Classification
 Nomenclature
Scientific Name:
Polystichum cystostegia (Hook.) J.B.Armstr., Trans. & Proc. New Zealand Inst. 13: 364 (1881) – as cystostigium
Synonymy:
  • Aspidium cystostegia Hook., Sp. Fil. 4, 26, t. 227 (1862)
  • Dryopteris cystostegia (Hook.) Kuntze, Revis. Gen. Pl. 2, 813 (1891)
Lectotype (selected by Brownsey et al. 2019): Middle Island, Discovery Peaks, 5800 feet, Travers, Herb. Hooker., K 001040190!
Etymology:
From the Greek kystis (bladder), and stege (roof, shelter), a reference to the swollen, umbrella-like indusia covering the sori
Vernacular Name(s):
alpine shield fern; mountain shield fern
 Description

Rhizomes erect, densely scaly. Rhizome scales ovate, 8–28 mm long, 3–8 mm wide, uniformly pale orange-brown, margins entire. Fronds 40–440 mm long, or rarely to 550 mm long. Stipes 5–240 mm long, pale orange-brown, densely scaly proximally but less abundant distally; scales ovate to narrowly ovate, uniformly pale orange-brown, apices acuminate, margins entire. Rachises pale brown, sulcate, narrowly winged only at distal end, abundantly scaly; scales on both surfaces narrowly ovate to linear, pale orange-brown. Laminae 2–3-pinnate, narrowly elliptic, 35–320 mm long, 13–75 mm wide, light green on both surfaces, coriaceous, bearing abundant pale brown scales on abaxial costae and lamina surfaces, sparser on adaxial surfaces. Primary pinnae in 12–27 pairs below pinnatifid apex, overlapping distally, widely spaced proximally, ovate; the longest at or near the middle, 6–42 mm long, 4–27 mm wide, apices acute, bases stalked; costae winged throughout except in largest fronds. Secondary pinnae decreasing in length along the primary pinnae to the distal end; the longest secondary pinnae ovate, 2–18 mm long, 2–10 mm wide, apices acute, margins serrate to deeply divided, bases stalked. Tertiary segments narrowly elliptic, up to 8 mm long and 2 mm wide. Sori round, medial or nearer margin than costa, confined to pinnae in distal half of lamina; indusia round, inflated, 1.0–1.9 mm diameter, ± concolorous, lacking a dark central area.

 Recognition

Polystichum cystostegia is distinguished from all other species in New Zealand by its alpine habitat, large inflated indusia, concolorous orange-brown stipe and rhizome scales, and lack of bulbils. The species is likely to be confused only with P. vestitum, which has bicolorous scales and lacks inflated indusia.

 Distribution

North Island: Volcanic Plateau, Taranaki.

South Island: Western Nelson, Sounds-Nelson, Marlborough, Westland, Canterbury, Otago, Southland, Fiordland.

Stewart Island, Auckland Islands, Campbell Island.

Altitudinal range: 900–2150 m.

In the North Island Polystichum cystostegia is confined to subalpine and alpine areas from c. 900 m to 2150 m on Mt Taranaki and at c. 1630 m on Mt Ruapehu. There is also a single specimen said to have been collected from the vicinity of Judd Ridge in the Tararua Ranges at 600 m in podocarp forest (WELT P025319), a locality which needs confirmation. Cheeseman (1906, 1925) erroneously recorded it from Mt Tongariro, and also cited a collection by Buchanan from the Tararuas, for which there is no extant supporting specimen. In the South Island it extends from north-west Nelson to southern Fiordland, occurring in alpine areas from 900 m, up to 1900 m in the Spencer Range, Nelson. It also occurs on Mt Anglem, Stewart Island, and on the Auckland and Campbell Islands.

 Habitat

Polystichum cystostegia occurs in alpine and subalpine herbfield, tussock grassland, flushes, dry creek beds, rock crevices, among boulders, on rock faces, and among shingle slips in alpine areas, rarely extending into forest at the treeline. It is one of the few truly alpine ferns in New Zealand and occurs at higher altitudes than most other New Zealand pteridophytes.

 Biostatus
Indigenous (Endemic)
 Hybridisation

There is evidence that Polystichum cystostegia hybridises with P. vestitum (Perrie et al. 2003​​​​).

 Cytology

n = 82 (Brownlie 1958).

 Notes

The name Polystichum cystostegia has sometimes been altered to P. cystostegium or even P. cystostigium (e.g. Armstrong 1881), but in the original description Hooker (1862b) treated the specific epithet as a noun in apposition (Art. 23.1), indicated by his spelling of Cystostegia with a capital letter. His original spelling should therefore be retained.

 Bibliography
Armstrong, J.B. 1881: A natural arrangement of the New Zealand ferns founded on the system of Smith's "Historia Filicum", with critical notes on certain species. Transactions and Proceedings of the New Zealand Institute 13: 359–368.
Brownlie, G. 1958: Chromosome numbers in New Zealand ferns. Transactions of the Royal Society of New Zealand 85: 213–216.
Brownsey, P.J.; Perrie, L.R.; Field, A.R. 2019: Taxonomic notes on the New Zealand flora: lectotypes in Dryopteridaceae and Nephrolepidaceae. New Zealand Journal of Botany 57(1): 32–38.
Brownsey, P.J.; Smith-Dodsworth, J.C. 2000: New Zealand ferns and allied plants. Edition 2. David Bateman, Auckland.
Cheeseman, T.F. 1906: Manual of the New Zealand Flora. Government Printer, Wellington.
Cheeseman, T.F. 1925: Manual of the New Zealand Flora. Edition 2. Government Printer, Wellington.
de Lange, P.J.; Rolfe, J.R.; Barkla J.W.; Courtney, S.P.; Champion, P.D.; Perrie, L.R.; Beadel, S.N.; Ford, K.A.; Breitwieser, I.; Schönberger, I.; Hindmarsh-Walls, R.; Heenan, P.B.; Ladley, K. 2018: Conservation status of New Zealand indigenous vascular plants, 2017. New Zealand Threat Classification Series No. 22. [Not Threatened]
de Lange, P.J.; Rolfe, J.R.; Champion, P.D.; Courtney, S.P.; Heenan, P.B.; Barkla, J.W.; Cameron, E.K.; Norton, D.A.; Hitchmough, R.A. 2013: Conservation status of New Zealand indigenous vascular plants, 2012. New Zealand Threat Classification Series 3. Department of Conservation, Wellington. [Not Threatened]
Hooker, W.J. 1862a: Species Filicum. Vol. 4 (part 14). Pamplin, London.
Hooker, W.J. 1862b: Species Filicum. Vol. 4, (part 13). Pamplin, London.
Kuntze, C.E.O. 1891: Revisio Generum Plantarum. Vol. 2. Arthur Felix, Leipzig.
Perrie, L.R.; Brownsey, P.J.; Lockhart, P.J.; Large, M.F. 2003: Morphological and genetic diversity in the New Zealand fern Polystichum vestitum (Dryopteridaceae), with special reference to the Chatham Islands. New Zealand Journal of Botany 41(4): 581–602.