Vaccinium exiguum (Ericaceae, Vaccinieae), a new species from the ultramafic summit of Mt. Victoria, Palawan Island, Philippines

Abstract 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 36th in the Philippines.


Introduction
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. 2009Robinson et al. , 2016Malabrigo 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 (1966Sleumer ( -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 (1966Sleumer's work ( -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.
Etymology. The epithet "exiguum" refers to the overall small stature and leaf morphology of the new species.
Distribution and habitat. Vaccinium exiguum is currently known from a single individual from a single location at the exposed ultramafic forest summit of Mt. Victoria, Palawan Island, Philippines at ca. 1700 m elevation.
Conservation status. Only a single flowering plant was documented from a single location at the summit of Mt. Victoria, Palawan. Other summits within the mountain range are similar to the type locality in elevation, but these have yet to be explored. As such, we propose the conservation threat status Data Deficient (DD) (IUCN Standards and Petitions Committee 2019) due to the scarcity of data with which to confidently assess the species against the IUCN guidelines.
Many species are endemic to Mt. Victoria (Robinson et al. 2009(Robinson et al. , 2016. Though harboring high biodiversity, Mt. Victoria is unfortunately not legislated as a protected area (PA). Mining activities within the area are considered a significant threat that poses the risk of habitat and forest degradation. Thus, the lack of legal protection is the greatest threat to this species and to the biodiversity of the area.
Phenology. The new species was observed flowering during the month of February.

Discussion
Vaccinium exiguum belongs to section Bracteata Nakai (Nakai and Koidzumi 1927) sensu Sleumer (Sleumer 1966(Sleumer -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).
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.