The flora of Samoa comprises a range of vascular and non-vascular plants (moss and liverworts) found in the Samoan Islands. The flora comprises 833 native and naturalised flowering plants, 482 bryophyte species (including 248 species of moss and 234 liverwort species), and 225 species of ferns and fern allies. The islands that support this flora are of volcanic origin, with peaks rising to . The volcanic landforms of the islands, combined with a tropical climate and high rainfall create a diverse range of habitats from mangrove forests along coastlines to lowland rainforests and cloud forests at higher elevations. The islands lie within the Polynesia-Micronesia biodiversity hotspot, an area with high levels of endemism. Most Samoan plants have origins in Southeast Asia and New Guinea, but a small number of species may have originated from the Americas. There are 540 native flowering plants and around one third of these are endemic. The largest family within the flowering plants of Samoa are orchids, with 101 native species, representing the highest diversity of orchid flora among all Polynesian islands.
Around 80% of original lowland forest has been lost to logging and agriculture. Plant habitats are also threatened by invasive species and eradication programmes are in place for some of these. Conservation initiatives focus on ecosystem conservation and sustainable development. In one example, of lowland rainforest at Falealupo on the island of Savaiʻi has been protected with a covenant by the traditional village owners. Specific conservation measures have been taken to preserve rare and endangered plants, with the establishment of a conservation garden at the Vailima Botanical Gardens on the island of Upolu. Many of Samoa's plants are important for practical and cultural purposes and for traditional medicine.
Context
Geography
The Samoan Islands are in the central Pacific Ocean and are volcanic in origin. The largest islands in the group are Savaiʻi and Upolu. The islands are politically divided between Samoa, an independent country; and American Samoa, an unincorporated territory of the United States.
The islands are located south of the equator, and have a humid tropical climate. Annual rainfall varies from at the coastline to inland. The highest point on the islands is Mount Silisili on the island of Savaiʻi, at an elevation of .
Samoa is highly forested, with trees covering 60.4% of the country, which is equivalent to roughly 171,000 hectares. The region experienced significant ecological growth between 1990 and 2000, expanding its forest cover by an average of 4,100 hectares, or 3.15%, each year. This rapid reforestation halted between 2000 and 2005. In total, between 1990 and 2005, Samoa expanded its forest cover by 31.5%, which is about 41,000 hectares, and achieved a net gain of 23.9% in its combined forest and woodland habitats.
Origins
Most Samoan plants have their origins in Southeast Asia and New Guinea. However, there are a few species which may have originated from the Americas, including Vaccinium whitmeei, a member of the Ericaceae family found on lava flows on Savaiʻi. The species Abutilon whistleri, Gyrocarpus americanus and Rhizophora mangle may also have American origins. Ancestors have been arriving for more than two million years, spreading their seeds through the wind, via birds and bats, or by floating on seawater. Although ocean currents are highly effective in dispersing seeds, this only works for coastal species like the coconut. To survive the long drift, these plants must produce strong and hard floating seeds or fruits.
Scientific study
The United States Exploring Expedition in 1839 marked the first major botanical study in Samoa. While the team gathered extensive plant collections, they did not record any local Samoan names. Over the next fifty years, three amateur European botanists—Swiss physician Eduard Graeffe and English missionaries Thomas Powell and Samuel J. Whitmee—built upon this work with their own large collections. Among them, only Powell documented the Samoan names for the plants. His comprehensive 1868 publication On Various Samoan Plants and Their Vernacular Names remains an important resource today, valued for its historical significance and Powell's abilities as a Samoan-speaking naturalist. Following this period, Samoan botany shifted into a second phase led by professional European scientists, beginning with F. Reinecke's 1897 publication Die Flora der Samoa-Inseln about local flora.
Plant diversity
Samoa is within the Polynesia-Micronesia biodiversity hotspot, a region in the oceania biogeographic realm with high levels of endemism. The flora of Samoa includes 833 native and naturalised flowering plants, and 225 ferns and allies. This is one-third the size of Fiji's flora, though the two regions are close enough to have similar floras. There is only one species of gymnosperm native to Samoa. There are high levels of biodiversity for mosses and liverworts across all the islands.
Flowering plants
There are 540 native flowering plants, and 250 more naturalised plants in the Samoan Islands. Two thirds of them are dicotyledons. The largest family of flowering plants is Orchidaceae (orchids), which includes 101 native species. This represents the highest diversity of orchid flora among all Polynesian islands. Species include large purple orchid, christmas orchid, shield orchid, Samoan bulbophyllum, and Mohl's dendrobium. The second largest family is Rubiaceae (the madder or coffee family), comprising 47 native and five naturalised species. Species include, Morinda citrifolia, Mussaenda raiateensis, Psychotria insularum, and Tarenna sambucina. The third biggest family is Urticaceae with 25 species. They include Elatostema species, Laportea interrupta, Pipturus argenteus, and Pipturus polynesicus. The largest genera are Cyrtandra (20 species), Psychotria (19 species), and then Syzygium (16 species).
