Nepenthes rhombicaulis — Vorstman's Guide to the Carnivorous Plants

Identification

A robust climber, according to Cheek & Jebb (2001) up to 20 m long, with a four-angled to rhomboid climbing stem 5 to 10 mm thick; short shoots and rosettes also occur. The stem is glabrous with prominent glands; the leaf margins carry reddish-brown hairs. Leaves of the climbing shoot are lanceolate, 12 to 22 cm long and 3 to 4 cm wide, with a nearly peltate to emarginate tip and a clasping base, and two or three longitudinal veins on each side. Lower pitchers are ventricose below and cylindrical above, 6 to 12 cm tall and 2.5 to 3.5 cm wide, with two fringed wings, a peristome of 3 to 5 mm with ribs about 0.5 mm apart, a lid of 2.5 by 1.7 to 2.5 cm and a spur split to the base into two threads, about 5 mm long. They are dull green to light red with purple blotches, with a peristome from light green to dark purple. Upper pitchers have rarely been seen in the field: ventricose below, tubular to slightly funnel-shaped towards the mouth, with a much reduced peristome; they are yellowish green with pale red spots.

Similar species

Nepenthes gymnamphora, which grows with this species on Pangulubao, has round stems, broader wings on the lower pitchers and a broader, toothed peristome. N. hirsuta from Borneo superficially resembles N. rhombicaulis but does not occur on Sumatra. N. spectabilis and N. ovata, on the same mountain, have respectively much longer, speckled upper pitchers and rounded lower pitchers with a hooked appendage under the lid. The rhomboid stem and the spur split into two threads are the handiest features.

The trap and how it works

The lower pitchers often lie half sunk in leaf litter and humus on the forest floor, and so catch almost only what walks over the ground: ants, beetles, springtails and other litter dwellers. The fringed wings guide these to the mouth, where the ribbed peristome turns slippery when wet. In the shade of the forest, where flying insects are scarcer than on open ridges, this ground-level strategy makes sense. The rarely formed upper pitchers catch flying insects.

Habitat and ecology

Dense, shady mountain forest (B1), especially lower montane forest on steep slopes, between about 1600 and 2000 m. The species grows on the ground and is one of the few Nepenthes that is common in the understorey of closed forest, rather than being tied to openings and forest edges. The lower pitchers often sit in the litter layer.

Distribution and elevation

Endemic to Sumatra, in North Sumatra province around Lake Toba: known from the Pangulubao complex and Gunung Lubukraya, with possible sightings on Gunung Bandahara in Aceh. The world checklist gives Sumatra as its only area.

Recorded (one-degree cells)

Distribution based solely on records at GBIF (Global Biodiversity Information Facility), not on TDWG regions — those are often far larger than the actual range. The light halo is the area around a record, the dark core the record itself. The darker cells are 3 one-degree squares — roughly a hundred and ten kilometres — in which the species has been recorded, from all GBIF records. This is coarser than for most species because hardly any record of this plant carries an accurate coordinate: the locality of poached species is obscured at source. At this scale that obscuring disappears into the noise. The cells show where it has been seen, not where it occurs: an empty square means nothing.

Forms and variation

No varieties have been described; lower pitcher colour ranges from dull green to light red, peristome colour from light green to dark purple. Natural hybrids are known with N. flava, N. gymnamphora, N. ovata, N. spectabilis and N. tobaica. On Pangulubao, where many species meet, such hybrids are readily found in the field.

Where and when to see it

Gunung Pangulubao near Lake Toba is the type locality and the best place. Look not only along the open ridge, where the other species grow, but also in the dense forest on the slopes below: that is where N. rhombicaulis stands, with its lower pitchers in the litter. Feel the stem: the four sharp angles are easy to recognise with the fingers. Anyone who finds upper pitchers has seen something rare; note and photograph them carefully.

Conservation status

Assessed on the Red List as Vulnerable, because of its small range around Lake Toba. The mountain forest in this area is under pressure from clearance on the lower slopes and from fire. As a member of the genus it is listed in CITES Appendix II. Because the species grows in closed forest, it is more sensitive to logging than ridge-top species, which may benefit from disturbance.

Scientific name

Nepenthes rhombicaulis | Kurata, 1973

Nepenthes rhombicaulis was described in 1973 by Shigeo Kurata, in The Gardens' Bulletin Singapore 26(2): 227-232. The holotype (Kurata 4300) was collected on 29 March 1972 on Mount Pangulubao, Sumatra — the same mountain as N. mikei and N. ovata (golf 27/28) — at 1,700-1,900 m elevation, and is held at the Nippon Dental College herbarium; an isotype is held at the National Herbarium of Singapore. Mount Pangulubao, Sumatra, Indonesia, at 1,700-1,900 m elevation. The specific epithet is formed from the Latin "rhombicus" ("rhomboid") and "caulis" ("stem"), referencing the characteristic cross-sectional shape of the stem internodes.

World Checklist of Vascular Plants (WCVP), via Plants of the World Online (POWO), Royal Botanic Gardens, Kew.