Introduction: family and genus

The Anacardiaceae (cashew or sumac) family consists of about 860 species grouped into 83 genera (Christenhusz and Byng 2016). The vast majority of the species in the family are distributed in warm subtropical and tropical areas in the Americas, Africa and Asia. Among the many trees, one can find economically important timber, fruit and nut trees, such as mangoes (Mangifera indica L. and other species in the genus Mangifera), cashew (Anacardium occidentale L.), and pistachio (Pistacia vera L.).

The genus Dracontomelon was first described by Blume (1850) based on two species known at the time. Utteridge and Jennings (2022) listed eight, POWO (2024) nine species within the genus, all of which are trees. These trees are distributed from tropical Asia and south–central China to the south–western parts of the Pacific region. They usually grow in primary lowland rainforests, swamp areas and secondary growth such as old coconut plantations (Utteridge and Jennings 2022). The evergreen, imparipinnate leaves are spirally arranged; the leaflets are opposite to alterative and usually distinctly unequal at the base. Leaf margins are entire. The five-merous flowers are arranged in terminal or axillary, paniculate inflorescences. They are bisexual (hermaphroditic), with articulated pedicels. The imbricate calyx lobes are more or less free. The petals are often puberulous. The ten stamens show subulate, alternatively short and long filaments (Hou 1978). The ovary possesses five carpels (some of which are aborting), with capitate stigmas (Utteridge and Jennings 2022). The subglobose, ovoid or ellipsoid fruit, botanically a drupe, has a hard endocarp (pyrene).

Widespread in the region and most widely known are the Argus pheasant-tree or Pacific walnut Dracontomelon dao (Blanco) Merr. & Rolfe and the Indochina Dragonplum Dracontomelon duperreanum Pierre, both species often being cultivated as economically important food and multipurpose trees (Lebot et al. 2008; Wang et al. 2017). Other species in the genus are confined to individual countries or islands, such as Dracontomelon lenticelatum H.P.Wilk, which is endemic to New Guinea (Utteridge and Jennings 2022).

Dracontomelon vitiense—general introduction

The valid name Dracontomelon vitiense Engl. was described by Engler (1883), referring to an earlier name Dracontomelon sylvestre Blume suggested by Seemann (1861) and shown on the original herbarium voucher ‘Seemann 99’ stored at K of a specimen collected by Seemann in Fiji in October 1860. This herbarium specimen was later designated as the lectotype for the species (Smith 1985). Attention should be given to the fact that both, Blume (1850) and Seemann (1865–1873) in his Flora Vitiensis made reference to the Herbarium amboinense (Rumphius 1741) and a line drawing of the species therein, which depicts today’s Dracontomelon dao. Seemann (1865–1873)’s statement within the description of Dracontomelon sylvestre “Also widely diffused through the East Indian Archipelago” [Netherlands East Indies = modern day Indonesia] lets one assume that Seemann (1865–1873) grouped individual trees that today belong to the two separate species Dracontomelon vitiense and Dracontomelon dao together into the same species. The scientific name Dracontomelon vitiense is derived from the two Ancient Greek words δράκων (drakōn) = dragon and μῆλον (mêlon) = apple or (generally) any fruit from a tree, and the Latin adjective vitiensis = from Fiji. Given that the fruit is a drupe, the English vernacular name Dragonplum is certainly more fitting than a possible alternative Dragonapple.

Dracontomelon vitiense can grow 8–20, rarely 30 m tall, with a stem diameter of up to 90 cm, with distinct, large and spreading, usually arched or kneed and thin buttresses and a light greyish to reddish-brown bark, which is smooth when young becoming rough and scaly with age (SPRIG 2003). The tree crown is large and spreading, broadly roundish to domed (Wheatley 1992). Young branches are greyish pubescent (Blume 1850). The compound (single pinnate) leaves are about 30–40 cm long, with a terminal leaflet. The petiole is 4–10 cm long. Each leaf consists of 6–9 pairs of (sub-) opposite or alternate, elliptic leaflets of 8–15 × 3–5.5 cm size, with an oblong, pointed but unequal base. The leaflets are glabrous but hairy in the vein axes of the ventral (lower) leaflet surface. The petiolules are 3–5 mm short (SPRIG 2003).

