Studies on Schismatoglottideae (Araceae) of Borneo XXII: The enigmatic Aridarum montanum refound

Aridarum montanum Ridl., the type species of the genus, has not been recollected since Cecil Joslin Brooks gathered a single specimen, purportedly on Gunung (Mt) Santubong, and now deposited in the Natural History Museum, London (BM). Boyce & Wong (2011) outlined the few facts known pertaining to A. montanum, emphasizing that numerous surveys of the supposed type locality, a large sandstone mountain situated on a peninsula jutting into the South China Sea approximately 35 km north of Kuching, the state capital of Sarawak, had failed to locate the plant, although another quite different Aridarum (A. nicolsonii Bogner) is abundant there. Recollection of A. montanum from a two quite separate very wet shale-dominated sites, one in Sri Aman Division and the other in Sarikei Division, coupled with the persistent failure to locate plants of this species at the supposed type locality on much drier sandstone ecology, raises the probability that Brooks’ label data are in error.
Brooks was in Sarawak during 1900–1910, employed as a metallurgical chemist by the gold-exploitation arm of the Borneo Co. Ltd. During this period he collected a wide range of wildlife, including plants, although mainly ferns. Data for Brooks’ fieldwork are often incomplete, and there are doubts over the numbering of at least part of his collections. Brooks’ Borneo fieldwork was concentrated almost exclusively in west Sarawak. He made collections on Gunung (G.) Santubong (c. 1º 44' N 110º20'E), G. Pueh (c. 1º48'N 109º41'E), G. Kapor (c. 1º23'N 110º7'E), G. Singai (c. 1º30'18.28"N 110º10'21.36"E), the Banjaran Bungo (c. 1º16'N 110º9'E), Bau (c. 1º25'N 110º9'E), Bidi (c. 1º23'N 110º8'E), and Tringgus (c. 1º14'N 110º5'E). With the exception of Banjaran Bungo, none of the above areas has exposed shales. Gunung Santubong, G. Singai, and Tringgus are predominantly sandstone; G. Kapor and Bidi are forested karst limestone, while G. Pueh is granite. Our new collections coupled with high levels of “geological endemism” present among aroids strongly favours A. montanum being a shale-obligate.
Brooks’ only documented excursion away from this general area was in 1908 when he ventured to the source of the true right tributary of the Sungai Sarawak, afterwards continuing to the headwaters of the Sungai Sambas where it rises in the Bengkayum (Bengkayang) Mountains (G. Niyut and G. Bentuang) NE of Pontianak, in modern Kalimantan Barat, Indonesian Borneo. Brooks returned to Sarawak from this expedition by way of Siluas and G. Penrissen (Brooks 1911). These areas do have exposed shales, and furthermore there are floristic links with Banjaran Bungo and via the Klingkang range to the border areas of Lubok Antu, close to one of the new collections of A. montanum. The Sarawak Banjaran Bungo is botanically reasonably well sampled and to date A. montanum has never been located there. On balance it seems most likely that Brooks’ collection was made during the trip to the headwaters of the Sungai Sambas.