Eucalyptus (From Greek, ευκάλυπτος meaning "well covered") is a diverse genus of trees (and a few shrubs), the members of which dominate the tree flora of Australia. There are more than seven hundred species of Eucalyptus, mostly native to Australia, with a very small number found in adjacent parts of New Guinea and Indonesia and one as far north as the Philippines islands. Species of Eucalyptus are cultivated throughout the tropics and subtropics including the Americas, England, Africa, the Mediterranean Basin, the Middle East, China and the Indian Subcontinent.
Many, but far from all, are known as gum trees in reference to the habit of many species to exude copious sap from any break in the bark (e.g. Scribbly Gum).
As a generalisation "forest trees" are single-stemmed and have a crown forming a minor proportion of the whole tree height. "Woodland trees" are single-stemmed although they may branch at a short distance above ground level.
"Mallees" are multi-stemmed from ground level, usually less than 10 metres in height, often with the crown predominantly at the ends of the branchlets and individual plants may combine to form either an open or closed formation. Many mallee trees may be so low growing as to be considered a shrub.
Apart from the forest tree, woodland tree, mallee and shrub habits two further tree forms are notable in Western Australia. One of these is the "mallet", which is a small to medium-sized tree, usually of steep branching habit, sometimes fluted at the base of the trunk and often with a conspicuously dense, terminal crown. It is the habit usually of mature healthy specimens of Eucalyptus occidentalis, E. astringens, E. spathulata, E. gardneri, E. dielsii, E. forrestiana, E. salubris, E. clivicola and E. ornata. The smooth bark of mallets often has a satiny sheen and may be white, cream, grey, green or copper.
Another habit category used in Western Australia is the "marlock". This has been variously applied but Brooker & Hopper (2001) defined the term and restricted the use to describe the more or less pure stands of short, erect, thin-stemmed "trees" that do not produce lignotubers. These are easily seen and recognised in stands of E. platypus, E. vesiculosa and the unrelated E. stoatei. The marlock is distinguished from mallets which are taller and have a characteristic steep branching habit. The origin and use of the term "morrel" is somewhat obscure and appears to apply to trees of the western Australian wheatbelt and goldfields which have a long, straight trunk, completely rough barked. It is now used mainly for E. longicornis (Red Morell) and E. melanoxylon (Black Morrel).
Tree sizes follow the convention of:
The leaves on a mature Eucalyptus plant are commonly lanceolate, petiolate, apparently alternate and waxy or glossy green. In contrast the leaves of seedlings are frequently opposite, sessile and glaucous. However there are numerous exceptions to this pattern. Many species such as E. melanophloia and E. setosa retain the juvenile leaf form even when the plant is reproductively mature. Some species such as E. macrocarpa, E. rhodantha and E. crucis are sought after ornamentals due to this lifelong juvenile leaf form. A few species such as E. petraea, E. dundasii and E. lansdowneana have shiny green leaves throughout their life cycle. E. caesia exhibits the opposite pattern of leaf development to most Eucalyptus, with shiny green leaves in the seedling stage and dull, glaucous leaves in mature crowns. The contrast between juvenile and adult leaf phases is valuable in field identification.
Four leaf phases are recognised in the development of a Eucalyptus plant - the ‘seedling’, ‘juvenile’, ‘intermediate’ and ‘adult’ phases. However there is no definite transitional point between the phases. The intermediate phase, when the largest leaves are often formed, links the juvenile and adult phases.
In all except a few species the leaves form in pairs on opposite sides of a square stem, consecutive pairs being at right angles to each other (decussate). In some narrow-leaved species - for example E. oleosa - the seedling leaves after the second leaf pair are often clustered in a detectable spiral arrangement about a five sided stem. After the spiral phase, which may last from several to many nodes, the arrangement reverts to decussate by the absorption of some of the leaf bearing faces of the stem. In those species with opposite adult foliage the leaf pairs, which have been formed opposite at the stem apex, become separated at their bases by unequal elongation of the stem to produce the apparently alternate adult leaves.
The appearance of Eucalyptus bark will vary with the age of the plant, the manner of bark shed, the length of the bark fibres, the degree of furrowing, the thickness, the hardness and the colour. All mature eucalypts put on an annual layer of bark, which contributes to the increasing diameter of the stems. In some species the outermost layer dies and is annually deciduous either in long strips (as in Eucalyptus sheathiana) or in variably sized flakes (E. diversicolor, E. cosmophylla or E. cladocalyx). These are the gums or smooth-barked species. The gum bark may be dull, shiny or satiny (as in E. ornata) or matt (E. cosmophylla). In many species the dead bark is retained. Its outermost layer gradually fragments with weathering and sheds without altering the essentially rough barked nature of the trunks or stems - for example E. marginata, E. jacksonii, E. obliqua and E. porosa.
