The flora of the Cook Islands (the associated state of New Zealand, including the Island of Rarotonga) consists of 104 native ferns and 187 flowering plants, with 26 of them being endemic. There are also 62 moss species, as well as 11 liverwort species. There are two main island groups, the Southern Cook Islands and the Northern Cook Islands. The two chains are geographically different, despite both being of volcanic origin. There is a diverse range of habitats from littoral forests along coastlines to montane and cloud forests at higher elevations. A unique uplifted coral reef limestone is found on some of the islands. Vegetation on this substrate is well preserved, as the rugged, uneven terrain is unsuitable for agriculture or human settlement.
Many plants of the islands are highly threatened to due human activity and other natural causes. There are eradication programmes targeting invasive species such as the giant reed Conservation efforts focus on the evaluation and propagation of native plant species, while also planting native coastal trees to reduce land erosion. There are plants used for medicines, tools, and everyday items, and plants that hold cultural importance.
Geography
The Cook Islands are an island group made up of 15 islands and atolls and are of volcanic origin. There are two island groups, the Northern Cook Islands, and the Southern Cook Islands. All islands spread out across the country are collectively called Pa Enua. Rarotonga is the largest and most populous island in the southern chain of the Cook Islands. Situated in the Ocean of the South Pacific approximately 3,400 kilometres northeast of New Zealand, this volcanic island has a rugged, mountainous interior that rises to at Te Manga, the highest point in the country. The islands enjoy a tropical climate, balanced by winds, with a dry season from April to November and a more humid seasonality from December to March.
The Northern Cook Islands form a rough, upside-down triangle with Penrhyn in the northeast, Pukapuka in the northwest, and Suwarrow in the south. A vast expanse of the Pacific Ocean separates the northern atolls from the southern group. The closest southern neighbour is Palmerston Island, located south of Suwarrow. The Northern Group is significantly older, with highly eroded and submerged volcanic cores. The islands are mostly coral atolls, and plant diversity is low. The Northern Group gets higher rainfall, ranging between 1,900 and 2,800 mm each year.
The Southern Group has volcanic interiors, which distinguishes these islands from the low-lying coral formations of the northern chain. The highest levels of floral biodiversity are found on the volcanic island of Rarotonga, followed by the raised coral islands (makatea) of Mangaia, Atiu, Mauke, and Mitiaro. The Southern Group has an average annual rainfall of about 2,000 mm.
Plant diversity The Cook Islands are a part of the Polynesia-Micronesia diversity hotspot, a region in the oceania biogeographic realm with high levels of endemism. The islands host a flora of 567 plants, 108 ferns and fern allies, and a total of 73 bryophyte species. There are no native gymnosperms, as all 4 recorded on the islands are introduced.
Flowering plants
There are 567 flowering plant species, but only 187 are indigenous. Some well known taxa include: Pipturus argenteus (orongā), Myrsine cheesemanii (ka‘ika makatea), Eugenia reinwardtiana, Melastoma denticulatum, Solanum nodiflorum (poroporo), and Morinda citrifolia (nono). Endemic plants include Rarotonga Ground-Orchid, Mitiaro Daisy, Cook Islands Myrsine, Te Manga Cyrtandra, Rarotonga Peperomia, Rarotonga Sclerotheca, Makatea Geniostoma, Cook Islands Myoporum, Rarotonga Haloragis, Rarotonga Psychotria, Rarotonga Garnotia-Grass, Rough Tree-fern, Cook Islands Oak-leaf Fern, Cloud Grass-fern, Asplenium parksii, Freycinetia wilderi, and Lepidium makateanum.
Ferns and allies
There are 104 ferns that are native to the Cook Islands. Tree ferns are scattered throughout Rarotonga and are collectively known as panga. Some native species include: Psilotum complanatum (flat-whisk fern), Angiopteris evecta (king fern), Phlegmariurus squarrosus (rock clubmoss), Hypolepis tenuifolia, (soft ground fern), Ophioderma pendulum (ribbon fern), and Oreogrammitis cheesemanii (cloud grass fern). There are also some endemic fern species, such as Sphaeropteris parksiae, (Rarotonga tree fern) and Asplenium parksii.
Moss and liverworts The bryophyte flora remains poorly documented. There are 62 species of moss and 9 known species of liverworts. moss spread across 9 moss families, and 11 genera. Liverworts make 6 families, and 8 genera. Native species include Dumortiera hirsuta, Fissidens zollingeri and Calymperes tenerum.
Vegetation There are four distinct types of vegetation in the islands, including littoral forests, montane and cloud forests that thrive at high elevations, and the unique Makatea forests (growing on uplifted coral reef limestone).
While its coastal and lowland plants have been heavily cleared or replaced by introduced species, its rugged interior remains largely untouched. A study of 19 upland forest sites revealed that over 92% of the trees are native, accounting for 95% of the total forest area.
Littoral forests The littoral zone of the Cook Islands are a vegetation found on the coastlines. These areas are dominated by a mix of specific tree species, including Barringtonia asiatica, Calophyllum inophyllum, Cordia subcordata, Guettarda speciosa, Hernandia nymphaeifolia, Hibiscus tiliaceus, Pandanus tectorius, Pisonia grandis, Sophora tomentosa, Timonius polygamus, and Tournefortia argentea. The open areas are dominated with plants such as the Morinda citrifolia, Pemphis acidula, Pipturus argenteus, Scaevola taccada, Schleinitzia insularum, and Suriana maritima.
