Vaccinium exiguum from the ultramafic summit of Mt. Victoria, Palawan Island, Philippines is here described as a new species of Ericaceae. It closely resembles V. hamiguitanense but is distinct by having much shorter petioles and leaves, longer and glabrous calyx lobes with serrate lobe margins, a larger corolla with deeper sulcations, and longer stamens with spurs oriented laterally. Vaccinium exiguum represents the third Vaccinium species found on the Island of Palawan and 36^th in the Philippines.

Ericales, endemic species, Malesia, sect. Bracteata, Vaccinioideae

The Island of Palawan is situated on the south-western side of the Philippine Archipelago and is bordered by the West Philippine Sea in the north and the Sulu Sea in the south. Palawan is a biodiverse area, regarded both biogeographically and geologically as a portion of the Sunda Shelf, with many of its species shared with Borneo (Dickerson 1928; Heaney 1986; Voris 2000; Esselstyn et al. 2004). The long and complex geohistory of the island which started during the mid-Oligocene, as well as its highly variable elevation and climate, has promoted high rates of speciation and endemism (Anacker 2011; Galey et al. 2017). In particular, the extensive areas of ultramafic substrates have resulted in the evolution of many endemic plant species in Palawan (e.g. Robinson et al. 2009, 2016; Malabrigo 2020; Quakenbush et al. 2020; Tandang et al. 2020) and it is to be expected that more species await discovery in these ultramafic regions as they are further explored and studied.

The tropical species of Vaccinium L. are predominantly montane inhabitants with a high degree of endemicity (Argent 2018). The genus is the most species-rich of the Philippine genera of the family and is currently represented by 35 species, 32 of which are endemic to the country (Argent 2008; Pelser et al. 2011 onwards). The highly regarded taxonomic treatments by Copeland (1930) and Sleumer (1966–1967) are the most comprehensive, thus far, for Philippine Vaccinium. However, gaps in our knowledge of Philippine Vaccinium remain, especially in various species complexes (e.g. V. caudatum Warb./V. benguetense S.Vidal) and many character ambiguities used for the treatments require clarification and resolution. After Sleumer’s work (1966–1967), three species have been added to the list, viz. V. cebuense Salares and Pelser, V. hamiguitanense P.W.Fritsch, and V. oscarlopezianum Co (Co et al. 2002; Salares et al. 2018; Fritsch et al. 2020).

During fieldwork on Mt. Victoria, Palawan Island in February 2021, author Bustamante documented a species of Vaccinium that grows at the ultramafic summit of the mountain (Fig. 1A) and closely resembles the recently described V. hamiguitanense from Mt. Hamiguitan, Mindanao Island. However, the inflorescence of this species differs from that of V. hamiguitanense in shape. After detailed morphological examination, it was confirmed that the specimen possesses distinguishing characters demonstrating its status as a species new to science, which we describe here under a biological species concept (Mayr 2000). Our discovery increases the number of species of Vaccinium in the Philippines to 36 and increases the number of known Vaccinium species from Palawan Island to three. Photographs and an illustration of the new species are also provided.

Figure 1. 

Vaccinium exiguum A ultramafic summit of Mt. Victoria B flowering branchlets in situ. Photo A by Alastair Robinson. Photo B by Rene Alfred Anton Bustamante.

The description was based on in-situ observations, herbarium specimens, and spirit samples preserved in Copenhagen solution. Microscopic details were described with the aid of a Swift SM100 stereo zoom microscope (30× magnification). Relevant literature and taxonomic keys (i.e. Copeland 1930; Sleumer 1966–1967; Argent 2018) were also consulted.

Vaccinium exiguum belongs to section Bracteata Nakai (Nakai and Koidzumi 1927) sensu Sleumer (Sleumer 1966–1967) as exhibited by its well-developed and often many-flowered racemes, corolla consisting of a single homogenous layer, and the absence of a membranaceous wing at the sinuses and anthers that open by short introrse slits or terminal pores (Co et al. 2002; Salares et al. 2018; Fritsch et al. 2020).

