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Metrosideros ()^[2] is a genus of approximately 50 trees, shrubs, and vines native to the islands of the Pacific Ocean, from the Philippines to New Zealand and including the Bonin Islands, Polynesia, and Melanesia, with an anomalous outlier in South Africa. 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 pohutukawa, (M. excelsa), northern rata (M. robusta), and southern rata (M. umbellata) of New Zealand, and ?ohi?a lehua, (M. polymorpha), from the Hawaiian Islands.

Distribution

New Caledonia has 21 species of Metrosideros, New Zealand has twelve, Hawai?i has five, and Papua has four. The remainder are scattere d across small islands of the Pacific, with one outlier described from South Africa. Metrosideros seeds can disperse on the wind, which accounts for their wide distribution from a presumed origin in a greater New Zealand continent, which at the time of the breakup of Gondwana in the late Cretaceous, included New Caledonia. How the genus reached Hawai?i appears puzzling because the prevailing trade winds blow from the east. However high altitude wind patterns may have brought seeds north from the Marquesas Islands, which molecular evidence suggests as the origin of the Hawaiian species from a single colonising event (the Hawaiian M. polymorpha is similar to the widespread M. collina found in the Marquesas Islands, and was long classified as a subspecies of it). Considering that the group likely spread north and east from New Zealand, counter to prevailing ground-level winds, this is not surprising.

Cultivation

Metrosideros are often cultivate d for their showy flowers, as street trees or in home gardens. The flowers are generally red, but some cultivars have orange, yellow or white flowers. Some names listed in horticultural catalogs and other publications, such as M. villosa and M. vitiensis, are actually the names of varieties or cultivars (usually of M. collina) rather than valid scientific species. The pohutukawa of New Zealand has several cultivars grown in Australia, Hawai?i and California and it has been planted successfully in the north of Spain^[3] and on the Scilly Isles off the south-west coast of Britain,^[4] but the species is considered an invasive pest in parts of South Africa. Metrosideros kermadecensis is recently naturalised in Hawai?i, 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

There are approximately 50 species of Metrosideros, in three subgenera: Mearnsia, 24 or 25 species, trees, shrubs (some epiphytic) and vines, with red, pink or white flowers; Metrosideros, 26 species, trees and shrubs, flowers mostly red, but some species have yellow or white flowers; and Carpolepis, 3 species of hemi-epiphytic rainforest trees from New Caledonia, all with bright yellow flowers.

of Gondwana in the late Cretaceous, included New Caledonia. How the genus reached Hawai?i appears puzzling because the prevailing trade winds blow from the east. However high altitude wind patterns may have brought seeds north from th e Marquesas Islands, which molecular evidence suggests as the origin of the Hawaiian species from a single colonising event (the Hawaiian M. polymorpha is similar to the widespread M. collina found in the Marquesas Islands, and was long classified as a subspecies of it). Considering that the group likely spread north and east from New Zealand, counter to prevailing ground-level winds, this is not surprising.

Cultivation

Metrosideros are often cultivated for their showy flowers, as street trees or in home gardens. The flowers are generally red, but some cultivars have orange, yellow or white flowers. Some names listed in horticultural catalogs and other publications, such as M. villosa and M. vitiensis, are actually the names of varieties or cultivars (usually of M. collina) rather than valid scientific species. The pohutukawa of New Zealand has several cultivars grown in Australia, Hawai?i and California and it has been plant ed successfully in the north of Spain^[3] and on the Scilly Isles off the south-west coast of Britain,^[4] but the species is considered an invasive pest in parts of South Africa. Metrosideros kermadecensis is recently naturalised in Hawai?i, 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

There are approximately 50 species of Metrosideros, in three subgenera: Mearnsia, 24 or 25 species, trees, shrubs (some epiphytic) and vines, with red, pink or white flowers; Metrosideros, 26 species, trees and shrubs, flowers mostly red, but some species have yellow or white flowers; and Carpolepis, 3 species of hemi-epiphytic rainforest trees from New Caledonia, all with bright yellow flowers.

References

• Simpson, P., 2005. Pohutukawa & Rata: New Zealand's Iron-Hearted Trees. Te Papa Press. 346 pp.
• Wagner, W.L., D. R. Herbst, and S.H. Sohmer. 1999. Manual of the Flowering Plants of Hawaii. Revised edition. University of Hawai?i Press and Bishop Museum Press, Honolulu. 1919 pp.
• Wright, S. D., C. G. Yong, S. R. Wichman, J. W. Dawson, and R. C. Gardner. (2001). Stepping stones to Hawaii: a trans-equatorial dispersal pathway for Metrosideros (Myrtaceae) inferred from nrDNA (ITS+ETS). J. Biogeography, 28(6): 769-774.
• Wright, S. D., R. D. Gray, and R. C. Gardner. (2003). Energy and the rate of evolution: inferences from plant rDNA substitution rates in the Western Pacific. Evolution, 57(12): 2893?2898.
• Wright, S.D.; C. G. Yong, J. W. Dawson, D. J. Whittaker, and R. C. Gardner (2000-03-21). "Riding the ice age El Ni?o? Pacific biogeography and evolution of Metrosideros subg. Metrosideros (Myrtaceae) inferred from nuclear ribosomal DNA". PNAS. http://www.pnas.org/cgi/reprint/97/8/4118.pdf. Retrieved 2007-06-13.