Many species are ‘half-barks’ or ‘blackbutts’ in which the dead bark is retained in the lower half of the trunks or stems - for example, E. brachycalyx, E. ochrophloia and E. occidentalis - or only in a thick, black accumulation at the base, as in E. clelandii. Some species in this category - for example - E. youngiana and E. viminalis - the rough basal bark is very ribbony at the top, where it gives way to the smooth upper stems. The smooth upper bark of the half barks and that of the completely smooth-barked trees and mallees can produce remarkable colour and interest, for example E. deglupta.
Hybrid individuals have not always been recognised as such on first collection and some have been named as new species, such as E. chrysantha (E. preissiana × E. sepulcralis) and E. "rivalis" (E. marginata × E. megacarpa). Hybrid combinations are not particularly common in the field, but some other published species have been suggested to be hybrid combinations and are frequently seen in Australia. For example, E. erythrandra is believed to be E. angulosa × E. teraptera and due to its wide distribution it is often referred to in texts.
An essential oil extracted from eucalyptus leaves contains compounds that are powerful natural disinfectants and which can be toxic in large quantities. Several marsupial herbivores, notably koalas and some possums, are relatively tolerant of it. The close correlation of these oils with other more potent toxins called formylated phloroglucinol compounds allows koalas and other marsupial species to make food choices based on the smell of the leaves. However, it is the formylated phloroglucinol compounds that are the most important factor in choice of leaves by koalas. Eucalyptus flowers produce a great abundance of nectar, providing food for many pollinators including insects, birds, bats and possums. Despite the fact that eucalyptus trees are seemingly well-defended from herbivores by the oils and phenolic compounds they do have their share of insect pests, such as the Eucalyptus Longhorn Borer Beetle, Phoracantha semipunctuata, or the aphid-like psyllids known as "bell lerps," both of which have become established as pests throughout the world wherever eucalypts are cultivated.
As with all large trees, eucalypt branches can be removed by high winds and people have been killed by branches falling from Eucalypt trees in extreme weather. Unlike other trees, the limbs may fall during the quiet period following the storm, when people may not expect such an event.
Eucalypts originated between 35 and 50 million years ago, not long after Australia-New Guinea separated from Gondwana, their rise coinciding with an increase in fossil charcoal deposits (suggesting that fire was a factor even then), but they remained a minor component of the Tertiary rainforest until about 20 million years ago when the gradual drying of the continent and depletion of soil nutrients led to the development of a more open forest type, predominantly Casuarina and Acacia species. With the arrival of the first humans about 50 thousand years ago fires became much more frequent and the fire-loving eucalypts soon came to account for roughly 70% of Australian forest.
The two valuable timber trees, Alpine Ash E. delegatensis and Mountain Ash E. regnans, are killed by fire and only regenerate from seed. The same 2003 bushfire that had little impact on forests around Canberra resulted in thousands of hectares of dead ash forests. However, a small amount of ash survived and put out new suckers as well. There has been some debate as to whether to leave the stands, or attempt to harvest the mostly undamaged timber, which is increasingly recognised as a damaging practice.
Eucalyptus have many uses which have made them economically important trees. Perhaps the Karri and the Yellow box varieties are the best known. Due to their fast growth the foremost benefit of these trees is the wood. They provide many desirable characteristics for use as ornament, timber, firewood and pulpwood. Fast growth also makes eucalypts suitable as windbreaks. Eucalypts draw a tremendous amount of water from the soil through the process of transpiration. They have been planted (or re-planted) in some places to lower the water table and reduce soil salination. Eucalypts have also been used as a way of reducing malaria by draining the soil in Algeria, Sicily, elsewhere in Europe, and California. Drainage removes swamps which provide a habitat for mosquito larvae, but such drainage can also destroy ecologically productive areas. This drainage is limited to the soil surface only since the eucalyptus roots have up to 2.5m length, not reaching the phreatic zone, meaning that rain or irrigation can wet the soil again.
Eucalyptus oil is readily steam distilled from the leaves and can be used for cleaning, deodorising, and in very small quantities in food supplements; especially sweets, cough drops and decongestants. Eucalyptus oil has insect repellent properties (Jahn 1991 a, b; 1992), and is an active ingredient in some commercial mosquito repellents (Fradin & Day 2002).
