Clearwater National Forest |
SIERRA WOOD-FERN Thelypteris nevadensis (Baker) Clute STATUS USFS Region 1: Sensitive USFWS: none ICDC: G4/S1 INPS: State Priority 1 TAXONOMY Family: Polypodiaceae (fern) Common name: Sierra wood-fern Synonyms: Aspidium nevadense D.C. Eat. Thelypteris oregana St. John Description Sierra wood-fern is a delicate, pale green, deciduous fern emerging in a small compact clump from a slender rhizome. The leaves are few, erect to arching, 3-8 dm tall and quite narrow (up to 15 cm wide). Leaves are twice-pinnate (twice divided leaflets) with 35-40 opposite or off-set leaflet pairs. The upper leaflets are larger and close together, the lower are greatly reduced in size and spaced quite far apart. Spore clusters (sori) are covered by an inconspicuous, horseshoe-shaped cover (indusium) on the leaflet undersides (Lichthardt 1999). Distinguishing Features and Similar Species One reason this species may be difficult to locate is its close resemblance to other northern Idaho ferns. It also occurs in typical fern habitats, thus it can easily be overlooked. The fronds of this species are distinctly narrower and more "delicate" than most of the similar fern species. This is subjective, but a frond averaging 5-8 dm tall and only 10-12 cm wide is not common. The second character for identification is the strongly reduced and distant lower pinnae. Both these characters are impossible to distinguish from a distance and when among dense ferns, identification can prove difficult. The best way to recognize this species is to become familiar with the overall appearance of the taxon. Observation of herbarium specimens, photographs, or live material is essential to expedite field identification. The fern species most resembling Sierra wood-fern is Athyrium filix-femina (lady-fern). This species is extremely common in moist habitats. Lady-fern is quite tall, averaging 2 to 3 feet and has reduced lower pinnae. However the pinnae are not distantly distributed along the lower stem. The fronds of lady-fern are much broader (up to 5 dm). The sori of lady-fern are more crescent shaped, while those of Sierra wood-fern are horseshoe shaped. DISTRIBUTION Range The main range of Thelypteris nevadensis is in the Cascade-Sierran region, chiefly the west slope, from Mt. Rainier in Washington to central California. It occurs from the foothills to mid-elevations in the mountains. There are only two known inland populations, both occurring on the Clearwater National Forest in the Isabella Creek drainage, a tributary of the North Fork Clearwater (Lichthardt 1999). Habitat Both Idaho populations occur in seeps where the soil is saturated for much of the growing season, at elevations of 3,000 to 3,200 ft. Both locations are notably cool, which separates the species from other coastal disjuncts. At the Elmer Creek population, the ground is seepy and the site probably is a Thuja plicata/Athyrium filix-femina (western red cedar/lady-fern) habitat type, however coverage of the lady-fern is quiet low, likely being replaced by the Sierra wood-fern. The surrounding community is a T. plicata/Gymnocarpium dryopteris (western red cedar/oak-fern) habitat type. Several species found at this location indicate saturated ground conditions and relatively cool temperatures. These include Equisetum sylvaticum (wood horsetail), Boykinia major (mountain boykinia), Habenaria saccata (slender bog-orchid) and Veratrum viride (green false hellebore). The Isabella Point population grows along a rocky streambank in an avalanche chute, mostly in an Alnus rubra (red alder) stand, but also under western red cedar (Lichthardt 1999). REMARKS This species if found at only two locations in Idaho. With this level of rarity, there is some concern for its viability. However, because the species requires wet seeps and would be expected to be tied to the riparian, most suitable habitat would be protected by standard riparian buffers. The Isabella Point population is in an avalanche chute and is very inaccessible. The Elmer Creek population is in a seep above the riparian. This site is near a large cedar grove and is under no threat from management activities. The Elmer Creek trial does pass through the population so there is some threat of recreational impacts, however this has proven to be very low over the years. When planning future management activities, that may have significant impacts, surveying for other upslope seeps would be appropriate. It is possible that there are additional populations of this species in Isabella Creek and neighboring drainages. However, due to the remoteness of the area and its similarity to the abundant lady-fern, finding any new populations would prove difficult. REFERENCES Lichthardt, J.J. 1999. Action plant for sensitive plant species on the Clearwater National Forest. Report to the Clearwater National Forest SO, Orofino, ID. Idaho Dept. of Fish and Game, Conservation Data Center, Boise, ID. 17 pages plus appendices.
|