Athyrium haleakalae differs from the only previously known Hawaiian Athyrium, Athyrium microphyllum, in having rhizomes 1–3 cm long and lanceolate blades 1- to 2-pinnate-pinnatifid, 3–8(–11) × 1–3(–4) cm, as compared to Athyrium microphyllum with rhizomes (10–)15–30 cm long and ovate to ovate-triangular blades 3-pinnate-pinnatifid to 4-pinnate, 30–82 × 20–50 cm.

United States of America. Hawaiian Islands, East Maui: Ko‘olau Forest Reserve, west fork of Helele‘ike‘oha Stream, 20°45'14.58"N; 156°5'23.496"W, 1247 m elev., 28 May 2014, Perlman et al. 23964, (holotype: PTBG-070914; isotypes: BISH, UC, US).

Lithophytic ferns. Rhizomes slender, erect to suberect, unbranched, radial, dark brown, 1–3 × 0.5–1.0 cm, closely set with roots and persistent, densely clothed by old stipe bases; scales covering rhizome tips, stramineous to dark brown, 2–4 × 1.0–1.5 mm, lanceate, margins entire, attenuate toward apex. Fronds 3–7 per rhizome; stipes medium brown, 20–50(–70) × 0.3–0.7 mm, swollen bases proximally thickened to 1 mm, well clothed with stramineous to dark brown basal scales 3.0–4.5 × 0.5–1.0 mm, sparser distally, thinning to glabrous; blades medium green, 1- to 2-pinnate-pinnatifid, 3–8(–11) × 1–3(–4) cm, lanceolate, rachises medium green to dark brown, glabrous, apex pinnatifid, acute, lobed ½ toward costae, pinnae 10–12 pair, lanceolate, alternate, petioled 1–2 mm, sessile near apex, fleshy spines 0.3–1.0 mm long on adaxial surface at bases of costae and costules, basal pinnae spaced 2–5(–8) mm, distal pinnae more closely spaced, not overlapping, lowest pinnae slightly reduced, second lowest pair usually largest, 0.7–2.0(–2.5) × 0.3–0.5(–0.8) cm, pinnules, lower with 6–8 pair, reduced distally, ovate to lanceolate, near alternate, serrate to lobed, veins 2–4 pairs in basal lobe, fewer pairs distally. Sori short linear, oblong, or J-shaped, 1.0–1.4 mm long, along acroscopic base of veinlets, 1(–2) per ultimate segment, indusia tan or brown, same shape as sori, entire, persistent.

The new species is named after Haleakalā, East Maui, a massive, dormant shield volcano (3,057 m tall) and the only known location of Athyrium haleakalae.

United States. Hawaiian Islands, East Maui: Hana Forest Reserve, Mokulehua drainage basin, Metrosideros-Cheirodendron-Dicranopteris montane wet forest, dissected by riparian vegetation, 1195 m elev., 21 Aug 2013, Wood & Oppenheimer 15624 (BISH, PTBG, US); loc. cit., 1161 m elev., 21 Aug 2013, Oppenheimer et al. H81332 (BISH, PTBG); Hana Forest Reserve, Kawakoe headwaters, 1183 m elev., 22 Aug 2013, Wood et al. 15637 (PTBG); loc. cit., 1164 m elev., 22 Aug 2013, Wood et al. 15639 (PTBG, UC); Ko‘olau Forest Reserve, west fork of Helele‘ike‘oha Stream, 1326 m elev., 28 May 2014, Oppenheimer et al. H51415 (NY, PTBG); loc. cit., 1367 m elev., 28 May 2014, Oppenheimer et al. H51418 (MO, PTBG); loc. cit., 1204 m elev., 29 May 2014, Oppenheimer et al. H51426 (PTBG, UC); Haleakalā National Park, Kīpahulu Valley, south of Palikea Camp, 1280 m elev., 28 Aug 2014, Welton et al. 2359 (HALE).

Athyrium haleakalae has only been documented on the mountain of Haleakalā, East Maui, the third highest prominence in the Hawaiian archipelago at 3,057 m. The volcanic island of Maui is ca. 1.2 million years old (Price and Elliott-Fisk 2004) and has an area of ca. 1,884 km². Hawaiian flowering plants on Maui include 518 plant taxa, with 422 endemic and 99 of those being single-island endemics (Sakai et al. 2002). Estimates on the number of endemic fern and lycophyte taxa on Maui vary only slightly (Palmer 2003, Vernon and Ranker 2013) and the present authors estimate there are 115, including five single-island endemics. Athyrium haleakalae now represents the sixth single-island endemic fern or lycophyte taxon found on Maui. It is the tenth endemic athyrioid species in the Hawaiian Islands, and the second in that group that is restricted to a single island, the other being Deparia cataracticola M. Kato, of Kaua‘i.

