A New Sonerila (Melastomataceae) from Central Kalimantan, Borneo

Abstract

Sonerila is a genus of ca. 100-175 species distributed from Sri Lanka and southern India to the Indo-Pacific region. It is the only trimerous genus (except for the monotypic Stussenia C. Hansen and Lithobium Bongard) in the Melastomataceae, with the flowers having one or rarely two whorls of stamens. Sonerila nagyana, described and illustrated here, has one staminal whorl and is unusual in its rosette habit with succulent and lanceolate leaves. It was discovered in heath forest in the Upper Barito Ulu region in Central Kalimantan, Borneo. Indonesian Borneo comprises the provinces of Kalimantan Barat (West), Kalimantan Tengah (Central), Kalimantan Timur (East), and Kalimantan Selatan (South). During a recent expedition I visited the area near the junction of the Rekut and Busang Rivers (Upper Barito Ulu, Central Kalimantan), tributaries of the Barito River. This is one of the largest rivers in Kalimantan, flowing from the hills of the province of Kalimantan Tengah to the South Java Sea, a distance of approximately 800 km (380 km direct). The landscape in the Upper Barito Ulu area consists of undulating terrain cut by numerous streams that ranges in elevation from ca. 150 to 500 m. The predominant vegetation type is mixed lowland dipterocarp forest with a few patches of heath forest (kerangas). The new Sonerila species was found growing in a patch of such forest, east of the Rekut basecamp (113'59'20E, 0003'40S; Fig. 1), at an elevation of ca. 500 m. It grew epiphytically on a low tree trunk in mossy substrate that in places was 40 cm thick; the soil pH was ca. 2.8. Statistics indicate that while the island of Borneo as a whole is among the better collected areas of the humid tropics, collecting in Indonesian Borneo (Kalimantan) is very limited, and Kalimantan is among the least collected regions of the world (Ashton, 1989). The flora of the Upper Barito Ulu region is extremely diverse and botanically understudied, especially as regards herbaceous plants. During my visit in the Rekut area, among other Melastomataceae I was able to collect five species of Sonerileae, a tribe of predominantly herbaceous plants of rainforests. Three species belonged to the genus Phyllagathis, which I am currently revising, and two to Sonerila. The latter are S. beccariana Cogniaux, a new record for the area, and S. nagyana, the species described here. Sonerila beccariana is fairly widespread and belongs to a group of Sonerila species that have obconical capsules which dehisce mainly apically. The fruit placenta in these species is stalked, and the column does not bend out of the capsule to facilitate seed dispersal (Cellinese, 1997). Sonerila nagyana is unusual in Sonerila in having a rosette of succulent and lanceolate leaves; all other species I have seen have membranaceous leaves, and rosette-species have broadly ovate to rounded leaves. Sonerila nagyana belongs to a second group of Sonerila species with triangular capsules and sessile or subsessile placentae. At maturity, the capsules dehisce along a longitudinal suture, and the placenta column bends out of the capsule and exposes the placentae with the attached seeds for rainwater