling from tree branches, that acquire common names like “tree hair” or “old man’s beard,” especially in places like the Pacific Northwest, where high humidity and tall trees support their robust expression. From a distance, these resemble the “Spanish moss” of southern states (which is neither moss nor lichen).

Common species of Usnea, such as U. hirta and U. subfloridana, occur on conifer bark and are small and shrubby.  At higher elevations with adequate moisture, larger species like U. cavernosa may be seen.

Common species of Usnea, such as U. hirta and U. subfloridana, occur on conifer bark and are small and shrubby. At higher elevations with adequate moisture, larger species like U. cavernosa (at top) may be seen.

More common in our Colorado forests are species of Usnea that display themselves less dramatically. Most are no more than a few inches tall (or long) and are frequently seen on bark of conifers like Douglas-fir and Colorado or Engelmann spruce. (Oddly, however, not on ponderosa pine.)

A closer view of this same specimen is below; click to enlarge further.

A close-up of the same lichen shows the tangled branches characteristic of the Usnea thallus. Each branch has a tough central cord, which distinguishes this genus from other light-green arboreal species.

A close-up of the same lichen shows the tangled branches characteristic of the Usnea thallus. Each branch has a tough central cord, which distinguishes this genus from other light-green arboreal genera, like Evernia and Ramalina.

Detailed description of Usnea cavernosa
More Usnea photos (at Ways of Enlichenment)
List of Usnea species and photos (at Sharnoff Lichens)