Rhododendron astrophorum M.N.Tamayo, Y.P.Ang & A.S.Rob., 2023

Tamayo, Maverick, Marcos-Langbao, Abrila, Bustamante, Rene Alfred Anton, Ang, Yu Pin, Schubert, Dale, Mustaqim, Wendy, Fritsch, Peter & Robinson, Alastair, 2023, RHODODENDRON ASTROPHORUM (ERICACEAE, RHODODENDRON SECT. SCHISTANTHE), A NEW MICRO-ENDEMIC SPECIES FROM CENTRAL PALAWAN, PHILIPPINES, Edinburgh Journal of Botany 80 (1979), pp. 1-14 : 2-9

Rhododendron astrophorum M.N.Tamayo, Y.P.Ang & A.S.Rob. , sp. nov.

Closely resembles Rhododendron wilkiei Argent but differs in having leaves with laminae narrowly elliptic-oblanceolate and c.5–7 times as long as wide (vs elliptic and c.2.5 times as long as wide), venation adaxially indistinct (vs prominent) with 4–8 active pseudowhorls of leaves along the length of the stem (vs 2–3(–4) mostly terminal pseudowhorls), bracts without scales on the margin (vs with scales), yellow corollas with the tube interior red (vs red throughout), the internal surface of the corolla tube densely hairy on the proximal 3/4 (vs glabrous), and filaments hairy at the base (vs glabrous).

– Type: Philippines, Palawan, Municipality of Narra, Victoria-Anepahan Mountain Range ( VAMR), Mt Shumkat , ridge on east-facing slope, in moss on bark, 1100–1200 m elevation, 11 vi 2022, AVAMR 85 ( Angiosperms of Victoria-Anepahan Mountain Range ) (holotype PNH!; isotypes BRIT!, CAHUP!, PPC [accession no. 1783]!). Figures 1–3, 4A .

Slender, sparingly branching epiphytic shrub up to c. 70 cm tall. Roots fleshy, slightly swollen, multibranching, up to c. 15 cm long, c. 6 mm in diameter. Stem distally pale green, becoming rapidly woody below, terete, 1–1.8 mm in diameter, with an indumentum of pale brown scales at a density of 2–4 mm –2. Leaves in tight pseudowhorls, generally 4–8 pseudowhorls per mature stem, each spread over c. 2 mm of stem and 18–38 mm apart, and comprising

4 or 5 larger and (0–)3(–5) smaller leaves; petiole terete, 1–4.5 × 0.3–0.9 mm, with a deep, central adaxial groove to 0.1 mm wide, base densely covered with scales, mostly overlapping; lamina entire, narrowly elliptic-oblanceolate, larger leaves 12–55 × 2.5–8 mm, and smaller, narrower leaves 8–14 × 1–2 mm, length-to-width ratio c.7:1, base cuneate, apex acute or obtuse, midrib narrowly impressed above, below marked by weak keel close to base, becoming impressed from midpoint to apex, lateral veins indistinct, 4–6 pairs, abaxial and adaxial surfaces lepidote; leaf scales scurfy, minute, buff to pale, golden brown, 110–240 μm in diameter, slightly pitted, sessile, lacerate, convex, cells radially arranged, each cell (40–)70–90(–120) μm long, with umbo-like inner cap of appressed polyhedral cells projecting unequally outwards forming irregularly margined annular zone, abaxial density (7–)9(–12) mm –2 or c.3 diameters apart, adaxial density (2–)3(–4) mm –2 or c.7 diameters apart. Inflorescence terminal, congested, 1–2(–3)-flowered; pedicels 9–18 × 1.1–2 mm, green, trichomes absent, scales few to scattered, c.110 μm in diameter, annular zone ± round rather than lacerate, two pedicels ± fused along one side for their entire length, giving rise to two otherwise normal flowers; flower buds initially pale green, rapidly becoming bright yellow, narrowly conical above, swollen at midpoint and cylindrical below, abruptly contracting into pedicel, where buds develop in pairs often flattened on side facing other bud; floral bracts 3–5, linear to lanceolate, up to half as long as corolla, slightly to widely spreading, largest bract c.24 × c. 3 mm, apex acute, lateral margins involute, entire, scales on abaxial surface only, and mostly distally situated towards apex; flowers 18–28 × 19–34 mm, erect to slightly pendent, unscented, bright yellow throughout but with deep red flush on corolla tube interior; calyx annular inconspicuous, 0.3–0.8 mm long, 2.8–3.5 mm

