A new species of stone oak, Lithocarpus tapanuliensis Harapan, W.H.Tan, Nurainas & Strijk from South Tapanuli, North Sumatra, Indonesia is described. We provide colour photographs, a distribution map and a new IUCN conservation status assessment for inclusion on the global Red List. The unique cupule morphology, particularly the shape, placement and distinctness of the cupule protuberances, are distinctive from other Lithocarpus species in the region. Ecological interactions (e.g. consumption and nesting) with Tapanuli orangutans were recorded in the field.

Batang Toru, Hoteng, Lithocarpus tapanuliensis, Sumatran Fagaceae, Tapanuli orangutan, food habits

The tropical rainforest of Sundaland is one of the most megadiverse regions on the Planet (Myers et al. 2000), with the island of Sumatra as one of the larger remaining land masses in this submerged continental shelf. Sumatra serves as a refugium for many Sundaic flora and fauna species (Woodruff 2010). Some of the world’s most critically endangered megafauna (Sumatran elephant (Elephas maximus sumatranus Temminck, 1847); Sumatran rhinoceros (Dicerorhinus sumatrensis Fischer, 1814); Sunda tiger (Panthera tigris sondaica Temminck, 1844) and Sumatran and Tapanuli orangutans ((Pongo abelii Lesson, 1827) and (Pongo tapanuliensis Nurcahyo, Meijaard, Nowak, Fredriksson & Groves, 2017)) are found here. Sumatra is also botanically diverse, with an estimated number of 10,600 plant species and more than 300 endemics (Roos et al. 2004). Moreover, the plant diversity of Sumatra is hypothesised to be as diverse as Borneo and much richer than the other neighbouring islands of Java and Sulawesi (Meijer 1981). However, floristic studies in Sumatra have been neglected in the past, with the mistaken assumption of being well documented due to its similarity with flora from the Malay Peninsula (Whitten 1984; Laumonier 1997). Recent discoveries in various taxa prove otherwise, sparking renewed interest in the island’s rich untapped diversity.

Lithocarpus Blume (Stone oaks) is the second largest genus in the Fagaceae family (Camus 1952–1954), with approximately 347 species recorded globally, including 32 found in Sumatra, of which five species are endemic to the island (POWO 2023; Strijk 2023). Species from this genus are commonly found throughout Sumatra, inhabiting many different attitudinal habitats, from lowland to montane forest (Laumonier 1997; Fujii et al. 2006). Fujii et al. (2006) found a great species diversity of Lithocarpus in Sumatra between 400 and 700 m above sea level, with several species having distributions limited to certain elevations. Along with several families like Lauraceae and Myrtaceae, Fagaceae are a major component of the lower tropical montane rainforest between 900 and1500 m above sea level (Cockburn 1972; Laumonier 1997). Species such as Lithocarpus pallidus (Blume) Rehder and Lithocarpus pseudomoluccus (Blume) Rehder often constitute the canopy layer of submontane forest in Sumatra (Laumonier 1997). Eight species of Lithocarpus were also recorded in Sumatra’s upper montane forest (1400–2500 m a.s.l.) by Fujii et al. (2006), further highlighting the flexibility of the genus in occupying different ecological niches.

South Tapanuli is one of the three forest blocks that make up the Batang Toru Ecosystem and is the last refuge for the recently described, Critically Endangered and extremely rare Tapanuli orangutans (Kuswanda et al. 2020). The land cover within the Batang Toru Ecosystem consists of a mosaic of mixed plantations and primary and secondary forests (Meylia and Mustari 2022). During a field survey conducted in South Tapanuli in February 2023, specimens of an unknown Lithocarpus were discovered. Further morphological comparisons with other relatives in Malesia clearly distinguish it as a new species due to its distinctive cupule morphology (Cockburn 1972; Soepadmo 2000; Phengklai 2008). Hence, we describe and name it as Lithocarpus tapanuliensis, providing a description, accompanied by photographs and a morphological comparison with closely-related species, as well as an exploration of its interactions with Tapanuli orangutans.

There are an estimated 32 species of Lithocarpus documented in Sumatra (Soepadmo 1972; Purwaningsih and Polosakan 2016; POWO 2023; Strijk 2023). Lithocarpus tapanuliensis sp. nov. is distinguished from other Lithocarpus in the region by its distinctly large acorn and a cupule covered in unique bullate protuberances and the distinct presence of a narrow ring of small denticulated plates around its rim. The possibilities of hybrids between stone oaks are rare because of the limited contact zone between closely-related taxa in our field sites (primary forest). A study in Mexico shows hybridisation of Quercus occurs in areas with high levels of disturbance (Tovar-Sánchez and Oyama 2004). Exposed and disturbed areas by humans may establish the hybrid zone (Howard et al. 1997; Tovar‐Sánchez and Oyama 2004) and enhance opportunities for contact and cross-pollination (Arnold et al. 1990; Klier et al. 1991).

