The Southeast Asian Journal of Tropical Biology Vol. 31 No. 2, 2024: 291 - 307 DOI: 10.11598/btb.2024.31.2.2023 ISSN: 0215-6334 | e-ISSN: 1907-770X 291 TROPICAL PALMS AND FACTORS INFLUENCING THEIR DISTRIBUTIONS AND DIVERSITY, WITH A FOCUS ON Pinanga DIVERSITY IN SOUTHEAST ASIA Mohamad Farid Abdullah1, Salwana Md Jaafar2, Norhayati Ahmad1,2, Rahayu Sukmaria Sukri2 1Environmental and Life Sciences Program, Faculty of Science, Universiti Brunei Darussalam, Bandar Seri Begawan, Brunei Darussalam. 2Institute for Biodiversity and Environmental Research, Universiti Brunei Darussalam, Bandar Seri Begawan, Brunei Darussalam ARTICLE HIGLIGHTS • The palm genus Pinanga is the most diverse in Southeast Asia. • Limited ecological studies on Pinanga show the influence of environmental factors. • High Pinanga endemism in Borneo requires conservation action. Article Information Received: 11 May 2023 Revised: 13 July 2023 Accepted: 14 July 2024 *Corresponding author, e-mail: rahayu.sukri@ubd.edu.bn Review Article ABSTRACT Palms represent some of the keystone species in tropical forests, providing numerous ecosystem services. They are widely studied by botanists because of their iconic and majestic appearance, although ecological studies of palm abundance, distributions, and diversity have only recently begun to gain attention. The abundance and diversity of palms at different spatial scales can be influenced by various factors, such as climate, soil properties, hydrology, topography and forest structure; understanding these influences is essential for conservation. This review details four abiotic factors (climate, soil chemistry, hydrology and topography) and one biotic factor (forest structure) affecting palm distribution and diversity. The genus Pinanga, one of the most diverse genera of palms, is discussed in terms of its distribution and diversity in Southeast Asia. Ecological studies on Pinanga diversity in the region are examined, revealing the influence of micro- and macro-scale variation in environmental factors, such as litter depth and thickness, canopy openness and crown cover, elevation, slope, aspect, light intensity, humidity and air temperature. The existing knowledge gaps on Pinanga ecology in Southeast Asia are discussed, and the need for more studies on population structures, functional traits and determinants of Pinanga diversity and distributions within different lowland forest types in the region are highlighted. The high endemism shown by Pinanga necessitates a more comprehensive ecological understanding of this genus to better inform its conservation and protection. Keywords:  borneo, environmental factors, monocots, palmae, plant distributions INTRODUCTION Palms (Arecaceae or Palmae) are among the most abundant and important plant life in tropical rainforests (Dransfield  et al. 2008; Browne & Karubian 2016; Elias  et al. 2019). They represent one of the oldest surviving monocot families, maintaining their presence in tropical rainforest-like biomes over geological timespans (Renninger & Phillips 2016). Palms occur in tropical and subtropical regions worldwide and are found in a wide range of ecosystem types, including rainforests, montane regions, dry forests, savannas and desert oases (Martins et al. 2014; Abdo 2017). In tropical forests, palms are remarkably majestic and can form either dense monodominant palm communities or species- rich communities with many forms of growth (Thonhofer et al. 2015; Elias et al. 2016; Balslev et al. 2016; Mehmud & Roy 2021). Palms display a variety of growth forms, including small shrubs, lianas and large trees (Muscarella et al. 2020). It is estimated that 40% of palm species are capable of growing stems of at least 10 cm diameter at 1.3 m above the ground (Kissling et al. 2019). Palms can take the form of tall trees entirely by primary growth because they lack secondary growth and vascular https://doi.org/10.11598/btb.2024.31.2.2023 BIOTROPIA Vol. 31 No. 2, 2024 292 cambium (Goodman et al. 2013). Some palm trees form thick and shallow root systems that provide better support than those of deep-rooted palm trees (Muscarella et al. 2020).  Palms are important to tropical ecosystems because they offer various ecosystem services (Cámara-Leret  et al. 2017a; Levis  et al. 2017; Muscarella et al. 2020; Montoya et al. 2021) and play an important role in forest structure and function (Boukili & Chazdon 2017). Within tropical ecosystems, palms are notable for their high biomass, effects on plant recruitment, interactions with other plants as well as dispersers, and pollinator and nutrient turnover (Montúfar et al. 2011; Sørensen 2012).  Palms are often considered keystone species in tropical forests due to the dependence of many animals on their flowers and fruits for food (Onstein et al. 2017; Macedo- Santana  et al. 2021). Palms have also functioned as a model system for geographical ecology and rainforest evolution (Kissling et al. 2012). With more than 2,500 recognized species in 183 genera worldwide, palms are one of the most diverse groups of plants (Eiserhardt et al. 2011; Baker & Couvreur 2012; Kissling et al. 2012; Göldel et al. 2015; Baker & Dransfield 2016; Cámara-Leret et al. 2017a; Levis et al. 2017; Muscarella et al. 2020). Palms are common and characteristic elements of tropical forests across the equatorial region (Dransfield et al. 2008; Svenning et al. 2008; Baker & Couvreur 2013; Reichgelt et al. 2018; Bogota- Angel et al. 2021). Palm diversity is the highest in tropical Asia, followed by the Americas and Africa (Blach-Overgaard  et al. 2010; Muscarella  et al. 2020). Approximately half of all palm species (c. 1,200 species in 57 genera) have been recorded in tropical Asia, most of which are found in Malesia, with almost 1,000 species in 50 genera (Dransfield  et al. 2008; Renninger & Phillips 2016). Although Malesia is known as the greatest palm diversity hotspot, there is a differentiation in