The flowers with a total size of about 8 mm are arranged in terminal paniculate inflorescences which are up to 35 cm long. Five ovate, ciliate sepals are about 3.5 mm in size and therefore twice shorter than the five white, oblong-lanceolate, ciliate petals which reach a size of about 7 mm (Blume 1850). Ten stamens are fused on the petal tub, with one central style. In Fiji, flowering occurs between January and March (Smith 1985). Wheatley (1992) recorded flowering in Vanuatu in March, June/July and November. The fruits are small (2.5–3.5 cm in diameter), (sub-) globose drupes, which are slightly flattened and show five small depressions below the apex (SPRIG 2003). Fruiting occurs from (March-) June to August; whereas immature fruits are green, the colour changes to yellowish-green, then to dark yellow or light brownish-yellow (SRPIG 2003) (Fig. 1). The fruits contain an edible, firm, yellow pulp (mesocarp), surrounding a single, lens-shaped pyrene with five small seeds (Walter and Sam 2002). Dispersal of the fruits takes place by large, frugivorous birds and fruit bats (Wheatley 1992).

Fig. 1
figure 1

Detail of leaves and ripening fruits of Dracontomelon vitiense. Tanna, Vanuatu, June 2024. Photo and copyright: Dominik M. Ramík, with kind permission

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Distribution and ecology

Dracontomelon vitiense is endemic to the western Pacific region, covering an area from Santa Cruz Islands (part of Solomon Islands) and Vanuatu in the west, to Fiji (including Rotuma Island) in the central parts to Futuna (part of the French Territory of the Wallis and Futuna Islands) and Upolu (part of Samoa) in the east (Whistler 2022), with its centres of distribution in Vanuatu and Fiji where it is moderately abundant (Fig. 2) (Smith 1985; Hancock and Henderson 1988; Walter and Sam 2002; SPRIG 2003). The occurrences in Tonga are considered human introductions in historic times (Walter and Sam 2002). The species does not occur west of Vanuatu, therefore a pre-historic introduction from SE Asia by humans seems unlikely (Smith 1985).

Fig. 2
figure 2

Distribution of the Fiji Dragonplum Dracontomelon vitiense in the western Pacific region (shaded area). Occurrences in Tonga are considered human introductions in historic times. After data in Smith (1985); Walter and Sam (2002); SPRIG (2003); Whistler (2022)

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Dracontomelon vitiense is a canopy forest tree growing naturally in wet tropical lowland forests in near-coastal areas and on lowland coral plateaus, dry or open forests, or in secondary forests. In its native area of distribution, it occurs at altitudes from sea level to 200–300 m asl. and becomes uncommon or rare at higher elevations of up to 700–800 m asl. in Fiji’s Viti Levu (Walter and Sam 2002; Utteridge and Jennings 2022). Climatically, the species is less frequent in areas with a pronounced dry season (Wheatley 1992). It is reasonably shade-tolerant and is found on a variety of acid to neutral, medium to heavy textured soils, including alluvial soils and swampy locations (SPRIG 2003).

Numerous local vernacular names exist across the range of the species. For Fiji, already Seemann (1865–1873) and Engler (1883) listed the name tarawau, Smith (1985) mentioned tarawau ndina and tarau. In Rotuma, the species is known as favrau or faorau (SPRIG 2003). The Bislama name for Dracontomelon vitiense in Vanuatu is Nakatambol (Harrison and Karim 2016). Walter and Sam (1993) listed 18 different local names, Wheatley (1992) two dozen local names on the various islands of Vanuatu.

Use of the species

Across its range, Dracontomelon vitiense is valued as an important fruit and timber tree, and to a lesser degree as a medicinal plant. Although the tree is a wild species and it is harvested from the wild for local use, within its native area of distribution it is often planted in tree groves, gardens and villages (Harrison and Karim 2016; Gardner 2018; Walter and Sam 2002), often together with citrus trees, mango, coconut, breadfruit, cutnut (Barringtonia edulis Seem.), June plum or naus (Spondias dulcis Parkinson), wild nandau (Pometia pinnata J.R.Forst. & G.Forst.), namambe or ifi (Inocarpus fagifer (Parkinson ex F.A.Zorn) Fosberg) (Müller 2024). In contrast, in Vanuatu the tree is considered an exclusively wild species (Walter and Sam 1993). In this aspect, Dracontomelon vitiense is not different from other locally important fruit trees, which continue to grow spontaneously in their natural environments, such as sea almonds (Terminalia catappa L.) and Burckella (Burckella obouata (G.Forst.) Pierre) in Vanuatu.

