|M. armillaris foliage and flowers|
L. nom. cons.
Melaleuca // is a genus of nearly 300 species of plants in the myrtle family, Myrtaceae, commonly known as paperbarks, honey-myrtles or tea-trees (although the last name is also applied to species of Leptospermum). They range in size from small shrubs which rarely grow to more than 1 metre (3 ft) high, to trees up to 35 metres (100 ft). Their flowers generally occur in groups, forming a “head” or “spike” resembling a brush used for cleaning bottles, containing up to 80 individual flowers. They are superficially like Banksia species which also have their flowers in a spike, however the structure of individual flowers in the two genera are very different.
Second only to members of the family Proteaceae, melaleucas are important food sources for nectarivorous insects, birds and mammals. Many are popular garden plants, either for their attractive flowers or as dense screens and a few have economic value for producing fencing and oils such as “tea tree” oil.
Most melaleucas are endemic to Australia, with a few also occurring in Malesia. Seven are endemic New Caledonia and one is found only on the (Australian) Lord Howe Island. Melaleucas are found in a wide variety of habitats. Many are adapted for life in swamps and boggy places, while others thrive in the poorest of sandy soils or on the edge of saltpans. Some have a wide distribution and are common whilst others are rare and endangered. Land clearing, exotic myrtle rust and especially draining and clearing of swamps threaten many species.
Melaleucas range in size from small shrubs like Melaleuca aspalathoides and Melaleuca concinna which rarely grow to more than 1 metre (3 ft) high, to trees like Melaleuca cajuputi and Melaleuca quinquenervia which can reach 35 metres (100 ft). (One specimen of M. cajuputi reached 46 metres (200 ft).)
Many, like Melaleuca linariifolia are known as paperbarks and have bark that can be peeled in thin sheets whilst about 20% of the genus, including Melaleuca bracteata have hard, rough bark and another 20% have fibrous bark. Every species in the genus is an evergreen and the leaves vary in size from minute and scale-like (Melaleuca micromera) to 270 millimetres (10 in) long (Melaleuca leucadendra). Most have distinct oil glands dotted in the leaves, making the leaves aromatic, especially when crushed.
Melaleuca flowers are usually arranged in spikes or heads. Within the head or spike, the flowers are often in groups of two or three, each flower or group having a papery bract at its base. There are five sepals, although these are sometimes fused into a ring of tissue and five petals which are usually small, not showy and fall off as the flower opens or soon after. The stamens vary greatly in colour, from white to cream or yellow, red or mauve with their yellow tips (the anthers) contasting with their "stalks" (filaments).
The fruits are woody, cup-shaped, barrel-shaped or almost spherical capsules, often arranged in clusters along the stems. The seeds are sometimes retained in the fruits for many years, only opening when the plant, or part of it dies or is heated in a bushfire. In tropical areas, seeds are released annually in the wet season.
Taxonomy and naming
The first known description of a melaleuca was written by Rumphius in 1741, in Herbarium amboinense before the present system of naming plants was written. The plant he called Arbor alba is now known as Melaleuca leucadendra. The name Melaleuca was first used by Linnaeus in 1767. Many species previously known as Metrosideros were then placed in Melaleuca. The genus Callistemon was raised by Robert Brown who noted its similarity to Melaleuca, distinguishing it only on the basis of whether the stamens are free of each other, or joined in bundles. Botanists in the past, including Ferdinand von Mueller and Lyndley Craven have proposed uniting the two genera but the matter is not decided. Evidence from DNA studies suggests that either Callistemon and some other genera be incorporated into Melaleuca or that at least ten new genera be created from the present genus.
In 2014, Lyndley Craven and others proposed, on the basis of DNA evidence, that species in the genera Beaufortia, Calothamnus, Conothamnus, Eremaea, Lamarchea, Petraeomyrtus, Phymatocarpus and Regelia be transferred to Melaleuca.
The name Melaleuca is derived from the Ancient Greek μέλας (mélas) meaning “dark" or "black” and λευκός (leukós) meaning “white”, apparently because one of the first specimens described had fire-blackened white bark.
Distribution and habitat
Most melaleucas occur naturally only on the Australian mainland. Eight occur in Tasmania but only two are endemic to that island. One (Melaleuca howeana) is endemic to Lord Howe Island and seven are endemic to Grande Terre, the main island of New Caledonia. A few tropical species also occur in Papua New Guinea and the distribution of one subspecies, Melaleuca cajuputi subsp. cumingiana extends as far north as Myanmar, Thailand and Viet Nam. The south-west of Western Australia has the greatest density of species and in the tropical north of the continent, species like Melaleuca argentea and Melaleuca leucadendra are the dominant species over large areas. Melaleucas grow in a range of soil types and many tolerate occasional or even permanent waterlogging. Some species, especially the South Australian Swamp Paperbark, Melaleuca halmaturorum thrive in saline soils where few other species survive. Many are fire tolerant, regenerating from epicormic buds or by coppicing but there are no melaleucas in rainforest and few species occur in the arid zone.
Melaleuca quinquenervia (Broad-leaved Paperbark) is the most damaging of sixty exotic species introduced to the Florida Everglades to help drain low-lying swampy areas. Introduced in the early 20th century, it has become a serious invasive species, with damaging effects including the displacement of native species, reduction in wildlife habitat, alteration of hydrology, modification of soil and changes in fire regimes. The area of infestation has increased fifty-fold over 25 years. A psyllid, Boreioglycaspis melaleucae has been trialled as a biological control agent. Some success has also been achieved with a weevil, Oxyops vitiosa. Scientists from the Agricultural Research Service in The United States Department of Agriculture continue to test Australian insects for their potential as biological control agents.
Traditional Aboriginal uses
Aboriginal people used several species of Melaleuca to make rafts, as roofing for shelter, bandages and food preparation. "Bee bread" and honey were collected from the hives of stingless native bee hive in melaleuca forests in the Northern Territory.
Melaleuca alternifolia is notable for its essential oil which is both anti-fungal and antibiotic, while safely usable for topical applications. This is produced on a commercial scale and marketed as Tea Tree Oil.
Melaleucas are popular garden plants, both in Australia and other tropical areas worldwide. The first to be cultivated were grown in England from seed in 1771. Some melaleucas are commonly cultivated, grown as trees for parks and large gardens (such as Melaleuca leucadendra) or as ornamentals (sometimes as Callistemon or Calothamnus) such as Melaleuca citrina (Callistemon citrinus), Melaleuca hypericifolia and Melaleuca wilsonii.
Melaleucas used in horticulture
Melaleuca leucadendra cultivated in parks and gardens
Melaleuca pungens useful as a hedge because of its prickly foliage
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