species richness and distribution patterns across the region (Baker & Couvreur 2012). High palm species richness was recorded in both western (302 species in Borneo, 162 species in the Philippines) and eastern Malesia (243 species in New Guinea; Baker & Couvreur 2012). In contrast, lower species richness has been recorded in Wallacea, which comprises Sulawesi, the Moluccas and the Lesser Sunda Islands, with estimates of at least 62, 40 and 6 species, respectively (Baker & Couvreur 2012). Palm abundance and diversity can have significant effects on tropical forest ecosystem functions as well as carbon sequestration (Muscarella  et al. 2020). Palms are widely accepted as suitable model organisms for understanding the influence of high tropical biodiversity and geographical variation (Eiserhardt et al. 2011; Couvreur & Baker 2013). Therefore, understanding the diversity, distribution and dispersal of palms is integral to their conservation (Dransfield et al. 2008; Eiserhardt et al. 2011; Bacon et al. 2013; Abdo 2017). However, quantitative analyses of the distribution patterns and factors associated with palm abundance and diversity are still lacking (Muscarella et al. 2020). Among the palms, the genus Pinanga is one of the most diverse (Govaerts et al. 2020) and has recently been gaining attention in scientific studies (Mooney 2020; Haji Ramlan 2020; Abdullah 2021). This review focuses on the factors influencing palm abundance and species diversity, as well as the diversity of Pinanga palms in Southeast Asia. FACTORS INFLUENCING PALM ABUNDANCE AND SPECIES DIVERSITY Global patterns in palm distributions are mainly limited by low temperatures (Moraes et al. 2014). An increase in temperature can extend palm distributions to higher elevations where precipitation can then exert a significant positive effect on palm species richness (Renninger & Phillips 2016). Several studies have reported that palm abundance and species richness are strongly correlated with environmental factors (Kahn 1987; Cintra  et al. 2005; Eiserhardt  et al. 2011; Kristiansen et al. 2011; Manggat 2012; Emilio et al. 2014; Scariot 2015; Benchimol  et al. 2017; Elias et al. 2019), and palm species distributions on continental and regional scales are associated with changes in climate, soil properties and dispersal (Blach‐Overgaard  et al. 2010). At smaller scales, palm distributions are associated with topography, hydrology, vegetation structure, species interactions and dispersal, but the relative importance of these factors differs between studies (Vormisto et al. 2004; Blach‐Overgaard et al. 2010; Eiserhardt et al. 2011; Sørensen 2012; Alvaro-Segura et al. 2012). Here, we review four abiotic factors (climate, soil chemistry, hydrology and topography) and one biotic factor (forest structure) that influence palm abundance and species diversity (Eiserhardt et al. 2011). Tropical palms and factors influencing their diversity, with a focus on Pinanga in Southeast Asia - Abdullah et al. 293 Climate Palm species distribution studies have primarily focused on the role of the environment, with climate often regarded as the main range-limiting factor, especially at large spatial scales (Blach‐ Overgaard et al. 2010). Climate plays a crucial role in global palm distribution (Muscarella et al. 2020; Bogota-Angel  et al. 2021), and the distribution of individual palm species and patterns of palm species richness and diversity across different vegetation types are strongly associated with climate (Svenning  et al. 2008; Eiserhardt  et al. 2011; Sevegnani et al. 2016). Previous studies also observed that climate is important in determining plant distribution and diversity (Pearson & Dawson 2003; Kreft & Jetz 2007). Palm diversity hotspots, such as the Sunda region, offer beneficial conditions for diversification in palms due to the prolonged periods of climatic stability, while poor palm diversity can be observed in the Lesser Sunda Islands due to their moderately dry, seasonal climate (Baker & Couvreur 2012). Palm diversity is strongly correlated with climatic factors, especially water (Bjorholm  et al. 2005), with precipitation having the strongest positive effect on palm species richness (Renniger & Philips 2016). Similarly, climate has the greatest influence on the distributions of palms, which appear to be more significantly influenced by water-related variables than by temperature (Blach‐Overgaard  et al. 2010; Sørensen 2012). Additionally, palm species richness is influenced by annual precipitation or the number of wet days (Sørensen 2012). This supports a broad-scale study of species richness conducted by Hawkins  et al. (2003), who concluded that water-related factors, such as rainfall and precipitation, are excellent drivers of plant richness in tropical and subtropical areas. Blach-Overgaard et al. (2010) concluded that climate was more important than habitat and human impact in determining the distribution of 25 out of 29 African palm species, with most of the palm species studied preferring humid climates, and few being influenced by temperature. In addition, palm species distributions within subtropical and temperate regions are often restricted due to low temperatures (Gatti et al. 2008). Therefore, palms are most diverse in warm and humid regions (Eiserhardt et al. 2011). Studies focusing on the influence of climatic variables on palm species richness and diversity are limited (Dransfield et al. 2008), with the influence of climatic factors on palm species distributions within tropical regions evaluated for African palms only (Blach-Overgaard  et al. 2010). Temperature seasonality has been observed to limit palm distributions and emphasizes climate as the main range determinant at large spatial scales (Pearson & Dawson 2003). However, it is increasingly understood that palm species distributions are sensitive to climate at multiple spatial scales (Blach‐ Overgaard et al. 2010), and therefore, more studies at both local and global scales are recommended (Eiserhardt et al. 2011).  