in diameter, with 10 lobes corresponding to stamens, upper part with numerous simple aciculate hairs 20–40 μm long, the same trichomes scattered at base of ovary; corolla tube 10–17 × 4–6 mm, internal surface densely hairy for proximal 3/4 tube length, hairs subulate, unicellular, (0.1–)0.3(–0.4) mm long; corolla lobes (8–)19 × 10.5–14 mm, obovate with a retuse apex, spreading widely up to c.70° from floral axis, overlapping in the lower 1/3 to 1/2; stamens 10, regularly arranged around ovary; filaments 13.5–22 mm long,

terete, proximally yellow, becoming red distally, 0.8–1.1 mm wide at base, c. 0.5 mm wide at anther, proximal c. 1.8 mm with subulate hairs, densest at base, 8–30 μm long; anthers dorsifixed, poricidal, introrse, oblong, 2–2.2 × c. 1 mm, dark brownish red, pores opening with yellow margins, pollen creamy white; ovary subcylindrical ellipsoid, 3.5–5 × 1.5–2.2 mm at anthesis, densely scaly, scales c. 22–30 mm –2, often clustered but not overlapping, similar to leaf scales but markedly convex-subconical, annular zone with more regular margins and straighter ray cells, simple hairs absent; style 13–15 mm long, c. 0.8 mm in diameter, glabrous; stigma 5-lobed, 1–1.2 mm in diameter, pale orangish red to red; fruits narrowly ellipsoid, 24–27 × c. 4 mm, valves spreading, straight or occasionally twisting slightly; seeds (4–)6(–7) mm, narrowly fusiform, winged, central portion c.900 × c.25 μm.

Distribution. Recorded only from the type locality.

Habitat and ecology. Rhododendron astrophorum is found as an epiphyte on the larger limbs of shrubs and small trees from mid-montane ultramafic forest at c. 1100–1477 m elevation at the periphery of a swamp located close to the summit of Mount Shumkat, where the vegetation was notably lower in stature and more compact than at lower elevations. Other plant species recorded in the area are Tristaniopsis oblongifolia (Merr.) Peter G.Wilson

& J.T.Waterh., Vaccinium palawanense Merr. and Leptospermum sp. in the immediate vicinity. Two other species of Rhododendron sect. Schistanthe were encountered, namely, R. javanicum (Blume) Benn. subsp. palawanense Argent ( Figure 5A–C) and R. vidalii Rolfe ( Figure 5D,E), the latter representing a new record for Palawan. Rhododendron vidalii was formerly known to occur only in Luzon and Mindoro Islands ( Pelser et al., 2011 –).

Etymology. The specific epithet astrophorum is derived from the Greek ἄστρΟΝ (ástron =

star), and the suffix -φόρΟΣ (- phóros = bearing) in reference to the bright yellow corolla, which resembles small clusters of stars in the forest canopy.

Phenology. The species was observed on 9 April 2015 mostly in bud, and in full anthesis in June 2022. This phenology suggests a tendency to flower following the dry season, which in Palawan occurs between January and April (Worldbank.org 2022).

Proposed IUCN conservation category. Despite numerous expeditions to surrounding peaks in the VAMR, Rhododendron astrophorum has been formally only identified from its type locality, with just six mature individuals observed within an extent of occurrence (EOO) of c. 3.4 km 2 (≤ 10 km 2). Direct observations of Rhododendron astrophorum made in situ satisfy the IUCN 3.1 Red List (2012) Critically Endangered (CR) Criteria B2ab (i, ii, v) (interpretation per IUCN Standards and Petitions Subcommittee, 2022). Plants believed to represent Rhododendron astrophorum occur on nearby Mount Victoria at c. 1400 m elevation as epiphytes on tall trees; however, they have not been vouchered. The only evidence of their presence is a single fallen flower. Should a formal determination of Rhododendron astrophorum be made for these plants, the conservation status must be reassessed.

Specimens of other taxa examined. Rhododendron vidalii . PHILIPPINES. Palawan: Municipality of

Narra, Victoria-Anepahan Mountain Range, Mt Shumkat, ridge on east-facing slope, in moss on bark, 1100–1200 m elevation, 11 vi 2022, AVAMR 101 (Angiosperms of Victoria-Anepahan Mountain

Range ) ( BRIT!; PPC [accession no. 2103]!) ; Rhododendron javanicum ssp. palawanense . PHILIPPINES. Palawan: Municipality of Narra, Victoria-Anepahan Mountain Range, Mt Shumkat , ridge on north-east to east-facing slope, in moss on bark, 1100–1200 m elevation, 11 vi 2022, AVAMR 102 ( Angiosperms of Victoria-Anepahan Mountain Range ) ( BRIT!) .