Montane forests
There are two primary types of relatively undisturbed montane rainforest found on Rarotonga.
Homalium forests are found on steep, mountain slopes. It is characterized by a dense, closed canopy standing tall. While the Homalium acuminatum is the dominant species, It is also dominanted by the Cyclophyllum barbatum, Elaeocarpus tonganus, and Ixora bracteata. Occasionally, species like Bischofia javanica, Macaranga harveyana. Pouteria grayana, Pterophylla affinis, and Terminalia glabrata can be found here as well.
Fagraea-Fitchia forest lives on rocky soils on elevations up to . It is characterized by thick growth from large shrubs. The plants are dominated by Fagraea berteroana and Fitchia speciosa. The strong prop roots of these plants help stop soil erosion. They are dominated by plants such as the Alyxia elliptica, Cyclophyllum barbatum, Coprosma laevigata, Hibiscus tiliaceus,Meryta pauciflora, and Metrosideros collina.
The montane habitats of Rarotonga are one of the most intact forests of any tropical Pacific montane ecosystems. They are in the steep, isolated interior of the largest of the Southern Cook Islands, so the vegetation is dominated by numerous endangered flora.
Cloud forests
2015 survey listed 107 vascular plant species within the island's cloud forest habitats. Among these findings were several threatened endemic species, including the Te Manga Cyrtandra, Sclerotheca viridiflora), and Radiogrammitis cheesemanii. Cloud forest habitats also support a rich abundance of indigenous non-vascular flora. This includes a diverse variety of mosses, and liverworts. The vegetation is primarily dominated by The Metrosideros collina, which reachs heights of up to . In higher-elevation zones characterized by increased vapor, Ascarina diffusa becomes the dominant canopy species. Associated woody plants in this vegetation include Coprosma laevigata, Elaeocarpus floridanus, Fitchia speciosa, Geniostoma rarotongensis, Morinda forsteri, Pittosporum arborescens, Pterophylla affinis, Xylosma gracile, and Vaccinium cereum.
Rarotonga's cloud forests serve as a protective cover for steep and wet slopes, ensuring high water quality in headwater streams. The vegetation acts as a giant sponge that traps and slowly releases precipitation, thereby maintaining the reliability of the island's water supply.
Makatea forests
Vegetation growing on makatea (uplifted coral reef limestone) surfaces remain generally well preserved, as the terrai is unsuitable for agriculture or human settlement. This vegetation can be found on Atiu, Mangaia, Maʻuke, and Mitiʻaro. The Tuamotu Islands has an island (Makatea) that shares the coral limestone formation. The biodiversity of the flora scales progressively with inland distance.
The plant formation of the coastal makatea vegetation changes as it extends inland. Closer to the shore, the canopy is composed of a mix of the Cocos nucifera Hernandia nymphaeifolia, Hibiscus tiliaceus, and Pandanus tectorius, alongside occasional occurrences of Barringtonia asiatica and Casuarina equisetifolia. Further inland, the tree range changes to a vegetation dominated by Elaeocarpus tonganus, Guettarda speciosa, Pisonia grandis, Schleinitzia insularum, and Sophora tomentosa. Fronting the maritime edge of this forest is a vegetation zone primarily dominated by beach naupaka, occurring alongside Euphorbia atoto, Timonius polygamus, and beach daisy. Next to the coastline, Pemphis acidula forms a narrow line.
The inland makatea vegetation is dominated by nearly all woody plants present in the coastal makatea zone, with the notable exception of Pemphis acidula. This interior forest features an elevated canopy layer dominated by larger tree species, including Aleurites moluccana, tamanu, and Hernandia moerenhoutiana. Additional woody plants documented within this inland vegetation include Cyclophyllum barbatum, Ficus prolixa, Ficus tinctoria, Glochidion sp., noni, and false stinger.
Threats
Centuries of clearing for housing and farming have destroyed most lowland native vegetation across the islands. Pristine montane and cloud forests survive only on Rarotonga, while other native plants persist exclusively in the rugged, inaccessible terrain of the makatea islands. Additionally, the region is increasingly vulnerable to transboundary climate impacts, including intensified floods, droughts, tropical cyclones, and rising sea levels. Furthermore, invasive species, particularly black rats, cats, and mynah birds, pose a severe and ongoing threat to local biodiversity. Invasive weeds and shrubs that outcompete native plants include, Cardiospermum grandiflorum (balloon vine), Spathodea campanulata (African tulip tree), and Sphagneticola trilobata (wedelia). There are eradication programmes focusing on other invasive plants such as the Giant reed (Arundo donax) and Dodder (Cuscuta campestris) stopping them from destroying habitats of the native flora.
Conservation There are 18 protected areas in the Cook Islands covering a total of . These conservation areas encompass a diverse range of ecosystems, including nature reserves, and uninhabited atolls, most of them being established under traditional customary law rather than modern statutory legislation. The Cook Islands lie within in two different ecoregions for conservation planning purposes: the Cook Islands tropical moist forests that covers the Southern Cook Islands group, and the Central Polynesian Tropical Moist Forests - a large ecoregion that includes the Northern Cook Islands group but extends northwards to just south of Hawaii.