Soil Properties Most tropical plant species show strong associations with edaphic conditions, and soil fertility can influence tree community composition (John et al. 2007; Sukri et al. 2012). Palms also show habitat associations governed by soil properties (Vormisto et al. 2004; Andersen et al. 2010; Viana  et al. 2021). Soil fertility is the second most important abiotic factor influencing palm species richness (Bjorholm  et al. 2006). For example, the diversity of palm subfamilies Arecoideae and Calamoideae tends to increase with increasing soil fertility (Bjorholm et al. 2006). Similarly, the link between soil properties and palm community composition in lowland rainforests is well studied (Vormisto  et al. 2004; Andersen  et al. 2010), and palm community compositions are linked with soil properties, such as exchangeable base concentration, aluminum concentration and soil texture (Cámara-Leret et al. 2017b). Studies have shown that soil chemistry can affect palms species’ abundance and richness (Ruokolainen & Vormisto 2000; Svenning 2001; Bjorholm et al. 2006; Poulsen et al. 2006; Eiserhardt et al. 2011; Muscarella et al. 2020). The distribution and abundance of several tropical American palm species are correlated with soil factors, such as nutrient concentrations, aluminum content and clay (Svenning 2001). Palm species in the western Amazon prefer either nutrient-poor or nutrient- rich soils (Ruokolainen & Vormisto 2000), and the composition of Amazon palm communities is significantly correlated with soil chemical properties, such as exchangeable cations and aluminum content (Poulsen et al. 2006; Dalling et al. 2016; Viana  et al. 2021). Macronutrients, such as exchangeable bases (calcium, potassium and magnesium) and phosphorus, were found to influence species abundances in palm communities in non-flooded Amazonian forests more BIOTROPIA Vol. 31 No. 2, 2024 294 significantly than micronutrients did (Cámara et al. 2017b). Similarly, exchangeable bases, phosphorus and nitrogen concentrations have been linked to the distribution of palms in various lowland tropical rainforests (Andersen et al. 2010; Baribault  et al. 2012; Condit  et al. 2013). Soil bulk density can also influence the composition of palm communities (Andersen et al. 2010; Viana et al. 2021), and palms can show variation across gradients of soil nutrient availability (Viana et al. 2021). Palm species composition can also vary with soil texture on local scales, likely due to a soil texture correlation with drainage (Kristiansen et al. 2012; Sørensen 2012). In contrast, some studies have also found that differences in soil properties may not be important in influencing individual palm species’ distributions at continental scales, and not all studies support a significant role for soil fertility in palm community turnover (Normand et al. 2006; Blach-Overgaard et al. 2010). Therefore, the strength of soil effects on palm community composition is variable and may depend on the spatial scale, ecosystem and habitat (Eiserhardt et al. 2011; Viana et al. 2021). Moreover, soil chemistry often interrelates with other environmental factors, such as topography, hydrology and vegetation structure, which requires further study (Eiserhardt et al. 2011). Hydrology Plant species distributions and diversity are also affected by hydrology, particularly through flooding and drainage (Eiserhardt  et al. 2011). As precipitation is one of the most important predictors of palm species richness (Kissling et al. 2012), this correlates with a higher abundance of palms in continually wet sites (Viana et al. 2021). The importance of hydrology for palm species distribution has been well demonstrated for Amazonian palms (Kahn 1987). Several studies have assessed the effects of flooding and drainage on the distribution and abundance of individual palm species (Svenning 2001; Montúfar & Pintaud 2006). Most palms are linked with either well-drained or poorly-drained soils (Henderson et al. 2019), while some palm species prefer swampy areas (Dransfield et al. 2008). For instance, most of the African palms occupy humid rainforests or swamplands, and are often in the riparian zone or areas where the water table is locally high, while very few species are found in dry and open habitats, such as savannas or deserts (Blach‐Overgaard  et al. 2010). Palm species abundance in French Guiana is correlated with a change in soil drainage along topographic transitions (Salm  et al. 2015). Additionally, Salm  et al. (2015) found that the abundance of the six most common palm species was correlated with humidity gradients from floodplains to terra firme, with palm distribution from the most flood-tolerant to the least flood- tolerant palm species.  Differences in soil moisture and drainage have been shown to affect palms individually (Svenning 2001). For example, five out of 23 palms responded individually to poor drainage in the Amazonian terra firme forest (Svenning 1999). Significant differences in palm community composition between well-drained and poorly-drained soils have been recorded (Balslev et al. 1987; Kahn 1987; Normand et al. 2006). For instance, palms in the terra firme (dry land) consisted of 17% of individuals and 11% of the basal area, compared with 30% of individuals and 19% of the basal area in the floodplain forest in Ecuador (Balslev et al. 1987). Palm species richness in seasonally flooded land and terra firme forests in the Amazon basin was found to be three times higher on well-drained soils (terra firme) than on seasonally flooded soils (Kahn & de Castro 1985). Similarly, in the western Amazon, higher species richness was recorded for palm communities in terra firme forests than in wetland forests (Eiserhardt  et al. 2011), and in Ecuador, palm species were found to be significantly more abundant in terra firme