Fruit

During the fruiting season, the fruits of Dracontomelon vitiense are widely consumed throughout the natural range of the tree and enhance local food security. The fruits are commonly sold in towns and local markets. Certainly in Vanuatu but also in Fiji, Dracontomelon vitiense is considered one of the main native fruit trees (Wheatley 1992; Walter and Sam 2002). Harvesting is done with the help of long bamboo poles (Walter and Sam 2002, see also Fig. 3). The fruits (Fig. 4) are described as sweet (or slightly acidic) to rather bland. They are eaten raw, often as a snack outside main meal times (SPRIG 2003). Alternatively, the fruits can be dried, ground before being cooked in coconut milk (Walter and Sam 2002). The fruits contain limited amounts of protein (2.1%), sugar and starch (9%) (English et al. 1996).

Fig. 3
figure 3

Fiji Dragonplum tree during fruiting season in a house garden in Lowanatom village, Tanna (Vanuatu), June 2024. On many islands in Vanuatu, the tree is regularly planted and common. Long bamboo poles are used to facilitate the harvest of the fruits. Photo and copyright: Dominik M. Ramík, with kind permission

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Fig. 4
figure 4

During harvest, entire infructescences are cut down from the tree branches. Fruits are (sub-) globose drupes, which contain an edible, firm, yellow pulp. Tanna, Vanuatu, June 2024. Photo and copyright: Dominik M. Ramík, with kind permission

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Timber and firewood

Dracontomelon vitiense produces commercially valuable, multipurpose timber whose properties and appearance vary between heartwood and sapwood (SPRIG 2003). The heartwood is pale brown drying greyish brown (Wheatley 1992), whereas the sapwood is creamy yellow and rather soft. The timber is used for light constructions, furniture (doors, tables, chairs), canoes, and wood carving (including food bowls) (Walter and Sam 2002; Siwatibau and Boland 2002). Walter and Sam (2002) reported on a particular use of the timber in Vanuatu, whereby a large board is cut from a stem buttress and placed over a hole in the ground to act as a type of sounding-box on which dancers perform, stamping their feet on the ground. The timber is also used as firewood.

Medicine

Although the use of various parts of the tree as a traditional medicine in Fiji and Vanuatu is reported in the literature, these references are of anecdotal character and not supported by pharmacological or ethno-pharmacological research (Cambie and Ash 1994). These medicinal uses are listed here for completeness. The bark is said to be used during delivery, whereby a hand-sized piece is soaked in cold water to soften it, and then placed onto the woman’s navel (SPRIG 2003). Walter and Sam (2002) mentioned that pulverised bark is administered internally to treat abdominal pain, and externally applied to treat pain in the head. Also, pulverised bark mixed with coconut milk is used as an anti-head lice shampoo.

Other uses

Other uses reported in the literature include the use of the large buttresses as two sides of an impromptu pig enclosure, or as a temporary bush shelter. On some islands of Vanuatu, the tree stem is used as a stake for growing yams (Walter and Sam 1993, 2002).

Cultural traditions

Seemann (1865–1873):52 mentioned an intriguing cultural aspect, which was widespread in Fiji at the time.

“The tree is not worshipped; but it is held to be the business of the dead to plant it, and it is believed to grow not only in this world, but also in the Naicobo-cobo, the Fijian nether-world, or perhaps, more correctly, the general starting-place for it. Hence arose the expression “Sa la’ki tei tarawau ki Naicobocobo,” literally, “He has gone to plant Tarawau at Naicobocobo;” i.e. he is dead.”

The fact that nowadays the species is often planted in gardens and near villages would point towards a loss of this cultural tradition in Fiji. However I did not find any information if and to which extent this cultural tradition is still being upheld in Fiji, and whether it would contravene any attempts to plant and cultivate the species more widely. Walter and Sam (1993) observed a loss of knowledge about e.g. local names from one generation to the next one, which might well extend to cultural traditions.

Cultivation and commercial production

Despite its local importance and potential for further planting and cultivation especially in mixed plantings near human settlements (SPRIG 2003), there are no production figures of Dracontomelon vitiense or statistics about its commercial use and yield available. Such data would be essential to answer the question to which degree Dracontomelon vitiense is a commercially under-used tree. Reports about the commercial use remain localised and largely anecdotal. Walter and Sam (1993) reported on attempts to identify and research local cultivars of Dracontomelon vitiense in Vanuatu, with the idea for further development and to establish in-situ conservation collections.