All parts of the eucalyptus may be used to make plant dyes that are substantive on protein fibres such as (silk and wool) simply by processing the plant part with water. Colours to be achieved range from yellow and orange through green, tan, chocolate and deep rust red. The material remaining after processing can be safely used as mulch or fertilisers..
In 1777, on Cook's third expedition, David Nelson collected a eucalypt on Bruny Island in southern Tasmania. This specimen was taken to the British Museum in London, and it was named Eucalyptus obliqua by the French botanist L'Héritier, who was working in London at the time. He coined the generic name from the Greek roots eu and calyptos, meaning "well" and "covered" in reference to the operculum of the flower bud. This organ protects the developing flower parts as the flower develops and is shed by the pressure of the emerging stamens at flowering.
The name obliqua was derived from the Latin obliquus, meaning "oblique" which is the botanical term describing a leaf base where the two sides of the leaf blade are of unequal length and do not meet the petiole at the same place.
In naming E. obliqua, L'Héritier caused to be perpetuated, most likely by accident, a feature common to all eucalypts - the operculum. In his choice of a specific name, he recognised not only the characteristic feature of E. obliqua, but one common to many other species as well. E. obliqua was published in 1788-89 and coincides with the date of the first official European settlement of Australia.
Between 1788-89 and the turn of the nineteenth century several more species of Eucalyptus were named and published. Most of these were by the English botanist James Edward Smith and most were, as might be expected, trees of the Sydney region. These include the economically valuable E. pilularis, E. saligna and E. tereticornis.
The nineteenth century saw the endeavours of several of the great botanists in Australian history, particularly Ferdinand von Mueller, whose work on eucalypts contributed greatly to the first comprehensive account of the genus in George Bentham's Flora Australiensis in 1867 - which today remains the only complete Australian flora. The account in Bentham is the most important early systematic treatment of the genus. Bentham divided the genus into five series whose distinctions were based on characteristics of the stamens, particularly the anthers (Mueller, 1879-84), elborated further by Joseph Henry Maiden (1903-33), and taken even further by William Faris Blakely (1934). By this time the anther system had become too complex to be workable and more recent systematic work has concentrated on the characteristics of buds, fruits, leaves and bark.
The first endemic Western Australian Eucalyptus to be collected and subsequently named was the yate (E. cornuta) by the French botanist La Billardiére, who collected in what is now the Esperance area in 1792.
Owing to similar favorable climatic conditions, Eucalyptus plantations often replace oak woodlands, the latter a favorable habitat for various wildlife due to an abundance of acorns for food for mammals and birds, and the presence of various insects feeding on dead and down oaks (in turn food for other life forms), and the usefulness of hollows in living oaks and down trees as shelter and nesting sites for birds and small mammals and for bee colonies. All of these favorable conditions disappear when an oak woodland is replaced with a eucalyptus plantation.
Oak woodlands in seasonally dry climates are often fire resistant, particularly if present in open grasslands, as a grass fire is insufficient to ignite the scattered trees. Such is not the case with a eucalyptus forest which tends to grow more densely, obtaining its long-term fire survivability from its ability to regenerate from residual root stock after a fire. Because of the bark litter both on living trees and on the forest floor and because flammable oils in eucalyptus leaves are released by the heat of a fire a dense planting is subject to catastrophic firestorms, unlike oak grasslands .
They went on to note that the promise of eucalyptus in California was based on the old virgin forests of Australia. This was a mistake as the young trees being harvested in California could not compare in quality to the centuries-old eucalyptus timber of Australia. It reacted differently to harvest. The older trees didn't split or warp as the infant California crop did. There was a vast difference between the two, and this would doom the California eucalyptus industry.
One way in which the eucalyptus, mainly the blue gum E. globulus, proved valuable in California was in providing windbreaks for highways, orange groves, and other farms in the mostly treeless central part of the state. They are also admired as shade and ornamental trees in many cities and gardens.
Eucalyptus forests in California have been criticised because they compete with native plants and do not support native animals. Fire is also a problem. The 1991 Oakland Hills firestorm which destroyed almost 3,000 homes and killed 25 people was partly fueled by large numbers of eucalypts in the area close to the houses.
In some parts of California eucalypt forests are being removed and native trees and plants restored. Individuals have also illegally destroyed some trees and are suspected of introducing insect pests from Australia which attack the trees.
Popular opposition soon developed that in 1913 a proclamation was issued ordering a partial destruction of all standing trees, and their replacement with mulberry trees. "The proclamation," Pankhurst notes, "however remained a dead letter; there is no evidence of eucalypts being uprooted, still less of mulberry trees being planted. The eucalypt remains a defining feature of Addis Ababa.