One third of the flowering plant species are endemic, meaning they are found nowhere else in the world. High rates of endemism are mostly on the montane and cloud forests, while rates on littoral habitats are zero percent. Over 108 plant species are rare in Samoa. Apart from rare endemic plants, other rare species include Acacia simplex, Amorphophallus paeoniifolius (elephant foot yam), Atuna racemosa (tabon-tabon), Benincasa hispida (wax gourd), Ceratophyllum demersum (coontail), Cucumis melo (melon), Dodonaea viscosa (hopbush), Gardenia taitensis (Tahitian gardenia), Gossypium hirsutum (upland cotton), Ipomoea indica (blue morning glory), and Sophora tomentosa (necklace pod). Centipeda minima has only been collected in Samoa five times, and three of these were documented over 100 years ago. Some endemic species such as the Corybas betchei, Dendrobium scirpoides, Cyrtandra campanulata, and Cyrtandra funkii are possibly extinct; the only collection was made over a century ago.
Samoa's grasses include the coastal beach dropseed (Sporobolus virginicus), basket grass (Oplismenus hirtellus), wild sugarcane (Saccharum maximum), Indian goosegrass (Eleusine indica), lemongrass (Cymbopogon citratus), and the burr grass Cenchrus calyculatus. Its sedges include the fragrant flatsedge (Cyperus odoratus), Javanese flatsedge (Cyperus javanicus), Chinese water chestnut (Eleocharis dulcis), and the endemic Samoan sedges Carex samoensis and Carex graeffeana.
Gymnosperms
The only gymnosperm native to Samoa is a cycad species, Cycas seemannii. It is native to regions from Queensland, Australia to the southwest Pacific.
Ferns and allies
The islands have over 225 species of ferns, more than New Zealand, a country that is 85 times larger. Despite this, endemism is quite low. According to the regional database on Plant Reference, the native flora of the islands includes a diverse selection of pteridophytes thriving across local ecosystems. This botanical collection shows several taxa. Species include the black maidenhair fern Adiantum capillus-veneris, the bird's nest fern Asplenium australasicum, the basket fern Drynaria rigidula, the pouched coral fern Gleichenia dicarpa, the water fern Histiopteris incisa, and the Kimberley queen fern Nephrolepis obliterata.
Mosses and liverworts
The Samoan Islands possess a rich bryoflora. In 1990, researchers reported a combined regional total of 248 moss species and 234 hepatic (liverwort) species, and estimated that the flora was 80% documented. The Independent State of Samoa has extensive high-elevation habitats for moss and liverwort; the island of Savaiʻi hosts 106 moss species, while Upolu supports 183 moss and 110 hepatic species. The smaller territory of American Samoa has fewer habitats but records notable diversity, with its principal island, Tutuila, hosting 114 moss and 37 liverwort species. The territory's Manuʻa Islands record 33 moss and 4 liverwort species, though this count remains incomplete as the high-altitude cloud forests of Mount Lata (over ) support specialized genera like Spiridens, Floribundaria, and Acroporium that indicate significant potential for future botanical discoveries. Representatives in the Bryaceae moss family include Brachymenium melanothecium, Bryum bicolor, Brachymenium indicum, Bryum pachytheca, Bryum chrysoneuron, and Bryum ramosum.
Vegetation zones
The Samoan Islands have many different vegetation zones, mainly composed of lowland rainforest, montane rainforest, and cloud forests. There are also swamp forests, mangrove forests, littoral scrub, and volcanic scrub, and littoral forest, montane meadow, and two types of upland scrub.
Lowland rainforest
Lowland rainforests represent the most complex and geographically extensive forest ecosystem in the islands. This ecosystem is divided into four distinct plant communities classified by their substrate composition, moisture levels, and specific species assemblages.
The Diospyros community consists of medium-statured canopy trees growing on well-drained soils, typically located just inland of littoral (coastal) forest zones. It is heavily dominated by Samoan Ebonys which grow alongside Syzygium clusiifolium and Syzygium dealatum. While Banyan trees serves as a primary canopy component, littoral species like the Devil's Claws tree are also occasionally present. Intact examples of this forest community survive on Apolima Island and the Aleipata Islands off the eastern coast of ‘Upolu.
The Pometia community is characterised by its massive emergent canopy layer, this community stands as one of the tallest rainforest types in the archipelago. It is defined by its primary emergent Tava Tree, which grows in association with Miva Mahogany Pacific Banyan, Myristica fatua, Neonauclea forsteri, and Planchonella torricellensis. This forest type represents a climax community growing on volcanically young, rocky soils. While it originally covered extensive lowland areas of both Savai‘i and ‘Upolu up to elevations of 450 metres, modern remnants are primarily restricted to the southern coasts of these two main islands.
The Syzygium community thrives on older, heavily weathered volcanic soils, typically occupying lowland ridges and steep interior slopes. The dominant canopy species is Syzygium inophylloides. Other major canopy contributors include Toi, Calophyllum neo-ebudicum, Canarium vitiense, Perfume flower tree, Tahitian Chestnut, Myristica hypargyraea, Planchonella garberi, and Syzygium samoense.