forests than in floodplain forests (Balslev  et al. 1987). Although flooding and drainage have strong effects on palm distribution on local to landscape scales, as well as on palm species richness on a landscape scale (Eiserhardt et al. 2011), the degree to which these effects are influenced by other factors, such as vegetation structure and light availability, requires further investigation (Svenning 2000). Topography Topography strongly affects the distribution of tropical plant species (Sukri et al. 2012; Wang et al. 2015; Limin et al. 2022) and is known as a key influencing factor for vegetation patterns at the landscape and regional scales (Sanders & Rahbek 2012; Moeslund et al. 2013). Plant species diversity is often related to topographic heterogeneity at broader scales (Kreft & Jetz 2007). Topography indirectly affects plant distributions by regulating other abiotic factors, such as soil conditions, wind exposure, hydrology, temperature, climate and forest structure (Svenning 2001; Balslev  et al. 2011; Eiserhardt et al. 2011; Lueder 2020). Tropical palms and factors influencing their diversity, with a focus on Pinanga in Southeast Asia - Abdullah et al. 295 Palm species abundance, richness and composition have a significant relationship with altitude (Rodrigues  et al. 2014). Palms decrease in abundance with increasing soil moisture and decreasing elevation, indicating that elevation and soil moisture influence palm community patterns (Bonetti  et al. 2017; Lueder 2020). Kreft  et al. (2006) found that rare palm species were more strongly influenced by topographic complexity, while climatic factors were strongly correlated with species richness for widespread species. At landscape scales, topography shows a correlation with palm abundance (Eiserhardt  et al. 2011). For instance, the abundance of canopy palms in Costa Rica and wet Amazonian forests was the highest on crests and steep slopes than at lower altitudes (Kahn 1987). In a seasonally dry forest in southern Amazonia, palm abundance decreased with elevation (Salm et al. 2007), while in Costa Rica, palms are abundant at lower elevations (approx. 2,500 m altitude; Lieberman et al. 1996). Eiserhardt et al. (2011) suggested that high palm abundance in steep and rugged terrain was due to a high number of canopy gaps that were favorable for palm recruitment, while the lower abundance of palms on steep slopes compared with crests may be due to the high water overflow causing those sites to be extremely dry for palms (Kahn 1987). Additionally, Salm  et al. (2007) showed that the high abundance of palms in low-lying areas was due to high moisture availability. At a local scale, palm species abundance differed independently of topographic position in both Amazonian lowland rainforests and seasonally dry forests (Kahn 1987; Salm et al. 2007). In Amazonian Ecuador, 10 out of 23 palm species were related to specific topographic positions, while altitude and inclination tended to be less important; the opposite pattern was seen in the wet lower montane forest in Ecuador, where altitude and aspect strongly influence palm species distributions (Svenning 1999, 2001). In terms of species richness, both palm species richness and genus richness decrease significantly with elevation in New Guinea and its adjacent islands (Bachman et al. 2004). However, topography may be less influential on palm species richness at a continental scale in the Americas (Bjorholm et al. 2005; Kreft et al. 2006). On a landscape scale, palm species richness tends to decline with elevation in the Amazon rainforest (Salm  et al. 2007) and along an altitudinal transect in Costa Rica (Lieberman  et al. 1996). In contrast, Poulsen  et al. (2006) found that the species richness of palms was the highest at the highest elevations in terra firme Amazonian rainforest. Overall, topography is important at local scales but may be less important in influencing palm species richness at broader spatial scales (Bjorholm et al. 2005; Kristiansen et al. 2011). Topography only influences palm species distributions and community composition indirectly; thus, its effects will vary for local scale differences depending on the geographical features of each location (Vormisto et al. 2004). It is also suggested that the relationship between diversity and elevation depends fundamentally on the interaction of environmental variables (Rahardjo 2020). Consequently, it is important to measure other environmental factors, such as soil water availability, soil nutrient conditions, hydrology, wind exposure and light availability, which are factors that directly underlie topographic gradients (Svenning 2001; Eiserhardt et al. 2011), and to determine how topographic variables can influence the population dynamics of palms (Pasion  et al. 2022). Forest Structure Forest structure reflects the role of plants and specific trees as ecosystem engineers that can influence environmental factors, such as light availability and litter fall (Svenning 2001). The surrounding vegetation and canopy gaps can influence microclimate and light availability (Carson & Schnitzer 2011), and light influences species distribution through competition for light and through shade tolerances (Carson & Schnitzer 2011). In palm communities, several palm species favor an open canopy due to light availability (Eiserhardt  et al. 2011). For example, rattan occurrence is positively correlated with the presence of treefall gaps (Siebert 2005). In addition, Svenning (2000) found that several palms prefer gaps and are distributed non-randomly with canopy openness. The importance of light on palms may differ between ontogenetic stages (Svenning 2001). Previous studies have suggested that large palms can reach the adult stage only in treefall gaps due to their light requirements increasing at the adult stage (Eiserhardt et al. 2011) There are no strong relationships between canopy heterogeneity or light