Since its discovery in August of 2013 ca. 300 plants of Athyrium haleakalae have been observed in several headwater drainage systems of East Maui, namely Mokulehua and Kawakoe in the Hana Forest Reserve, Helele‘ike‘oha in the Koolau Forest Reserve, and Kīpahulu, near Palikea in Haleakalā National Park (Figure 2). Perhaps the combination of its small size, remoteness of preferred habitat, and the extreme physical geography of its surroundings can explain why Athyrium haleakalae has been overlooked to date. Modern access by helicopter and careful floristic inventories around large waterfalls and rugged plunge pools have led to its recent discovery by botanists of the

National Tropical Botanical Garden

The current distribution of Athyrium haleakalae has an elevational range of 1,161–1,326 m. The dominant plant community of those regions is a Metrosideros Banks ex Gaertn. (Myrtaceae)-Cheirodendron Nutt. ex Seem. (Araliaceae) montane wet forest. Large colonies of matting ferns such as Dicranopteris linearis (Brum. f.) Underw. and Diplopterygium pinnatum (Kunze) Nakai (both Gleicheniaceae) are associated with these forests, being especially expansive near forest borders where steep slopes drop down to deep dissecting streams. Observations to date indicate that Athyrium haleakalae is an obligate rheophyte which prefers concave moss-matted basalt walls along the waterline of perennial streams, forming colonies over wet basalt rock faces especially under and around the ledges of waterfalls and hollows of large plunge pools (Figures 3, 4A). These stream sites average ca. 10–15 m broad and have exposed basalt bedrock and large strewn boulders. Associated ferns occurring with Athyrium haleakalae include Athyrium microphyllum, Cyclosorus sandwicensis (Brack.) Copel. (Thelypteridaceae), Selaginella arbuscula (Kaulf.) Spring (Selaginellaceae), and Hymenasplenium unilaterale (Lam.) Hayata and Vandenboschia davallioides (Gaudich.) Copel. (both Hymenophyllaceae). Significantly, Athyrium haleakalae grows in association with one of the rarest Hawaiian endemic rheophytes, Cyclosorus boydiae (D.C. Eaton) W.H. Wagner.

Adjacent riparian angiosperm vegetation, usually outside the rheophyte zone, includes Broussaisia arguta Gaudich. (Hydrangeaceae), numerous species of Clermontia Gaudich. and Cyanea Gaudich. (both Campanulaceae), several species of Cyrtandra J.R. Forst. & G. Forst. (Gesneriaceae), Deschampsia nubigena Hillebr. (Poaceae), Dubautia plantaginea Gaudich. and Dubautia scabra (DC.) D.D. Keck (both Asteraceae), Gunnera petaloïdea Gaudich. (Gunneraceae), Kadua affinis DC. and Kadua axillaris (Wawra) W. L. Wagner & Lorence (both Rubiaceae), Labordia venosa Sherff (Loganiaceae), Machaerina angustifolia (Gaudich.) T. Koyama (Cyperaceae), Melicope clusiifolia (A. Gray) T.G. Hartley & B.C. Stone and Melicope molokaiensis (Hillebr.) T.G. Hartley & B.C. Stone (both Rutaceae), Myrsine sandwicensis A. DC. (Primulaceae), Nertera granadensis (Mutis) Druce (Rubiaceae), Phyllostegia ambigua (A. Gray) Hillebr. (Lamiaceae), Polyscias oahuensis (A. Gray) Lowry & G.M. Plunkett (Araliaceae), Scaevola chamissoniana Gaudich. (Goodeniaceae), and Vaccinium dentatum Sm. (Ericaceae).

Pigs (Sus scrofa L.), landslides, and invasive weeds such as Ageratina adenophora (Spreng.) R.M. King & H. Rob. (Asteraceae), Axonopus fissifolius (Raddi) Kuhlm. and Paspalum urvillei Steud. (both Poaceae), Juncus planifolius R. Br. (Juncaceae), Hedychium gardnerianum Ker Gawl. (Zingiberaceae), and Tibouchina herbacea (DC.) Cogn. (Melastomataceae) threaten the immediate habitat of Athyrium haleakalae.

IUCN Red List Category. When evaluating the conservation status of Athyrium haleakalae utilizing the World Conservation Union (IUCN) criteria for endangerment (IUCN 2001), Athyrium haleakalae falls into the

Critically Endangered