pitchers are yellowish-green with pale red spots in the upper part and pitchers produced on offshoots from the climbing stem have clear red speckles throughout. Ecology Nepenthes rhombicaulis is known from a number of peaks in the Indonesian province of North Sumatra, particularly around Lake Toba. Plants that appear to match the description of N. rhombicaulis have also been recorded from Mount Bandahara in Aceh. The species is known with certainty only from the Mount Pangulubao complex and Mount Lubukraya, although Shigeo Kurata suggested that it is likely to be more widespread in the Lake Toba region. Nepenthes rhombicaulis has an altitudinal distribution of 1600–2000 m above sea level. Nepenthes rhombicaulis grows terrestrially in dense, shady montane forest. It is usually found in lower montane forest above steep slopes, but has also been recorded from upper montane forest. It is one of the few Nepenthes species that are common in the understory. Lower pitchers frequently develop embedded in detritus and leaf litter on the forest floor, resulting in their often deformed appearance. Bruce Salmon noted that lower pitchers which develop in this way grow around twice as large as those which develop completely above ground. He postulated that the species is adapted to trapping subterranean insects, although he did not examine the contents of these pitchers to test the hypothesis. In its natural habitat, N. rhombicaulis occurs sympatrically with N. flava, N. ovata, N. spectabilis, and N. tobaica. Natural hybrids with all of these species have been recorded. On Mount Pangulubao, N. gymnamphora (N. xiphioides) and N. mikei grow around 100 m above populations of N. rhombicaulis. Due to its somewhat restricted distribution, N. rhombicaulis is listed as Vulnerable on the 2006 IUCN Red List of Threatened Species. Related species Nepenthes rhombicaulis is very similar to N. gymnamphora in both morphology and growth habit. It is sympatric with this species on Mount Pangulubao. Nepenthes rhombicaulis also bears a close resemblance to N. hirsuta from Borneo. In 2001, Charles Clarke performed a cladistic analysis of the Nepenthes species of Sumatra and Peninsular Malaysia using 70 morphological characteristics of each taxon. The resultant cladogram placed N. rhombicaulis in an unresolved polytomy at the base of the Montanae/Nobiles clade, together with N. benstonei. Natural hybrids At least four natural hybrids involving N. rhombicaulis have been recorded and at least three of the parent species (excluding N. rhombicaulis) occur on Mount Pangulubao. N. flava