Metrosideros

Metrosideros 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 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

New Caledonia has 21 species of Metrosideros, New Zealand has twelve, Hawaii has five, and Papua has four. The remainder are scattered 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 Hawaii 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, 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 pōhutukawa of New Zealand has several cultivars grown in Australia and Hawaii and is very popular in California and it has even been planted successfully in the north of Spain , but the species is considered an invasive pest in parts of South Africa. 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 50º 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 Carpolepsis, 3 species of hemi-epiphytic rainforest trees from New Caledonia, all with bright yellow flowers.

subgenus Metrosideros

• M. bartlettii Bartlett's rātā (New Zealand)
• M. boninensis (Bonin Islands)
• M. cherrieri (New Caledonia)
• M. collina (from Vanuatu in the southwest to French Polynesia in the east)
• M. engleriana (New Caledonia)
• M. excelsa - pōhutukawa (New Zealand)
• M. gregoryi (Samoa)
• M. humboldtiana (New Caledonia)
• M. kermadecensis (Kermadec pōhutukawa) (Kermadec Islands)
• M. macropus - lehua mamo (Hawaii)
• M. microphylla (New Caledonia)
• M. nervulosa (Lord Howe Island)
• M. nitida (New Caledonia)
• M. ochrantha (Fiji)
• M. oreomyrtus (New Caledonia)
• M. polymorpha - ōhia lehua (Hawaii)
• M. punctata (New Caledonia)
• M. robusta - northern rātā (New Zealand)
• M. rugosa - lehua papa (Hawaii)
• M. salomonensis (Solomon Islands; this species has been variously placed in both subgenera)
• M. sclerocarpa (Lord Howe Island)
• M. tetrasticha (New Caledonia)
• M. tremuloides - lehua āhihi (Hawaii)
• M. umbellata - southern rātā (New Zealand)
• M. waialeaiae (Hawaii)

subgenus Mearnsia

• M. albiflora (New Zealand)
• M. angustifolia (South Africa)
• M. brevistylis (New Caledonia)
• M. cacuminum (New Caledonia)
• M. carminea - Carmine rātā (New Zealand)
• M. colensoi (New Zealand)
• M. cordata (New Guinea)
• M. diffusa (New Zealand)
• M. dolichandra (New Caledonia)
• M. fulgens - Scarlet rātā (New Zealand)
• M. halconensis (Philippines)
• M. longipetiolata (New Caledonia)
• M. operculata (New Caledonia)
• M. ovata (New Guinea)
• M. paniensis (New Caledonia)
• M. parkinsonii (New Zealand)
• M. patens (New Caledonia)
• M. perforata (New Zealand)
• M. porphyrea (New Caledonia)
• M. ramiflora (New Guinea)
• M. rotundifolia (New Caledonia)
• M. scandens (New Guinea)
• M. whitakeri (New Caledonia)
• M. whiteana (New Guinea)
• M. n. sp. (un-named) (Solomon Islands)

subgenus Carpolepsis

• M. elegans (New Caledonia)
• M. laurifolia (New Caledonia)
• M. tardiflora (New Caledonia)

References

• Simpson, P., 2005. Pōhutukawa & Rātā: 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 Hawaii 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 Riding the ice age El Niño? Pacific biogeography and evolution of Metrosideros subg. Metrosideros (Myrtaceae) inferred from nuclear ribosomal DNA. PNAS. Retrieved on 2007-06-13..