Distinguishing Polystichum Ferns

Other than the "usual" sword fern of the dense forests of western Washington, Oregon, and British Columbia, there are many other native Polystichum genus ferns that can be hard to distinguish. Here is a simplified key for the common ones (work in progress, partly based on my own observations):

P. munitum: Large, dark green, wide spreading fronds, in moist forest on west side of Cascade crest or near crest, pinnae often curved roughly like a scimitar and held in a flat array, almost certainly the sword fern P. munitum, our most common fern. It is also evergreen, so this is the main fern you will see in winter. In spring the fiddleheads have an unusual folded form rather than the classic fern coiled fiddlehead. Sometimes smaller and in subalpine or alpine locations among rocks, when it is easy to confuse for other species, especially P. imbricans.

P. lemmonii: Small to medium sized (6-14 inch fronds), usually severely upright fronds in full sun, with pinnae twisted at base and overlapping so that fronds appearing almost cylindrical, pinna edges rounded not spined, almost always on serpentine rock, almost certainly P. lemmonii, though also look at P. krueckebergii, which has sharper serrations on leaflets. Because of the difference in serrations, P. lemonii feels soft when you run your fingers outward on the frond, whereas P.k. feels stubbly. Early in growth season the tip of the fronds maybe be curved over and growing in shadier spots fronds will be less vertically oriented. Pinna tip is rounded and edges are not sharply toothed, though usually divided. https://www.inaturalist.org/observations/62228715

P. lonchitis: Small to medium sized (6-12 inch fronds) with narrow pointed teeth protruding (not closely appresssed) on both edges of pinnae, pinnae held flat or slightly twisted at base and often sightly cupped, often found in fairly sunny places among rocks in mountains, P. lonchitis. The pinnae are also relatively short and wide compared to P. munitum. Other Polystichum have toothed leaves, but none others in the PNW have narrow pointed teeth that stick out from the leaf. Ferns in the genus are often called "holly ferns" but this is the one with leaflets (pinnae) that look closest to the classic English Holly. https://www.inaturalist.org/observations/126889259. Sometimes very spiny looking https://www.inaturalist.org/observations/129153106.

P. imbricans: Medium sized (10-20 inch fronds, H&C says can be much longer), often relatively erect fronds with pinnae sometimes twisted at base to face toward tip of frond (tending to overlap when viewed along the frond axis), usually with many flaky tan scales along bottom side of rachis, often in relatively sunny locations in mountains but below alpine, P. imbricans. Personally I find this one really hard to tell apart from the much more common P. munitum, which also has all of the above characters some of the time especially when higher in mountains, though the pinnae of P. munitum are usually nearly planar with the frond plane. On average P. imbricans has shorter wider pinnae relative to the frond length, making the frond look narrower than P. munitum (hence the common name Narrowleaf Swordfern), and the leaflet tips are less elongated (except that young P. munitum has similarly blunter leaflet tips), and usually an apiculate tip.

P. scopulinum: Medium sized, pinnae shorter near base of frond, can be erect or spreading with pinnae variably held flat or twisted at base, usually in rocky sunny locations east of Cascade crest in mountains, tolerant of serpentine. P. scopulinum in Washington is rare outside the central Cascade Wenatchee Mountains. Pinnae are often somewhat toothed or divided, especially those at the base of the leaf. Can look similar to P. lemmonii but pinnae have spiny teeth instead of rounded edges and are less dissected (never to the costa). P. krueckbergii is similar but roughly intermediate between this and P. lemmonii.

P. andersonii not yet observed by me. Deeply divided pinnae (making it look half way between a lady fern and sword fern).

comments from frondsinhighplaces:

on https://www.inaturalist.org/observations/129150807 and closeby

Yes, 1650 meters is right at the upper limit for P. munitum. That's the same elevation as the observation I mentioned above. The plant is obviously growing in unfavorable conditions and is stunted.

To elaborate on my description above:

Leaves once pinnate, pinnae undivided. (Narrows it down to P. lonchitis, P. imbricans or P. munitum.)
Spinulose teeth of pinnae appressed, closely ascending, or incurved. (Eliminates P. lonchitis.)
Petiole with persistent, brown, ovate-lanceolate scales to 5 mm wide. (Eliminates P. imbricans.)

P. imbricans is also less likely to be found at that elevation than P. munitum.

Miniature sword ferns can sometimes be found at the upper elevation limit of the species. They usually lack sori.

When growing in open dry conditions P. munium pinnae will display an imbricate growth pattern.

[Not native. Deeply divided pinnae (making it look somewhat like a lady fern), scaly rachis, P. setiferum. Does not grow wild in the western U.S. but it is sometimes grown as an ornamental.]

Publicado el 31 de julio de 2022 17:27 por jhorthos jhorthos


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