Racomitrium occidentale (Ren. & Card.) Ren. & Card.
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Synonym: Bucklandiella occidentalis (Ren. & Card.) Bednarek-Ochyra & Ochyra
Racomitrium heterostichum var. occidentale Ren. & Card.
Special status: NONE
Recognition: This species was treated as a variety of Racomitrium heterostichum by Elva Lawton with good reason: with long hair points they are very similar in the field. In the field, the characteristic yellow olive green of R. occidentale is usually quite distinctive, readily distinguishable from the blackish, dark green of R. heterostichum. The yellow olive green color is shared with R. obesum, from which it must be distinguished by careful observation.
Positive identification depends on leaf anatomy. With bistratose margins consistently present in the distal part of the leaf, it is in the group of species similar to R. sudeticum. Its distinctive character is the robust costa which is is quadrate in its distal half, the corners with pronounced ridges. The ridges between the grooves are typically well developed, high enough to be described as wings and confusable with Dryptodon patens. Indeed, the ridges or wings are often visible under the handlens, as are those of Dryptodon; differentiation in the field is easy because Dryptodon is muticous while R. occidentale has hairpoints.
It is important to pay attention to the part of the leaf from which a section has been made. Near the base of leaves many of the species of Racomitrium are likely to be confusable. Margins in species with distal bistratose margins are usually unistratose at the leaf base. Here the costa is often quite wide and may occasionally display indentations that indicate grooves on the back of the costa. Specimens with ambiguous costa sections may be troublesome. Only in R. occidentale are the costal grooves and ridges regularly present and prominent in the distal half of the leaves. A good clue to distinguish R. sudeticum is noting the longer awn of R. occidentale.
The closest look alike is R. obesum, a species with similar color and long hairpoints. Anatomical differences are subtle: grooves in the lower third of the costa are characteristic of R. obesum, similar to grooves in basal costa sections in R. occidentale, but the costa of R. obesum is much wider than R. occidentale and not as abrubtly set off from the lamina. Differences in costal anatomy is greatest in the distal costal sections. Not only does R. occidentale have a quadrate, winged costa but the adaxial costal surface is narrow and flat in R. occidentale vs. wide and U-shaped in R. obesum.
Critical to appreciating the taxonomic value of costa anatomy is mastery of sectioning. This is because it is necessary to study a series of leaf sections, from distal to proximal portions of the leaf, to get a grasp of the the nature of the costa anatomy that characterizes the specimen. Making a determination from a preparation with only one or two decent sections will not provide an acceptable of confidence.
Distribution: On rock and also on wood, on roof shingles in Portland; widespread in western Oregon at low to middle elevations; apparently more common in the north than in the southern part of the state.
Comments: For some time I failed to perceive the critical differences between R. occidentale and the equally common but very similar R. obesum. I had confused these two species in earlier versions of this key. The majority of images previously used to illustrate R. occidentale, from Slagle Creek, Jackson County (Wagner 9488a) are actually R. obesum. I did not recognize that having low grooves in the lower third of the costa is characteristic of R. obesum and was under the mistaken impression that costal grooves were found only in R. occidentale. And I failed to see that the distal costal anatomy was so different between these species.
Racomitrium occidentale; Mt Baker-Snoqualmie National Forest, Washington. R. Lesher 2687a.
Racomitrium occidentale; Mt Baker-Snoqualmie National Forest, Washington. R. Lesher 2687a.
Racomitrium occidentale; Mt Baker-Snoqualmie National Forest, Washington. R. Lesher 2687a.
Racomitrium occidentale; Marys Peak, Benton Co., OR. DHW m2475.
Racomitrium occidentale; Marys Peak, Benton Co., OR. DHW m2475.
Racomitrium occidentale; Marys Peak, Benton Co., OR. DHW m2475.
Racomitrium occidentale; Marys Peak, Benton Co., OR. DHW m2475.
Racomitrium occidentale; Marys Peak, Benton Co., OR. DHW m2475.
Racomitrium occidentale; Marys Peak, Benton Co., OR. DHW m2475.
Racomitrium occidentale; Marys Peak, Benton Co., OR. DHW m2475.
Racomitrium occidentale; Marys Peak, Benton Co., OR. DHW m2475.
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Guide to Racomitrium of Oregon
Created 2007 Northwest Botanical Institute