Syntrichia is one of the easiest moss genera to recognize. The broad leaves in a pinwheel arrangement, often with a red-orange hue around older growth, in combination with the prominent "awn" (a clear, discrete hairpoint at the end of the leaf) make it unmistakeable. If you are marginally lucky, the erect sporophytes with the elongate and twisted teeth are another feature that help define this group. After that, however, it gets a little hazy. Of the roughly 1000 iNaturalist observations of the genus in my immediate region (coastal SW BC & WA), about 60% of them purport to be Syntrichia ruralis, with S. princeps being the second-most observed species in the genus. These two are harder to distinguish that I thought. The problem is compounded by other very similar-looking taxa, like S. ruraliformis, S. papillosa and S. norvegica. This post attempts to seperate the species of Syntrichia based on hand-lens level features and habitat. Each linked species hereafter takes you to an iNaturalist observation that I think is representative. At the end of this post is a link to Syntrichia ID resources.
Habitat: This common species can be found epiphytic on hardwoods, concrete, mortar, limestone and limey soils from sea level to above the tree line. According to herbarium records, it is predominant species of arid regions.
Distinguishing characters: When wet, the leaves are squarrose-recurved, meaning they arch back to a 90 degree angle with the stem. The leaf margin is rolled over at the edges almost all the way to the apex of the leaf.
Advanced Characters: This species, unlike similair-looking S. princeps, has antheridia and archegonia on seperate plants ("dioicous"). To see this you can peel back the leaves around a developing sporophyte, place the base of the sporophyte in water on a slide and then squash it with a cover slip. Under the microscope, it will look like this if it only has archegonia, whereas it will look like this if it has both archegonia and antheridia.
One Line ID: leaves bent back to 90 degrees, margins rolled almost to the tip.
addendum: @terrymcintosh has chimed in below with his considerable experience and mentions that three other species are now verified as occuring in the coastal regions. All three of them are part of the Syntrichia ruralis complex treated in this A Scandinavian study and as best I can tell, a microscope would be needed to tease them apart. Syntrichia virescens is much like ruralis, but the leaf margins are plane in the distal half and under the microscope, the upper surface of the costa in cross section is covered by 1-2 rows of "stereid" cells (small, thick walled, tiny lumen), whereas in S. ruralis this is 3-6 rows thick. The other two species can be distinguished based on mamillae and papillae. Syntrichia papillosissma has tall bulging mamillae crowned by 1-2 papillae, whereas Syntrichia subpapapillosima, is TBD
Habitat: Concrete walls, mortar, cliffs crevices, sandy areas with mineral soil. Typically low elevation. Likes urban areas.
Distinguishing characters: These plants produce lots of sporophytes (reproduction is easier when every plant has the right parts). Schofield's guide notes that from the side, annual growth increments are visible as clusters of leaves along the stem. Unlike S. ruralis, S. princeps is supposed to have leaves that are constricted near mid-leaf. Even still I find this is a hard one to be sure of without a microscope.
Advanced Characters: A cross section through the leaf of S. princeps should show hyroids in the costa which is absent in S. ruralis and S. norvegica. Hydroids are wide-lumened water-conducting cells that are dead at maturity. I find these hard to recognize but you can look here for a iNaturalist discussion about it with links to good examples.
This species, unlike similair-looking S. ruralis, has antheridia and archegonia on the same plant in the same regions ("synoicous"). To see this you can peel back the leaves around a developing sporophyte, place the base of the sporophyte in water on a slide and then squash it with a cover slip. Under the microscope, it will look like this, with round antheridia mixed with occasional long-necked bottle-like archegonia. In S. ruralis, you will only find archegonia in these regions.
One Line ID: SYNOICOUS DISTURBOPHILE WITH CONSTRICTED MIDLEAF
Habitat: Subalpine to Alpine on mineral soil and rocks.
Distinguishing characters: almost identical to S.ruralis, but with a red awn. Sometimes said awn can be green, in which case you'll need a microscope and some razors to distinguish it from S. ruralis. The leaves are purported to be "recurved" (rolled over at the edges) along most of the margin expect the distal 1/4 of the leaf, where the margin is plane/flat.
One line ID: RED AWN AT HIGH ELEVATION
Habitat (in this region): Garry Oak Bark
Distinguishing characters: This is a tiny little Syntrichia with plane margins. It can be confused with small forms of S. princeps, but the margins on the latter are rolled back.
One line ID-TINY ON OAK WITH PLANE MARGINS
Habitat: Epiphytic -- found once on a street tree in Vancouver
Distinguishing characters: This is going to look like standard Syntrichia except the margins are incurved (not rolled back or plane) and gemmae are purported to be present.
One Line ID: EPIPHYTIC WITH INCURVED MARGINS
Habitat: Epiphytic on tree trunks (especially street trees in urban areas)
Distinguishing characters: The lack of a hairpoint, rounded leaf tips and medium size distinguish it from other species in the province. Some forms of S. laevipila lack the hair point, but these are much smaller. Under the microscope, S. latifolia a
One Line ID: MEDIUM-SIZED ROUND LEAVES NO HAIRPOINT ON TREES
This species is not recognized in the Flora of North America treatment of Syntrichia, and all collections are synonymized with Syntrichia ruralis. In Europe, this taxon if variably recognized as a species, subspecies and variety. I mention it only because @michael-lueth pointed out an observation of similair form here on the eastern Vancouver Island
Habitat: Sand dunes, stony and rocky flats.
Distinguishing characters: Gradually tapering leaves that come to a point and end in the awn. The tips of the leafy section of the leaf tend to be clear (non-photosynthetic).
One Line ID: ACUMINATE SAND FORM WITH IMPOSTER SYNDROME AND HYALINE LEAF APICES
Links to Syntrichia Treatments
Flora of North America Treatment
California Moss eFlora Treatment
A Scandinavian study taking a multi-pronged but granular approach to teasing apart the very similar species of Syntrichia. Includes revised key and phylogeny but based on Scandinavian material.