The flora of Sumatra has garnered more interest over the last decade with a variety of plant species being described, like the iconic rafflesia (Susatya et al. 2017), enigmatic pitcher plants (Victoriano 2020), several begonias (Hughes et al. 2015) and a peculiar pipevine (Mustaqim and Arico 2022). To the authors knowledge, the discovery of L. tapanuliensis is the first new Sumatran Lithocarpus to be described in the last decade. This is a major contrast with other neighbouring botanical countries, in which a significant number of Fagaceae have been described in the last decade (e.g. Tan et al. (2023); Zhang et al. (2023)). The hotspots of Lithocarpus diversity are well-known to be in Indochina and NE Borneo, but the diversity of Sumatran Fagaceae has remained understudied. With further efforts in the future, the number of species confirmed for this island is expected to significantly increase.

During fieldwork, acorn remains of L. tapanuliensis and C. tungurrut consumed by Tapanuli orangutans were collected (Fig. 3A). It has been well documented that the acorns, leaves and bark of Fagaceae (i.e. Lithocarpus, Castanopsis and Quercus) are consumed by all three orangutan species (Russon et al. 2009; Kuze et al. 2011; Payne and Zainudin 2023). Fruit scarcity is a common occurrence in the seasonal Sundaic rainforest, due to the supra-annual fruiting cycle (within a period of 2–10 years) of many major tree families (e.g. Wong et al. (2005); Tan et al. (2021)). Although often overlooked, Sundaic Fagaceae are often a fall-back resource for many animals in periods of fruit scarcity, being one of the few families to have annual asynchronous fruiting (Araye et al. 2022). Besides the well-documented rodents, several large Asian megafaunal species, such as the Asian black bear (Ursus thibetanus Cuvier, 1823) and Malayan sun bear (Helarctos malayanus Raffles, 1821) (Wong et al. 2002; Fredriksson et al. 2006; Steinmetz et al. 2013) often consume fruits of Fagaceae, especially in times of famine. The great migrations of the bearded pig (Sus barbatus Müller, 1838) across Borneo are dictated by the fruiting of several species, including those from Fagaceae (Lusking and Ke 2017). The importance of Fagaceae as a food supply for many species in times of famine should warrant greater protection for the family throughout its range in Southeast Asia as the loss of matured Fagaceae would have detrimental trophic effects.

An orangutan nest was also observed in a neighbouring tree to L. tapanuliensis (Fig. 3B). Orangutans are known to make their nest close to food sources and can be very selective of the tree species used. Species from families like Fagaceae and Dipterocarpaceae, with dense branching and tall, straight boles with thick diameters, are often favoured by orangutans for nest building, as they are relatively stable and provide good vantage points across the canopy (Kuswanda et al. 2020; Patana et al. 2021; Meylia and Mustari 2022). However, orangutans will avoid building nests in fruiting trees to avoid disturbance from other animals that also used the same resources (Sugardjito 1983; Van Casteren et al. 2012). The ecological interactions we recorded in the field further highlight the importance of Fagaceae to orangutans. Hence, we strongly recommend future conservation plans in the region to incorporate the family, not only for orangutans, but also for the myriad of other species that rely upon it for resources.

We provide an initial IUCN conservation assessment for L. tapanuliensis as Critically Endangered, as only two individuals were recorded in a small section of South Tapanuli, specifically in Pilar Forest (West Block of Batang Toru Ecosystem). The Batang Toru ecosystem suffers from habitat fragmentation and habitat loss due to massive infrastructure projects, such as mining, agroforestry plantations and hydropower in important corridor areas (e.g. Rahman et al. (2019)). Most of the Batang Toru area has been gazetted as protected forest, but some key forest areas are still unprotected (Fredriksson 2017; Sloan et al. 2018; Rahman et al. 2019). Recent infrastructure development near the study area could further fragment important habitats, increasing the risk of damaging the unique biodiversity found within this landscape. Conserving the remaining forest within the Batang Toru ecosystem is important to safeguard orangutans and many other mammals that depend on seasonal fruit production in highland and lowland forest areas of Batang Toru (Buij et al. 2002).

Our findings make an important contribution to the discovery of new Fagaceae species, highlighting the importance of preserving Indonesia’s unique habitats of the Batang Toru landscape. A comprehensive understanding of the species, along with further surveys and spatial distribution analysis, is crucial for protecting against potential extinction. Future strategies must focus on the long-term survival of the species through ex situ conservation in suitable habitats combined with in situ conservation efforts (Volis and Blecher 2010; Harapan et al. 2022).

We are grateful to Diansyah and Edi for their help in collecting the specimens in the field. We thank Nazifah Rahmi for preparing and digitising the specimens. We thank Ahimsa Campos-Arceiz for comments on the early draft. We sincerely appreciate three anonymous reviewers and subject editor Dr. Norbert Holstein for their constructive comments and insightful suggestions.