Areca riparia Heatubun, 2011

Heatubun, Charlie D., 2011, Seven New Species of Areca (Arecaceae), Phytotaxa 28, pp. 6-26 : 20-21

Areca riparia Heatubun , sp. nov.

A ceteris speciebus Cambodianis distinctissima; palma rheophytica , A. triandrae Roxb. ex Buch. -Ham. similis, foliolis angustissimis semper simplicem plicatis, inflorescentiis in 2 ordines ramosis, floribus femineis semper singulis in rachilla differt.

Type:–– CAMBODIA. Koh Kong Province: Tatai Chveng waterfall, 11º 35' N 103º 06' E, 27 September 2008, Evans 174 (holotype K!) .

Clustering, small and slender palm, snaking, rarely vertical. Stem to 2.5 m tall, 1−1.5 cm diam.; internodes 2.5−3 cm long, green near the crown, shiny, brown near the base, nodal scars conspicuous, white to whitish brown. Leaves about 7 in the crown, pinnate, to 80 cm long (including petiole); sheath tubular, ca. 19 × 4 cm long, smooth, greenish cream, speckled with reddish scurf-like ramenta; crown shaft well defined, 28−30 cm long and 1−2 cm diam.; petiole short, 12−13 cm long, 5 × 5 mm at the base, channelled adaxially, rounded abaxially; rachis somewhat arching, very slender with adaxial longitudinal ridge, rounded abaxially, covered by rusty brown indumenta; blade with regularly arranged leaflets, about 25 leaflets on each side; leaflets somewhat arching, single fold, linear, 20−30 cm long, acuminate, with pointed tips, papery, green adaxially and light green abaxially, slightly discolorous when dried; continuous white woolly ramenta present along the mid-ribs on abaxial surface. Inflorescence infrafoliar, erect, small and slender, 13−15 cm long and 6−8 cm wide, protandrous, branching to 1 order and sometimes the basalmost rachilla branches to 2 order; peduncle 1−1.5 cm long; prophyll ca. 14 × 4.5 cm, lanceolate, 2-keeled, papery, cream coloured, entirely enclosing the inflorescence, then splitting longitudinally and falling before staminate flower anthesis; rachis cream to greenish; rachillae ca. 16, divaricate, elongate, yellowish. Floral clusters uniseriate, only one complete triad including female flower occurring at the base of each rachilla, rarely several triads. Staminate flowers small, triangular, ca. 2 × 0.75 mm (young stage), asymmetric, cream to pale brown; calyx low, triangular cup-shaped, ca. 0.75 mm wide and 0.5 mm high (at young stage), 3-lobed; petals 3, strongly keeled, valvate, ca. 1.5 × 0.75 mm (at young stage); stamens 3, about 1 mm long (at young stage), basifixed; anthers ca. 0.8 mm long (at young stage), white to cream-coloured, longer than the filaments; filaments ca. 0.2 mm (at young stage), orange to brown; pistillode lacking. Pistillate flowers larger than the staminate, triangular, ca. 6.2 mm long and 4 mm wide (at young stage); sepals 3, strongly imbricate, ca. 4 × 3 mm (at young stage), triangular; petals 3, imbricate, triangular, ca. 5 × 3 mm at anthesis; gynoecium about 3 mm (at young stage), lacking staminodes. Fruits 2–2.8 × 1–1.5 × 1–1.5 cm, elongate, fusiform, apical stigmatic remains, epicarp smooth, mesocarp fleshy, endocarp fibrous, bright crimson when ripe. Seeds ca. 1.6 × 1.2 cm, obovoid, endosperm ruminate, embryo basal. ( Figures 8 & 9 ).

Distribution: —This palm is only known from one collection made by Dr. Tom Evans in Tatai Chveng waterfall in Koh Kong Province, Cambodia.

Habitat: ––This species grows as a rheophyte in a wet, vegetated rocky river bank at elevation less than 20 m above sea level, downstream of the falls and outside the splash zone, but below the peak flood level.

Local Name and uses: —Not recorded.

Conservation Status: —Data Deficient. Further population studies are needed to assess the conservation status of this palm, although the collector had stated that this palm is common on the river system near the type locality.

Etymology: —The specific epithet reflects the habitat where this species grows.

Discussion: –– Areca riparia is the third species of Areca that has been reported from Cambodia and the fifth for Indo-China ( Govaerts & Dransfield 2005, Henderson 2009, Henderson et al. 2010). This palm differs from other Areca species in the region by its growth-form being truly rheophytic ( van Steenis 1981, Dransfield et al. 2008) in the flood zone of fast-flowing rocky rivers. This species has three stamens, similar to A. songthanhensis Henderson et al. (2010: 34) , A. triandra Roxb. in Buchanan-Hamilton (1826: 310) and A. montana Ridley (1907: 136) —this last species was synonymized by Andrew Henderson in his recent monograph of palms of Southern Asia ( Henderson 2009), a taxonomic decision questioned here. However, A. riparia can easily be distinguished from A. songthanhensis , A. montana and A. triandra by having narrow and flexible single-folded leaflets adapted to flowing flood water. Additional morphological characters found in A. songthanhensis and A. montana , such as solitary habit and pistillate flowers borne only on distal rachillae, further distinguish this new species, which is clustering and bears pistillate flowers on all rachillae. Differences from A. triandra are in dimension and inflorescence structure in general. Areca riparia is a small and slender palm with a stem diameter to 1.5 cm and stem height to 2.5 m, inflorescences branched to two orders, rachillae relatively few with only one pistillate flower on each rachilla. In contrast, A. triandra is typically medium-sized with a stem diameter to 7 cm or more and stem height to 6 m, inflorescences branched to three orders, numerous rachillae and 3 to 5 (sometimes up to 10) pistillate flowers on each rachilla. Also the inflorescence of A. triandra also has a powerful lemon scent as emphasized by Lim & Whitmore (2001), which has not been recorded for A. riparia .

A rheophytic form of A. triandra has been reported recently from Vietnam ( Henderson 2009, Henderson et al. 2010). One herbarium specimen collected by Andrew Henderson and his colleagues (Henderson et al. 3623) from a lowland area of Vietnam has been studied at Kew and is clearly not the same taxon as A. riparia . In this specimen, the typical features of A. triandra can be observed, such as the relatively broad leaflets with multiple folds (at least two-folded with prominent nerves in the leaflet, in contrast to the single-folded, smooth leaflets without prominent nerves in A. riparia ), the relatively large inflorescence (about 2.5 times longer and wider than A. riparia ) that is branched to three orders, as opposed to two orders in A. riparia .

Areca riparia is the second rheophytic species in the genus, the other species is A. rheophytica Dransfield (1984: 18) from Borneo. Besides being rheophytes, these two species are different from each other in many ways.

Figures

FIGURE 8. Areca riparia. A. Stem, leaf and inflorescence. B. Detail of rachilla showing staminate and pistillate flowers in bud. C. Staminate flower in section. D. Pistillate flower. E, F. Fruit whole and in section. Scale bar: A = 4 cm; B = 7 mm; C = 1 mm; D = 3 mm; E, F = 1 cm. All from Evans 174. Drawn by Lucy T. Smith.

FIGURE 9. Areca riparia in its natural habitat in Cambodia. A. Close up view of the crown with leaves and leaf sheath, inflorescence and the ripe red fruits. B. Areca riparia surrounded by other riverside vegetation. C. The flowing rocky river bank is the habitat of Areca riparia. All photo by Tom D. Evans.