Pinanga tenella (H.Wendl.) Scheff. is a small, thicket-forming feather palm endemic to Borneo and one of the few true rheophytes among palms: it lives on the banks of rivers, in the band of rock, pebbles and silt that lies above normal water level but is regularly swept by floods. Its clumps of very slender stems carry pinnate leaves with narrow, grass-like, dark green but tough leaflets, a leaf form that lets fast flood water pass without tearing the plant apart. It was first described by Wendland as Ptychosperma tenellum, and two varieties are accepted, separated by whether the leaflets have one main fold or two. Although several palms share this habit in Borneo, the closely similar Pinanga rivularis turns out to be only distantly related. It is rarely cultivated.
Distribution and habitat
Kew records Pinanga tenella as native to Borneo only, with no introduced range. A recent review of Bornean Pinanga lists it from all four political units of the island: Brunei, Sabah, Sarawak and Kalimantan. The type of the species came from the Bungal area of north-eastern Borneo, in present-day Sabah, and another early collection was made on the Tawaran River near Bungol. The varieties described by Beccari came from the Rejang and from the Sekrang, a tributary of the Batang Lupar, both in Sarawak. In Brunei the palm is abundant along the Sungai Temburong at Kuala Belalong.
It is strictly a plant of river margins in lowland rainforest. It is found mainly on rocks along the banks, on pebbly banks that seem to stay stable from year to year and on banks of silt, and less often wedged into crevices between boulders. It shares this zone with other rheophytes such as a narrow-leaved form of Osmoxylon borneense. Its occurrence along a river is patchy, and in some stretches that look suitable it is replaced by a rheophytic screw pine, Pandanus cf. dorystigma.
Description
- Habit and stem: a densely clustered dwarf palm forming tight clumps, usually up to about 2 m tall; the type specimen was a plant of only about 60 cm (2 ft). Stems are very slender. In older clumps the stems lean over in the direction of the current. New shoots arise only at the very base of the clump, and always from buds in the leaf axils, so the clump becomes thickly branched at ground level.
- Leaves: pinnate, with very narrow but tough, dark green leaflets. In var. tenella each leaflet has a single main rib (single-fold), whereas in var. tenuissima the leaflets are two-fold, with two ribs.
- Inflorescence and flowers: borne below the leaves, with 1–4 branches, the number varying even within one population. The flower clusters (a female flower between two males) are arranged strictly in two rows, with the female flowers placed laterally relative to the stem. In the pistillate flower the calyx is made up of 3 free, overlapping (imbricate) sepals; in the staminate flower two petals are about equal and the third is smaller.
- Fruit and seed: almost-ripe fruits have been observed, but the structure of the fully ripe fruit has not yet been described.
Var. tenella
The typical variety has leaflets with one main rib. Dransfield (1980) found it in Sabah in the herbarium material then available; the large, well-studied population on the Temburong River in Brunei was later identified as this variety.
Var. tenuissima
Pinanga tenella var. tenuissima (Becc.) J.Dransf. has two-fold (bicostate) leaflets. In Dransfield's 1980 account all specimens with this leaflet form came from Borneo outside Sabah, chiefly from Sarawak. It includes the plants that Beccari named Pinanga calamifrons and Pinanga calamifrons var. tenuissima.
How to tell it apart
Pinanga rivularis forms very similar riverside thickets of narrow leaves, but differs in several ways. In Pinanga rivularis the pistillate calyx is a low tube ending in 3 blunt lobes rather than 3 free sepals, the stems often branch from the internodes along their whole length, all three petals of the staminate flower are about equal, the rachillae are thick and densely hairy, and the flower clusters are often not strictly in two rows. Pinanga rivularis also has a shorter, stouter inflorescence, which Beccari described as unbranched, and leaflets that are less drawn out at the tip. Pinanga salicifolia, with which Pinanga tenella was once confused, is a very different palm of ridges and heath forest.
Ecology
The narrow, leathery leaflets are assumed to be an adaptation that reduces drag when the river floods over the plants, as in other rheophytes. At Kuala Belalong in June 1989 most fertile clumps carried fruit that was not quite ripe, together with some flowers, which suggests that flowering is to some extent seasonal. Seedlings establish on silt ledges on the steep river bank, inside the zone reached by floods. How the seeds are dispersed has not been observed.
A genetic study of five Pinanga species growing together at Kuala Belalong found var. tenella to be the densest of them, with an average of 343 plants per hectare along its riverside habitat. It also had the lowest genetic diversity of the five (expected heterozygosity 0.133), and like the others it was strongly inbred (fixation index 0.753), a pattern consistent with dense, clonal clumps and limited gene exchange.
Cultivation
This palm is very rarely grown. Grower experience treats it as a strictly tropical plant needing warmth, shelter and permanent moisture, in keeping with its flooded river-bank habitat. Long-standing grower data give USDA zone 10b (average annual minimum +1.7 to +4.4 °C) as its limit, which should be read as a survival limit rather than a growing temperature.
Conservation
Pinanga tenella depends on rivers with a natural flood regime. Because it lives in the flood zone it is exposed to extreme floods: a re-survey of the Kuala Belalong Pinanga populations about 20 years after the first study found that the species had vanished completely from one study site. Dam building or other changes to river flow would remove its habitat.
Name history
Wendland described the species as Ptychosperma tenellum in Botanische Zeitung (Berlin) 17: 63 (1859). Scheffer moved it to Pinanga as Pinanga tenella in Natuurkundig Tijdschrift voor Nederlandsch-Indië 32: 179 (1872). The holotype is a specimen collected by Lobb, labelled as a shrub of about 60 cm (2 feet) growing by the river at Bungal in north-eastern Borneo; a second collection of the same plant was made by Burbidge on the Tawaran River near Bungol, labelled as a grassy-leaved, tufted and possibly subaquatic palm. Scheffer and Miquel confused Pinanga tenella with Pinanga salicifolia, an error that became clear once Pinanga salicifolia was better understood.
Beccari collected narrow-leaved river-bank Pinanga in Sarawak three times. One collection, from the Tubau in Ulu Bintulu (P.B. 3773), became Pinanga rivularis; the second, from the Rejang (P.B. 3841), became Pinanga calamifrons Becc., Malesia 3: 132 (1886); the third, from the Sekrang in the Batang Lupar basin (P.B. 3848), became Pinanga calamifrons var. tenuissima Becc., Malesia 3: 133 (1886). All three holotypes are in Florence (FI). Beccari separated Pinanga calamifrons from Pinanga tenella only by its two-ribbed leaflets. In 1980 Dransfield found that the types of Pinanga tenella and Pinanga calamifrons share the same flowers, fruit and leaflet texture, and that the two leaflet forms, though normally an unreliable character in Pinanga, were consistent and geographically separated in these rheophytes. He therefore reduced Pinanga calamifrons to a variety, publishing the combination Pinanga tenella var. tenuissima (Becc.) J.Dransf. in Kew Bulletin 34: 773 (1980).
Dransfield also noted that the calyx difference between Pinanga tenella (free sepals) and Pinanga rivularis (tubular calyx) is the same character Burret used to separate the genus Pseudopinanga from Pinanga; following Moore (1973), Dransfield did not accept Pseudopinanga.
The epithet tenella is the Latin diminutive of tener, "tender" or "delicate", referring to the slender, fine-leaved habit. Pinanga comes from the Malay pinang, the name for the betel nut palm and its relatives. Pronunciation: pih-NAHN-gah teh-NELL-ah. The genus belongs to subfamily Arecoideae, tribe Areceae, subtribe Arecinae.
