The tropical rainforests of Sundaland (Bali, Borneo, Java, Sumatra, the Malay Peninsula, and the surrounding small islands) form one of the most biologically megadiverse regions on Earth, with each landmass containing fragments of a larger forest which once covered the now largely submerged Sundaland landmass. The island of Sumatra, once considered to be too similar to Peninsula Malaysia to merit much study, is now recognised as one of the most diverse regions within Sundaland, possibly containing as many Plant species as the much larger island of Borneo; significantly more than the islands of Java or Sulawesi. The island is also home to a variety of Critically Endangered megafauna, including Sumatran Elephants, Elephas maximus sumatranus, Sumatran Rhinoceros, Dicerorhinus sumatrensis, Sunda Tiger, Panthera tigris sondaica, and Sumatran and Tapanuli Orangutans, Pongo abelii and Pongo tapanuliensis.
Stone Oaks, Lithocarpus spp., are currently considered to be the second largest genus within the Family Fagaceae, with about 347 named species (only the True Oaks, Quercus spp., have more, with around 600), including 32 species found on Sumatra, five of which are entirely endemic to the island. Stone Oaks are found across the island of Sumatra, found in all forest environments, although individual species tend to have limited elevation ranges, and the group is at its most diverse between 400 m and 700 m above sealevel. Fagacean trees, along with Laurels, Lauraceae, and Myrtles, Myrtaceae, form major components of the lower montane forests of Sumatra, between 900 m and 1500 m above sealevel, and eight species are known from the Sumatran upper montain forest, 1400-2500 m above sealevel.
The Batang Toru Ecosystem consists of a mosaic of mixed plantations and primary and secondary forests, with three remaining large forest blocks, the largest of which is the South Tapanuli Block, which forms the last refuge of the Critically Endangered Tapanuli Orangutan.
In a paper published in the journal PhytoKeys on 20 October 2023, Try Surya Harapan of the Southeast Asia Biodiversity Research Institute and Center for Integrative Conservation at the Xishuangbanna Tropical Botanical Garden, the Yunnan International Joint Laboratory of Southeast Asia Biodiversity Conservation and Yunnan Key Laboratory for Conservation of Tropical Rainforests and Asian Elephants, and the Herbarium at Universitas Andalas, Wei Harn Tan of the Faculty of Science at Universiti Brunei Darussalam, Thoriq Alfath Febriamansyah, Nurainas, and Syamsuardi, also of the Herbarium at Universitas Andalas, and Joeri Sergej Strijk of the Alliance for Conservation Tree Genomics at the Pha Tad Ke Botanical Garden, describe a new species of Stone Oak from the South Tapanuli Block, observed to be a food source utilised by Tapanuli Orangutans.
The new species is named Lithocarpus tapanuliensis, where 'tapanuliensis' means 'from Tapanuli'. It is a large forest tree, reaching 35 m without any buttresses. It's bark is greyish-green with whitish lenticels, fissured in place to form longitudinal slits exposing a dark red inner bark. Leaves are elliptic to oblong in shape, reaching 20 cm in length and 8.5 cm in width, dark green above, and fawnish green on the underside, which is covered with woolly hair. Flowering of the tree has not been observed, but acorns 1.9–2.3 cm in length are born in cups 2.8–3.4 cm in diameter.
The species was discovered during a field survey carried out in February 2023; only two trees were observed, growing at an altitude of 894 m within Pilar Forest of the South Tapanuli Block. A number of collected acorn remnants showed signs of having been fed upon by Orangutans, and an Orangutan was observed nesting in a neighbouring tree (Orangutans tend to nest near to, but not in, food trees, to avoid being disturbed by other Animals when they sleep). The consumption of the acorns, leaves and bark of Fagaceaen trees by Orangutans has previously been recorded, although these is generally overlooked as a food source in studies of the Apes. It is likely that acorns serve as a reliable source of food during periods of fruit-scarcity, caused by the multi-annual fruiting cycles of many rainforest trees. Asian Black Bears, Ursus thibetanus, and Malayan Sun Bears, Helarctos malayanus, have also been observed consuming acorns during periods of low fruit abundance.
Only two specimens of Lithocarpus tapanuliensis has been observed, both within Pilar Forest. This forest is small, lacks any legal protection, and extensive habitat alteration and forest clearance are occurring within the immediate area to where the trees were found. For this reason Harapan et al. recommend that Lithocarpus tapanuliensis be classified as Critically Endangered under the terms of the International Union for the Conservation of Nature's Red List of Threatened Species.
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