availability and palm community composition (Eiserhardt  et al. 2011). For example, canopy openness and the presence of non-palm trees in Amazonian terra firme rainforest does not significantly influence palm species richness (Cintra et al. 2005). BIOTROPIA Vol. 31 No. 2, 2024 296 However, the community composition of palms is associated with forest structure (Browne & Karubian 2016), where adult palm abundance and species richness tend to increase with canopy height, while juvenile abundance decreases with canopy openness. Juvenile species richness was also found to increase with distance to the forest edge and decrease with canopy openness (Browne & Karubian 2016). Hilário and Toledo (2016) observed that forests with taller and larger trees, a closed canopy and an open understory contain higher palm abundance. However, Eiserhardt  et al. (2011) argued that the influence of vegetation structure on the species richness of palms may be weak, as only a few palm species respond to canopy openness or light availability, likely due to species- specific preferences. GENUS Pinanga The genus  Pinanga  Blume (1838) is a genus of palms in the family Arecaceae (Syauqina  et al. 2017; Randi  et al. 2019) and is one of the most diverse genera of palms (Govaerts  et al. 2020). Arecaceae are persistent climbers, bushes and trees (Mainasara  et al. 2019). The growth forms of  Pinanga  are varied, including shrubs and tree-like or stemless plants (Dransfield et al. 2008). The genus shows significant differences in morphological and quantitative characteristics, such as size, form, stem color, crown shaft, leaves and pollen (Randi et al. 2019). Pinanga palms can be distinguished from other palms by the entire margins of the blades of their leaves or leaflets, and by their inflorescences, flower arrangements and fruits (Ang et al. 2010). Pinanga can be found in a variety of habitats because each species typically prefers different environments (Dransfield 1980; Fernando et al. 2020). For example, Pinanga simplicifrons (Miq.) Becc. is found in shaded and swampy areas (Ang et al. 2010), while Pinanga javana Blume prefers hill slopes (Zulkarnaen et al. 2019). Some Pinanga species prefer higher elevations (Syahida-Emiza et al. 2018; Fernando et al. 2020). Local communities often use Pinanga palms as tools and sources of food. For example, the stalk of Pinanga auriculata Becc. is used to make fish traps, while edible fruit from Pinanga patula Blume var. microcarpa Becc. can be eaten with betel (Haji Ramlan 2020). In Bali (Indonesia), Pinanga arinasae Witono (locally known as Nyabah or Jabah) leaves, fruits as well as stems are culturally important for Balinese people (Sutomo et al. 2018). Local people mostly use P. arinasae for ornamental purposes (Syauqina et al. 2017; Sutomo et al. 2018), such as the production of traditional umbrellas called ‘Cukup’ from the P. arinasae leaf sheath (Sutomo et al. 2018). The extract from Pinanga limosa Ridl. may function as a potential anticancer agent, although its efficacy requires verification (Mainasara et al. 2019). Distribution of Pinanga Species Currently, a total of 145 species of Pinanga have been recorded worldwide (POWO 2024) and their distributions range from the Himalayas and South China to New Guinea, with high diversity at the Sunda Shelf, while they are poorly represented in Papuasia (Coode et al. 1996; Randi et al. 2019). Within Southeast Asian tropical rainforests, Pinanga  typically displays prominent levels of endemism and high species diversity (Dransfield et al. 2008). The Southeast Asian region is the center of Pinanga diversity worldwide, with 140 species of Pinanga recorded (Govaerts et al. 2020; Zulkarnaen  et al. 2022a). The highest Pinanga species richness has been recorded in Malaysia and Indonesia (61 and 49 species, respectively), while Cambodia and Laos are the least species- rich countries (1 species each; Fig. 1). No records of Pinanga species have been published to date for Timor-Leste. These differences in species richness of Pinanga in the Southeast Asian countries partly reflect the larger amounts of sampling effort and studies that have been conducted in Malaysia and Indonesia compared to countries, such as Cambodia, Laos and Timor-Leste. As studies on palm communities have gained increasing attention in the past ten years,  Pinanga  palms have been discovered in various locations in Southeast Asia during this period (Ang et al. 2010; Loo et al. 2014; Syauqina et al. 2017; Randi  et al. 2019; Fernando  et al. 2020; Lim  et al. 2022). In Singapore, Ang et al. (2010) rediscovered P. simplicifrons, which was widely regarded as nationally extinct in Singapore before this rediscovery. P. simplicifrons  was also rediscovered near Lornie Trail at the edge of MacRitchie Reservoir, Singapore (Lim et al. 2022). These rediscoveries in specific localities have led to the assessment of P. simplicifrons as ‘critically endangered’ in Singapore (Ang et al. 2010; Loo et al. 2014; Lim  et al. 2022), although it remains unassessed in the IUCN Red List (IUCN 2024). In Malaysia, the Pinanga genus was the third most dominant palm genera recorded in the Dered Tropical palms and factors influencing their diversity, with a focus on Pinanga in Southeast Asia - Abdullah et al. 297 Figure 1 Variation in species richness of Pinanga across Southeast Asia Notes: The total number of Pinanga species recorded in each country was compiled from Coode et al. (1996), Ang et al. (2010) and POWO (2024). Krian National Park after the genera Calamus and Daemonorops, while Pinanga chaiana J.Dransf., Pinanga crassipes Becc. and Pinanga jambusana C.K.Lim were found scattered along trails in this national park (Syauqina  et al. 2017). During an expedition to Long Banga, Sarawak, at least 11 palm species were found at higher ridges, including an unidentified species of  Pinanga  (Syahida- Emiza  et al. 2018). In Indonesia, P. arinasae was found in