Dicksonia brackenridgei Mettenius (1861: 81)

Noben, Sarah & Lehnert, Marcus, 2013, The genus Dicksonia (Dicksoniaceae) in the western Pacific, Phytotaxa 155 (1), pp. 23-34 : 28-29

Dicksonia brackenridgei Mettenius (1861: 81) .

Type:— FIJI. Ovalau: Without locality, 1838–1842, Wilkes’ Exploring Expedition #3 (holotype US).

Trunks to 2.8 m tall, to 11 cm diameter, with persistent old petiole bases, these relatively long, forming overlapping whorls; adventitious buds not observed. Fronds to 280 cm long, ascending to arching, strongly hemidimorphic, fertile parts starting from the penultimate basal pinnae to the center. Petioles to 70 cm long, dark to medium brown, scabrous. Petiole hairs to 2.5 cm long in lower parts, with dark brown bristly bases and soft, pale brown ciliform tips, becoming shorter and more setiform in upper petiole parts, dense undercoat of relatively stiff, dark reddish brown hairs, which may extend to the lower rhachis. Laminae to 210 x 110 cm, obovate, widest above the middle, with gradually reduced apices. Frond axes [rachises, costae and costules] inermous or rhachises weakly scabrous, mostly green in fresh material, abaxially turning reddish to atropurpureous with maturity, green parts turning light brown when dried, adaxially sparsely covered with reddish, erect hairs to 1.2 mm long, in most glabrescent. Pinnae subsessile to stalked to 3 cm, patent to weakly ascending, 12–14 pairs per frond, mostly subopposite, usually alternate towards the lamina base, the proximal pinnules of the pinnae covering the rhachsies; lowest pinnae about half the size of largest ones. Sterile pinnae truncate-lanceolate, to 55 × 20 cm, shiny dark green adaxially, pale grey-green abaxialy; fertile pinnae lanceolate, truncate at base, to 28 × 15 cm, always paler green than the sterile ones. Sterile pinnules to 10.0 × 2.3 cm, lanceolate, sessile to subsessile (stalked to 2 mm), fully pinnate in lower third with sessile segments, in distal parts adnate, ending an attenuate tip with serrate margins; fertile pinnules to 9 × 2 cm, sessile to subsessile (stalked to 2 mm), fully bipinnate (skeletonized), their tips filiform, simple or trifurcate, to rhomboid with serrate margins. Sterile segments to 16 × 6 mm, oblong, straight to distally weakly falcate, with crenate margins, their tips obtuse to rounded, rarely acute (mainly artifact from drying), the basal ones free, sessile, equilateral, usually with basal auricles, the distal ones adnate, inequilateral with strongly decurrent basiscopic bases; fertile segments to 15 × 4 mm, with reduced green tissue, the tip filiform, simple or trifurcate, to rhomboid with serrate margins. Veins paler than laminar tissue, pale green to stramineous, glabrous on both sides. Sori 1.2–2.0 mm wide, at the end of unbranched lateral veins, subcostal to costal, slightly kidney shaped when closed, circular when open; indusia bivalved, outer one greenish, with a thin cartilaginous margin, inner one brown with entire margins, even at maturity; paraphyses of the same length as sporangia (ca. 0.6 mm), abundant, pale, catenate with red-brown acicular tip. Spores tetrahedral-globose, 63.6 µm × 52.3 µm, exospore smooth, perispore granular.

Distribution and habitat: —Evergreen mountain rainforests at (100–) 400–1300 m in Fiji, Samoa and American Samoa ( Whistler 1998), also reported from Vanuatu on Espiritu Santo at 1000 m ( Matsumoto et al. 1998).

Etymology: —The name commemorates W.D. Brackenridge (1810–1893), assistant naturalist on Wilkes’ U.S. Exploring Expedition (1838–1842).

Additional specimens examined:— FIJI. Ovalau, Summit of Mt. Ndelaiovalau and adjacent ridge, [ca. 17°40'12''S, 178°48'36''E,] 575–626 m, 11–25 May 1953, A. C . Smith 7572 ( UC). Vanua Levu : Tholo North , ridge between Mt. Nanggaranambuluta ( Lomalangri ) and Mt. Namama, east of Nandarivatu, 1050–1120 m, 30 June–18 August 1947, A. C . Smith 4959 ( UC). Viti Levu : Naitardamu mountain at Navuni taruilan, 900 m, 29 September 1927, J. W . Gillepsie 3339 ( UC); District Ba , above village Navai , track towards top of Tomaniivi (Mt. Victoria), 17°37'12''S, 178°00'01''E, 795 m, 5 August 2011, M . Lehnert 2567 ( SUVA); District Naitasiri , along road between Navai and Udu, 17°42'49''S, 178°01'23''E, 865 m, 5 August 2011, M . Lehnert 2573 ( SUVA); eastern slopes of Mt. Koroyanitu , Mt. Evans Range, 950–1050 m; 1–2 May 1947, Smith, A. C .4155 (UC).— SAMOA. Savaii: Above Matavanu, rim of old crater, [ca. 13°33'37''S, 172°24'56''W,], 1000 m, E . Christophersen & E. P . Hume 2033b ( UC) .

Discussion:— Dicksonia brackenridgei can be found frequently in cool montane forests above 400 m on Fiji, but also occurs rarely in shaded gorges as far low as 100 m (personal observation; Keppel et al. 2005). It has a similar elevational range in the Samoan islands (Savi’i, Tutuila, Olosega, Ta’u), and may be found on smaller islands in between that have mountains that high, e.g. the Tongan island of Tafahi. In Vanuatu, it has only been recorded for the largest and northernmost island Espiritu Santo at 1000 m, but is expected to occur farther south as well. Biogeographically interesting are Erromango, Tanna and Aneityum: because of their proximity to New Caledonia, D. brackenridgei may be replaced here by a congener from the latter island, but this remains to be confirmed.

Brownlie (1977) states that Dicksonia brackenridgei is “doubtfully distinct from other species described from the South Pacific”. From the leaf, this species is most similar to the New Caledonian D. thyrsopteroides , but has a unique pattern of the hairy indument, with soft pale brown hairs with darkened indurated bases. These hairs build a wooly cover especially on croziers but are usually worn off on the petioles and frond axes of mature fronds. The persisting indurated bases can be slightly irritating to the skin. Remnants of the soft parts of the hairs may bleach to a hyaline white and resemble matted cobwebs, adding to the usually untidy look of the plants. This morphology separates Dicksonia brackenridgei from the New Caledonian species, which have either spreading hairs (persistent in D. baudouinii , glabrescent in D. thyrsopteroides ) or exclusively matted hairs (pale brown to white in D. munzingeri , orange-brown in D. perriei ).