Data about the storage behaviour of the seeds are not available. However, some information about the storage behaviour of seeds of two more widespread, congeneric species exists. Dracontomelon duperreanum is said to possess seeds which show orthodox storage behaviour (SER et al. 2023), whereas seeds of Dracontomelon dao are known to be recalcitrant (Orwa et al. 2009).

Published results of propagation trails do not exist, so looking at congeneric species might give some indication of possible propagation techniques for Dracontomelon vitiense. For arboricultural propagation of Dracontomelon dao, Orwa et al. (2009) recommended to extract the seeds immediately after fruit collection. These authors observed a 33% germination of freshly sown seeds but reported 85–95% germination after 28–67 days from a second propagation trail in Malaysia. As an alternative to seed propagation, Gunn (2013) mentioned the possibility of using grafting techniques and shoot tips and leaf cuttings. Ultimately, it remains to be seen in how far such information relating to Dracontomelon duperreanum and Dracontomelon dao might be inferred to Dracontomelon vitiense itself.

Conservation and availability in gene bank collection

National conservation assessments of Dracontomelon vitiense do not exist. The global IUCN Red List lists the species as “data deficient” (Sirikolo et al. 2022). Although the species is common in parts of its area of distribution, to my knowledge there are no data available about its current population status, nor data on regeneration patterns in the wild. Additional data would be required to assess future populations trajectories, which could anticipate potential higher threat levels as a result of an increased use from naturally regenerated trees.

Ex-situ gene bank accessions of Dracontomelon vitiense are not available. An online search on global databases for plant genetic resources (Genesys PGR 2024; FAO WIEWS 2024; GRIN Global of the U.S. National Plant Germplasm System (NPGS 2024), and the seed list of the Royal Botanic Gardens Kew’s Millennium Seed Bank (RBGK 2024)) showed that no active, accessible gene bank collection exists at all, anywhere in the world.

Breeding potential and outlook

The genetic variation of Dracontomelon vitiense has yet to be investigated (SPRIG 2003). In Vanuatu, local people distinguish between two cultivars on the basis of the fruit size (Walter and Sam 1993, 2002). The larger fruited trees have fruits with a 3 cm diameter and the smaller-fruited ones have fruits < 3 cm diameter. Some trees have fruits with a sweeter pulp, which are much preferred for consumption, a fact that might constitute a small, continuous selection pressure towards sweeter fruits (Walter and Sam 2002). Apart from this reference from Vanuatu, very little else is known about existing cultivars, and data on growth, yield, and quality seem to be almost entirely lacking. This scarcity of published results stands in sharp contrast to the tree’s potential to increase the food resources and food security (Evans 1996), in particular for small-scale farmers in Melanesia.

The tree is still used locally and perceived as a valuable multipurpose tree by local village communities across its area of distribution. It is currently impossible to quantify its present underuse. Nevertheless, the species retains a great potential for commercialisation and income generation for those village communities and as a component in sustainable agroforestry production systems (Clarke and Thaman 1993; Pauku 2005; Shukla and Dhyani 2023). Traditional farming and forestry systems of the Pacific region are inherently resilient. Climate-smart agriculture, using traditional multicrop garden systems is an approach being recommended to achieve sustainable agricultural development for food security under climate change. Dracontomelon vitiense might play a significant role in this.

As it currently stands and to the best of my knowledge, we must assume that there is currently no active breeding programme going on. One has to critically ask whether a breeding programme should be implemented for Dracontomelon vitiense at all. A breeding programme with the aim to introduce a new commercial fruit tree to be cultivated in monoculture would be culturally out of context, and any new breeding should be kept in line with traditional systems (Müller 2024). Hammer et al. (2001) defined “neglected crops” as “neglected crops have been ignored by science and development but are still being used in those areas where they are well adapted and competitive.” Following this definition, Dracontomelon vitiense can be considered a neglected tree for the Pacific region. I recommend further work on agronomy, production systems, breeding, diseases and pests, storage, utilisation, and marketing for this species. Further research efforts should aim at: (I) expanding our knowledge of its local and traditional uses, (II) a standard characterisation and inventory project including genetic and chemical features should map existing local cultivars across its entire area of distribution; (III) long-term conservation of the existing  cultivars and landraces in germplasm collections, considering no accessions of Dracontomelon vitiense are currently conserved ex-situ; and (IV) increase the productivity through research on better agronomic management, breeding and strengthening of local value chains.