the Pilan Forest in Bali (Wahyuni  et al. 2017), although the species was previously suggested to be present only in Mt. Tapak Bedugul, Bali (Witono et al. 2002; Sutomo et al. 2018). Pinanga schwanerensis Randi, Hikmat & Heatubun was newly discovered in Kalimantan, growing on undulating grounds and steep slopes (Randi  et al. 2019). Pinanga  lepidota Rendle was rediscovered on Palawan Island in 2020, increasing the number of Pinanga  species recorded in the Philippines to 27 (Fernando et al. 2020). In Vietnam, Pinanga spiralis A.J.Hend. & N.Q.Dung was discovered by Henderson and Dung (2017). Within Borneo, a total of 41 species of Pinanga have been recorded (Table 1), mostly from Sarawak, Brunei Darussalam and Sabah (Odufuwa 2019; Govaerts et al. 2020; POWO 2024). Fewer records of Pinanga have been found in Kalimantan, Indonesian Borneo, despite the larger land area and greater habitat diversity (Randi et al. 2019; Odufuwa 2019). The higher species richness in Malaysian Borneo likely partly reflects greater sampling efforts compared to Kalimantan. Notably, despite having the smallest land area within Borneo, 23 species of Pinanga have been recorded in Brunei Darussalam. Almost all of the Pinanga species recorded in Borneo are endemic to the island (POWO 2024), with the exception of three species (P. auriculata, P. patula and P. simplicifrons). BIOTROPIA Vol. 31 No. 2, 2024 298 Table 1 Checklist of Pinanga species recorded in Borneo No. Species Brunei Sabah Sarawak Kalimantan 1 Pinanga albescens Becc.   X X X 2 Pinanga angustisecta Becc. X X X   3 Pinanga aristata (Burret) J.Dransf. X X X X 4 Pinanga arundinacea Ridl.   X X   5 Pinanga auriculata Becc. X X X   6 Pinanga borneensis Scheff. X X X X 7 Pinanga brevipes Becc. X X X X 8 Pinanga capitata Becc. X X X X 9 Pinanga chaiana J.Dransf. X X X   10 Pinanga crassipes Becc.   X X   11 Pinanga cucullata J.Dransf.     X   12 Pinanga decora L.Linden & Rodigas   X X   13 Pinanga dumetosa J.Dransf. X X X   14 Pinanga gracillima Merr.   X X   15 Pinanga jambusana C.K.Lim     X   16 Pinanga keahii Furtado   X X X 17 Pinanga lepidota Rendle X X X X 18 Pinanga ligulata Becc.   X X X 19 Pinanga limbangensis C.K.Lim     X   20 Pinanga minuta Furtado. X X X   21 Pinanga mirabilis Becc. X X X X 22 Pinanga mooreana J.Dransf. X   X   23 Pinanga pachyphylla J.Dransf.     X   24 Pinanga patula Blume X X X X 25 Pinanga pilosa (Burret) J.Dransf.   X X   26 Pinanga ridleyana Becc. X X X X 27 Pinanga rivularis Becc. X   X   28 Pinanga rupestris J.Dransf.     X   29 Pinanga salicifolia Blume X X X X 30 Pinanga schwanerensis Randi, Hikmat & Heatubun       X 31 Pinanga sessilifolia Furtado X X X X 32 Pinanga simplicifrons (Miq.) Becc. X X X X 33 Pinanga stricta Becc.   X X   34 Pinanga subterranea Randi & W.J.Baker     X X 35 Pinanga tenacinervis J.Dransf.     X   36 Pinanga tenella (H.Wendl.) Scheff. X X X X 37 Pinanga tomentella Becc. X X X X 38 Pinanga trichoneura Becc.   X X   39 Pinanga variegata Becc. X X X X 40 Pinanga veitchii H.Wendl. ex H.J.Veitch X   X   41 Pinanga yassinii J.Dransf. X         Total number of species 23 29 39 19 Notes: Presence of each palm species in a country or state in Borneo was indicated by an ‘X’. All species are Bornean endemics, with the exception of three species highlighted in bold. Tropical palms and factors influencing their diversity, with a focus on Pinanga in Southeast Asia - Abdullah et al. 299 Ecological Studies on Pinanga in Southeast Asia In Southeast Asia, most published studies on Pinanga palms have focused on checklists and descriptive notes on  the Pinanga  species found within a country (Ang et al. 2010; Henderson et al. 2010; Adorador et al. 2020; Fernando et al. 2020). Despite increased attention on documenting Pinanga diversity within Southeast Asia, studies focusing on Pinanga ecology remain limited. Similarly to other palms, Pinanga distributions are often shaped by both micro- and macro-scale variation in environment. Within Southeast Asia, studies have shown the influence of a variety of environmental factors, such as litter depth and thickness, canopy openness and crown cover, elevation, slope, aspect, light intensity, humidity and air temperature (Syauqina et al. 2017; Astuti et al. 2018; Randi et al. 2019; Zulkarnaen et al. 2019, 2022a,b; Iryadi & Sutomo 2020; Yudaputra et al. 2021). A study by Yudaputra et al. (2021) investigating the ecology of endemic P. arinasae in Bali showed that litter depth, canopy openness, elevation, slope and aspect were the most important environmental factors influencing this species’ occurrence. In addition, microclimatic conditions, such as light intensity, humidity and air temperature, were recorded as significant influencing factors, with light intensity considered the main factor influencing the distribution and abundance of P. arinasae (Iryadi & Sutomo 2020). Similarly, the population structure and habitat preferences of P. javana  at Mt. Slamet (Central Java, Indonesia) were investigated (Zulkarnaen et al. 2019). The study showed that the population of P. javana was dominated by adult palms on the southern slope of Mt. Slamet and that slope, litter thickness and crown cover influenced the species’ density. Pinanga palms are also frequently found in humid areas covered by high canopy cover, near small rivers and in soil with thick litter layers, as well as in limestone areas (Syauqina et al. 2017; Randi et al. 2019). Another study by Zulkarnaen et al. (2022b) found that Mt. Slamet was dominated by P. javana and Pinanga coronata Blume, with P. javana found above waterfall areas or on hill slopes. The growth of P. javana in Mt. Slamet appears to require specific abiotic factors, such as slope, litter thickness and canopy cover (Astuti  et al. 2018).  