In the key of Sleumer (1996–1967) of the Malesian Vaccinium section Bracteata, V. exiguum best keys to V. gitingense Elmer (endemic to the Philippines). However, the new species differs from V. gitingense by having shorter petioles (0.5–1.0 mm vs. ca. 2.0 mm), smaller leaf blades (4.9–9.3 × 3.0–6.0 mm vs. 15–35 × 8–15 mm), glabrous calyx lobes (vs. finely ciliate), a non-glaucous and glabrous corolla (vs. glaucous and occasionally bears few hairs), and longer filaments (1.9–2.0 mm vs. ca. 1.5 mm) (Elmer 1912). In the key to the Bornean species of Vaccinium (Argent 2018), the species best keys to V. coriaceum var. stapfianum (Sleumer) Argent (restricted to Mt. Kinabalu, Malaysia); however, Vaccinium exiguum differs by having shorter leaf blades (4.9–9.3 mm vs. 6.0–13.0 mm), a crenate leaf margin (vs. entire), longer calyx lobes (1.8–2.0 mm vs. ca. 1.3 mm) that are triangular (vs. ovate), and a corolla with deep sulcations (vs. no sulcations). Likewise, the presence of translucent lanceolate glands on the crenations along the margins of the leaves, bracts, bracteoles, and calyx lobes of V. exiguum (Fig. 3) is a character shared by the three species.

Although the species exhibits morphological similarities with V. hamiguitanense, the flowers of V. exiguum are unique amongst all other Philippine Vaccinium in having a broadly urceolate and strongly 5- to 7-ribbed corolla and calyx lobe margins that are serrate. Vaccinium exiguum also possesses the smallest leaves amongst the Philippine Vaccinium species with a size range near V. hamiguitanense and V. microphyllum Reinw. The latter species is easily distinguished from V. exiguum by its axillary solitary flowers (vs. terminal multi-flowered inflorescences). In addition to the characters distinguishing V. exiguum from V. hamiguitanense as specified in the diagnosis, the two species have distinct geographical ranges.

Currently, two species of Vaccinium are recorded in Palawan: V. brachytrichum Sleumer and V. palawanense Merr. The new species can be easily differentiated from V. brachytrichum by bearing fewer flowers per inflorescence [(3 to)5 or 6 vs. 5 to 8)], longer anthers (1.5–2.1 mm vs. ca. 1 mm), and a glabrous (vs. pubescent) hypanthium. It differs from V. palawanense in the short and densely flowered inflorescences that are much shorter than the leaves (vs. flowers in racemes about as long as the leaves), a shorter pedicel (2.0–5.0 mm vs. 5.0–7.0 mm), and a 5- or 6-(or 7-) locular ovary (vs. 5) (Merrill 1908; Sleumer 1961). Moreover, the strongly ribbed corolla and the small leaves with crenate margins are also remarkable characters of the new species that easily distinguish it from the two Palawan species. At its type locality, V. exiguum was observed to be sympatric with V. palawanense.

Due to the paucity of collections and relative lack of study, intraspecific morphological variation within the species of Philippine Vaccinium (Salares et al. 2018), as well as sectional boundaries and composition, are imprecisely known (Vander Kloet and Dickinson 2009; Fritsch et al. 2020). A detailed monographic study of this group is warranted.

We thank Mr. Crispin O. Lumba Jr., acting Municipal Mayor of Narra, Mr. Ronico Masbang, Chief of the Tagbanua Tribe, and the Palawan Council for Sustainable Development (PCSD) for issuing the gratuitous permit no. 2018-28, transport permit no. PPC-LTP-AO12-2021-316 and certificate of inspection no. 1614239583882; Prof. Lea Camangeg, Jehson Cervancia, William Cabanillas, and Prof. Marjorie delos Angeles for their invaluable help and support; Mr. Jonah van Beijnen and Ms. Kyra Hoevenaars of Fins & Leaves for their generous sponsorship of the research fieldwork; Dr. Alastair Robinson for the use of his photograph of the Mt. Victoria summit; and John Charles Altomonte (PTI), Yu Pin Ang (PTI) and reviewers Dr. Jasper Obico, Mr. Val Salares, and Dr. Wendy Mustaqim for helpful comments and suggestions that greatly improved the manuscript. We gratefully acknowledge the support of Palawan State University (PSU), especially Dr. Ramon M. Docto (President), Dr. Muriel Toquero (Vice President for Research and Extension), Prof. Imelda R. Lactuan (Dean, College of Sciences), and the U.S.-National Science Foundation (grant DEB-1754697 to PWF).