Metrosideros (; ) is a genus of approximately 60 trees, shrubs, and vines in the family Myrtaceae, mostly found in the Pacific region. Most of the tree forms are small, but some are exceptionally large, the New Zealand species in particular. The name derives from the Ancient Greek metra or "heartwood" and sideron or "iron". Perhaps the best-known species are the pōhutukawa (M. excelsa), northern rātā (M. robusta) and southern rātā (M. umbellata) of New Zealand, and ōhia lehua (M. polymorpha) from the Hawaiian Islands.
Distribution
left|thumb|A young pōhutukawa (M. excelsa) in flower at [[Ōhope, in the Bay of Plenty, New Zealand]]
Metrosideros is one of the most widely spread flowering plant genera in the Pacific. New Caledonia has 21 species, New Zealand has 12, New Guinea has seven and Hawaii has 5. The genus is present on most other high Pacific Islands, including Solomon Islands, Vanuatu, Fiji, Samoa, Cook Islands, French Polynesia, Bonin Islands and Lord Howe Island, but absent from Micronesia. The genus is also represented by one species in the Philippines, one in South America (Chile and Argentina), and one outlier in South Africa.
Metrosideros seeds are very lightweight and easily dispersed by wind. The seeds can also survive freezing temperatures, and up to 30 days submerged in salt water, and still germinate, which probably accounts for their wide distribution. They are often found as pioneer trees on lava flows and on mountain ridges. Despite the clear propensity towards long-distance dispersal, the genus does not occur in mainland Australia.
Fossil record
For some time, it had been hypothesised that Metrosideros evolved in New Zealand, and dispersed from there throughout the Pacific. This was due to the long fossil record of Metrosideros in New Zealand coupled with the absence of any Metrosideros fossils on other Gondwanan landmasses. The oldest conclusive fossil evidence of Metrosideros in New Zealand is fossil fruits from the Miocene aged Manuherikia sediments of Central Otago. There is a fossil pollen record going back much further, but it has been shown that Metrosideros pollen is very similar morphologically to many other genera within the family Myrtaceae and as such, fossil pollen cannot be reliably used as the oldest record of the genus. and on the Scilly Isles off the south-west coast of Britain, but the species is considered an invasive pest in parts of South Africa and in the Azores. Metrosideros kermadecensis is recently naturalised in Hawaii, and has the potential to become a pest. In turn, various cultivars of M. collina and M. polymorpha are widely grown in New Zealand under various names. Metrosideros umbellata occurs naturally south of mainland New Zealand in the Auckland Islands at 50° South latitude, and is the hardiest member of the genus, and a few cultivated specimens are growing in Scotland.
Metrosideros species
;New Caledonia
- Metrosideros brevistylis
- Metrosideros cacuminum
- Metrosideros cherrieri
- Metrosideros dolichandra
- Metrosideros elegans
- Metrosideros engleriana
- Metrosideros humboldtiana
- Metrosideros laurifolia
- Metrosideros longipetiolata
- Metrosideros microphylla
- Metrosideros nitida
- Metrosideros operculata
- Metrosideros oreomyrtus
- Metrosideros paniensis
- Metrosideros patens
- Metrosideros porphyrea
- Metrosideros punctata
- Metrosideros rotundifolia
- Metrosideros tardiflora
- Metrosideros tetrasticha
- Metrosideros whitakeri
South America
- Metrosideros stipularis
South Africa
- Metrosideros angustifolia
;New Zealand
- Metrosideros albiflora - Large white rātā
- Metrosideros bartlettii - Bartlett's rātā
- Metrosideros carminea - Carmine rātā
- Metrosideros colensoi - Colenso's rātā
- Metrosideros diffusa - White rātā
- Metrosideros excelsa - Pōhutukawa
- Metrosideros fulgens - Scarlet rātā
- Metrosideros kermadecensis - Kermadec pōhutukawa (Kermadec Islands)
- Metrosideros parkinsonii - Parkinson's rātā
- Metrosideros perforata - Small white rātā
- Metrosideros robusta - Northern rātā
- Metrosideros umbellata - Southern rātā
New Guinea
- Metrosideros arfakensis
- Metrosideros cordata
- Metrosideros ovata
- Metrosideros parallelinervis
- Metrosideros ramiflora
- Metrosideros regelii
- Metrosideros whiteana
Philippines
- Metrosideros halconensis
;Hawaii
- Metrosideros macropus - Lehua mamo (Oahu)
- Metrosideros polymorpha - Ōhia lehua
- Metrosideros rugosa - Lehua papa (Oahu)
- Metrosideros tremuloides - Lehua āhihi (Oahu)
- Metrosideros waialealae (Kauai, Molokai, and Maui)
French Polynesia, Pitcairn and the Cook Islands
- Metrosideros collina
Fiji, Samoa and Vanuatu
- Metrosideros gregoryi (Samoa)
- Metrosideros ochrantha (Fiji)
- Metrosideros tabwemasanaensis (Vanuatu)
- Metrosideros vitiensis
Solomon Islands
- Metrosideros salomonensis
- Metrosideros tetragyna
Lord Howe Island
- Metrosideros nervulosa - Mountain rose
- Metrosideros sclerocarpa
Ogasawara Islands
- Metrosideros boninensis