In contrast, P. coronata individuals were recorded under the heavy shade of the canopy (Zulkarnaen et al. 2022a). In Mt. Ungaran (Central Java), P. javana was found Figure 2 Selected Pinanga species found in Brunei Darussalam: A. Pinanga aristata (Burret) J.Dransf.; B. Pinanga aff. brevipes Becc.; C. Pinanga mooreana J.Dransf.; D. Pinanga lepidota Rendle; E. Pinanga veitchii H.Wendl. ex H.J.Veitch; F. Pinanga mirabilis Becc. and G. Pinanga chaiana J.Dransf. A B C D E F G BIOTROPIA Vol. 31 No. 2, 2024 300 to be strongly correlated with a stable and humid microclimate, similarly to the P. javana found in Mt. Slamet (Zulkarnaen et al. 2019, 2022b). This suggests that humidity levels can significantly influence P. javana populations. Furthermore, P. javana  seems to prefer locations with steep to very steep slopes (Zulkarnaen et al. 2022a). In Kalimantan, Randi et al. (2019) found that P. schwanerensis  grows on uneven terrain and slopes between 150-550 masl (meters above sea level), and prefers a humid area, high forest cover and thick leaf litter, as well as locations near streams. Other studies, such as those in the Philippines, Vietnam and Singapore, provide little information on the ecology of Pinanga palms, typically only including information on their altitude occurrences and locality (Ang  et al. 2010; Henderson & Dung 2017; Fernando et al. 2020; Lim et al. 2022). For instance, on Palawan Island, P. lepidota is found in Dipterocarp forest at an elevation of 600 m, while P. lepidota in Borneo can be found up to an elevation of 1,500 m (Fernando et al. 2020). P. spiralis in Vietnam is found in lowland rainforests at elevations between 200-550 m (Henderson & Dung 2017). P. simplicifrons  favors an open area caused by tree falls, and under high canopy cover (Lim et al. 2022), and can also survive in swampy areas with rich alluvial soil (Ang et al. 2010). In Brunei Darussalam, a total of 23 species of  Pinanga  have been recorded (Fig. 2), and most of these were recorded in Ulu Temburong National Park (Coode et al. 1996; Haji Ramlan 2020). These Pinanga species are found in different habitats, from gentle to steep slopes, sandy to clay soil, and at various altitudes; additionally, most of the species are shrubs and are found near running streams (Coode et al. 1996). This indicates that Pinanga may have specific preferences for soil and environmental conditions.  Several recent studies have focused on the ecology of Pinanga  palms in Brunei Darussalam. The distributions of Pinanga species in Brunei Darussalam are shaped by many factors, such as topographic positions, vegetation structure and soil properties. Shapcott et al. (2022) investigated the populations of five co-occurring  Pinanga  species at Kuala Belalong, Ulu Temburong, and found that the population sizes of most of these species were maintained 20 years later, with two species (P. brevipes  and  P. aristata) declining in overall abundance. They attributed the decline of understory Pinanga palms to the possible influence of climate change due to increased rainfall fluctuations at Kuala Belalong (Shapcott et al. 2022). Also at Kuala Belalong, Mooney (2020) investigated the landscape distribution of Pinanga and concluded that one group of  species (P. mooreana and P. minuta) was strongly associated with ridges, while a second group (P. aristata, P. brevipes, P. dumetosa and P. veitchii) was more strongly associated with valleys. The complex topography of the Kuala Belalong area (Sukri et al. 2012) appears to have created a range of microhabitats that enabled Pinanga species to coexist within the landscape. At two contrasting mixed Dipterocarp forest sites in Brunei Darussalam (Belalong and Teraja), Haji Ramlan (2020) recorded the influence of soil properties on Pinanga species richness, with higher Pinanga species richness recorded in Belalong due to higher concentrations of soil nutrients (total P, Mg, Ca, total and exchangeable K) and higher soil organic matter content, soil pH and soil gravimetric water content at this site. Abdullah (2024) recorded distinct Pinanga community composition between heath and mixed Dipterocarp forest types in Brunei Darussalam, with five Pinanga species (P. aristata, P. chaiana, P. mooreana, P. variegata var. hallieriana and P. veitchii) more strongly linked with the mixed Dipterocarp forest sites and two Pinanga species (P. lepidota and P. salicifolia) more strongly linked with the heath forest sites. Focusing specifically on the foliar nutrient content of  Pinanga  species, Abdullah (2021) examined its variation in three Pinanga species (P. lepidota, P. mooreana and P. mirabilis) with contrasting habitats and leaf formations. The study found that those foliar nutrient concentrations differed between the three  Pinanga  species and differed between different leaf formations of the same species. However, further investigations are required to relate the relationship between foliar nutrient concentrations and soil variables to further examine the ecology and habitat adaptations of these three Pinanga species. Knowledge Gaps on Pinanga Ecology in Southeast Asia Most studies of the genus Pinanga in Southeast Asia have focused on descriptive notes (Dransfield 1980; Coode et al. 1996; Witono et al. 2002; Zulkarnaen et al. 2019; Fernando et al. 2020). Although descriptive notes on Pinanga species are abundant (Dransfield 1980, 1991; Keat et al. 1998; Henderson et al. 2010; Fernando et al. 2020; Kuhnhäuser et al. 2023), these typically include Tropical palms and factors influencing their diversity, with a focus on Pinanga in Southeast Asia - Abdullah et al. 301 limited information, focusing on taxonomic descriptions and brief notes on, for example, locality and habitat. Studies that focus on population structures of Pinanga species are still lacking, and to date, only two studies in Southeast Asia are known to have focused on this. In Indonesia, Yudaputra et al. (2021) studied the population structures of P. arinasae in Bali Island and found that the population comprised many seedlings, but a much lower abundance of juvenile and mature individuals. Meanwhile, Zulkarnaen et al. (2019, 2022a) studied the population structure of P. javana at two sites, Mt. Ungaran and Mt. Slamet, Java, Indonesia. They found that P. javana populations in Mt. Ungaran mainly comprised individuals at the seedling stage (Zulkarnaen et al. 2019), while those at Mt. Slamet mainly comprised adult individuals with stem heights of 6.1-8.1 m and stem diameters of 7-8.9 cm (Zulkarnaen et al. 2022a). In Brunei Darussalam, one of the first studies of Pinanga was done on five Pinanga species (P. aristata, P. brevipes, P. dumetosa, Pinanga tenella var. tenella, and P. veitchii) by Shapcott (1999). The study compared the population genetics and densities of these five Pinanga species at Kuala Belalong, Brunei, and showed that all species of Pinanga palms studied had quite prominent levels of genetic diversity, which were correlated with population density. Shapcott (1999) further reported that the densest species had less genetic variation than less-dense species, with P. dumetosa exhibiting the greatest gene flow between its populations and P. brevipes exhibiting the lowest.  The lack of studies on Pinanga population structure in Southeast Asia underscores the need for increased ecological investigation into the populations of these species, particularly since the region holds the highest diversity of Pinanga species globally. Understanding the population biology of Pinanga species, particularly those that are endemic to Borneo, is of priority to better inform and design management plans and conservation efforts for these palms. Although environmental factors and soil properties have been shown to influence palm communities at different spatial scales (Eiserhardt et al. 2011), their influence on Pinanga palm communities are only beginning to be understood. Descriptives notes provide anecdotal evidence to demonstrate that Pinanga species display habitat preferences (Dransfield 1980; Coode et al. 1996; Witono et al. 2002; Fernando et al. 2020), but the significant influence of environmental factors and soil conditions still requires investigation. Although a few studies have statistically shown that Pinanga species appear to exhibit habitat specificity, these remain few and should be expanded to the various forest types that are known to occur throughout Borneo and in the wider Southeast Asian region. To complement investigations of drivers of Pinanga diversity in different forest types, an increased understanding of functional traits of Pinanga species, such as their growth forms, stem traits, fruit traits and leaf traits (Kissling et al. 2019), which could explain their habitat specificity, should also be aimed for. It is notable that only three Pinanga species that are recorded as Bornean endemics have been assessed and listed in the IUCN Red List: P. chaiana (Near Threatened), P. mooreana (Least Concern) and P. patula (Least Concern) (IUCN 2024). Pinanga palms are often prized by illegal collectors and also face threats from deforestation and forest fragmentation (Bellot et al. 2022). The threats faced by Pinanga populations in Borneo and Southeast Asia highlight a need for more studies of these palms to guide the conservation management of this genus throughout the region. CONCLUSION It is widely accepted that palm abundance is influenced by various factors, such as climate, soil chemistry, hydrology, topography and vegetation structure. Currently available studies on the relationship between palm abundance and species richness are still lacking because not all determinants have been quantitatively studied at all scales and because some studies involved more comprehensive investigations than others. Palm species richness has been comprehensively studied only at a continental to global scale, and studies on community abundance are often conducted at local scales but less frequently at landscape to regional scales, being especially limited at a continental to global scale. Therefore, it is crucial to assess the relationship between palm abundance and species richness with biotic and abiotic factors at all spatial scales. For Pinanga palms, there is still insufficient ecological understanding of the factors that act as determinants of their abundance and species richness. Studies to date have shown that Pinanga palms typically prefer humid conditions, thick litter layers, high canopy cover and locations near water sources, such as rivers or small streams. Pinanga species abundance and richness, as well as its foliar BIOTROPIA Vol. 31 No. 2, 2024 302 contents, can be influenced by soil properties. Detailed studies focusing on different lowland forest types with contrasting environmental conditions, and on how these factors affect Pinanga species abundance and richness, coupled with studies investigating their functional traits, are needed. Several Pinanga species are endemic to Borneo, and this highlights the need to investigate their populations and habitat preferences, particularly for the conservation of these iconic palms. ACKNOWLEDGEMENTS The authors would like to thank Universiti Brunei Darussalam for research funding (grant reference number: UBD/RSCH/1.13/FICBF(b)/2022/033) and the Ministry of Education, Brunei Darussalam for the lead author’s graduate scholarship. We thank the Brunei Forestry Department for field and herbarium support for Pinanga surveys in Brunei Darussalam. We also thank Alison Shapcott and Sidonie Bellot for advice on Pinanga species in Brunei Darussalam. REFERENCES Abdo ME. 2017. A floristic study of Halmahera, Indonesia focusing on palms (Arecaceae) and their seed dispersal [Dissertation]. Retrieved from Florida International University. Abdullah MF. 2021. Variation in foliar nutrient contents of three Pinanga species with contrasting habitats and leaf formations [Dissertation]. Retrieved from Universiti Brunei Darussalam archive. Abdullah MF. 2024. 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