The most readily recognisable characteristics of Eucalyptus species are the distinctive flowers and fruits. The name Eucalyptus means "well-covered"; it describes the bud cap (technically called an operculum). This cap forms from modified petals and falls off as the flower opens. Thus flowers have no petals, decorating themselves instead with many showy stamens. The woody fruits are roughly cone-shaped and have valves at the end which open to release the seeds.
Nearly all eucalypts are evergreen but some tropical species lose their leaves at the end of the dry season. As in other members of the Myrtle family, eucalypt leaves are covered with oil glands. The copious oils produced are an important feature of the genus. Eucalypts also exhibit leaf dimorphism. When young, their leaves are opposite and often roundish and occasionally without petiole. When several years old, the leaves become alternate, quite slender and with long petioles. Plants do not flower until adult foliage starts to appear, except in E. cinerea.
The bark dies annually and species can be roughly grouped based on its appearance. In smooth-barked trees most of the bark is shed, leaving a smooth surface that is often colourfully mottled. With rough-barked trees the dead bark persists on the tree and dries out. Many trees, however, have smooth bark at the top but rough bark on the trunk or its bottom. The types of rough bark is often used to broadly label a group of eucalypts. They are:
Stringybark - consists of long-fibres and can be pulled off in long pieces. It is usually thick with a spongy texture.
Ironbark - is hard, rough and deeply furrowed. It is soaked with dried kino (a sap exuded by the tree) which gives a dark red or even black colour.
Tessellated - bark is broken up into many distinct flakes. They are corkish and can flake off.
Box - has short fibres. Some also show tessellation.
Ribbon - this has the bark coming off in long thin pieces but still loosely attached in some places. They can be long ribbons, firmer strips or twisted curls.
A small genus of similar trees, Angophora, has also been known since the 18th century. In 1995 new evidence, largely genetic, indicated that some prominent eucalypt species were actually more closely related to Angophora than to the other eucalypts; they were split off into the new genus Corymbia. Although separate, the three groups are allied and it remains acceptable to refer to the members of all three genera Angophora, Corymbia and Eucalyptus as "eucalypts".
Specimens of the Australian Mountain-ash, Eucalyptus regnans, are among the tallest trees in the world at 92 metres tall, making them the tallest of all flowering plants; other taller trees such as the Coast Redwood are all conifers.
Most eucalypts are not tolerant of frost, or only tolerate light frosts down to -3C to -5C; the hardiest, are the so-called Snow Gums such as Eucalyptus pauciflora which is capable of withstanding cold and frost down to about -20C. Two sub-species, E. pauciflora niphophila and E. pauciflora debeuzevillei in particular are even hardier and can tolerate even quite severe continental type winters.
Several other species, especially from the high plateau and mountains of central Tasmania such as E. coccifera, E. subcrenulata, and E. gunnii have produced extreme cold hardy forms and it is seed procured from these genetically hardy strains that are planted for ornament in colder parts of the world.
The coolibah trees, referred to in Waltzing Matilda, are eucalypts E. coolabah and E. microtheca.
An essential oil extracted from eucalypt 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.
Eucalypts support the larvae of a number of Lepidoptera species - see list of Lepidoptera which feed on Eucalyptus.
Eucalypts have a habit of dropping entire branches off as they grow. Eucalyptus forests are littered with dead branches. The Australian Ghost Gum Eucalyptus papuana is also termed the "widow maker", due to the high number of pioneer tree-felling workers who were killed by falling branches. Many deaths were actually caused by simply camping under them, as they shed whole and very large branches to conserve water during periods of drought. For this reason, one should never camp under an overhanging branch. This may be the real reason behind the drop bear story told to children - the idea is to keep them away from under dangerous branches.
On warm days vapourised eucalyptus oil rises above the bush to create the characteristic distant blue haze of the Australian landscape. Eucalyptus oil is highly flammable (trees have been known to explode) and bush fires can travel easily through the oil-rich air of the tree crowns. The dead bark and fallen branches are also flammable. Eucalypts are well adapted for periodic fires, in fact most species are dependent on them for spread and regeneration, both from reserve buds under the bark, and from fire-germinated seeds sprouting in the ashes.
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,
Eucalypts regenerate quickly after fire. After the 2003 Canberra fires, hectares of imported species were killed, but in a matter of weeks the gum trees were putting out suckers and looking generally healthy.
The two valuable timber trees, Alpine Ash E. regnans and Mountain Ash E. delegatensis, are killed by fire and only regenerate from seed. The same fire that has had little impact on forests around Canberra has resulted in thousands of hectares of dead ash forests. There has been some debate as to whether to leave the stands, or attempt to harvest the mostly undamaged timber.
Gum trees are also very accomplished at scavenging water at the expense of other plants.
Cultivation and uses
Eucalypts have many uses which have made them economically important trees. Due to their fast growth the foremost of these is the wood. They provide many desirable characteristics for use as ornament, timber, firewood and pulpwood. Fast growth also makes eucalypts suitable as windbreaks.
The roots absorb lots of water and so eucalypts have been planted (or re-planted) 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 and also in Europe and California. Drainage removes swamps which provide a habitat for mosquito larvae.
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.
The nectar of some eucalypts produces high quality honey. Perhaps the Karri and the Yellow box are the best known.
The ghost gum's leaves were used by Aborigines to catch fish. Soaking the leaves in water releases a mild tranquiliser which stuns fish temporarily.
Plantation and ecological problems
Eucalypts were first introduced to the rest of the world by Sir Joseph Banks, botanist on the Cook expedition in 1770. They have subsequently been introduced to many parts of the world, notably California, Brazil, Morocco, Portugal, South Africa, Israel and Galicia. Several species have become invasive and are causing major problems for local ecologies. In Spain, they have been planted in pulpwood plantations, replacing native oak woodland. As in other such areas, while the original woodland supports numerous species of native animal life (insects, birds, salamanders, etc.), the eucalypt groves are inhospitable to the local wildlife which is not adapted to them, leading to silent forests and the decline of wildlife populations. On the other hand, eucalypts are the basis for several industries, such as sawmilling, pulp, charcoal and others.
Eucalypts have been imported into California, primarily for use in windbreaks enclosing large tracts of arid western San Joaquin valley farmland. While some of the Australian gumwoods make beautiful furniture wood, resembling Teak, an inappropriate type for this purpose was imported into California. This type is neither attractive as a veneer (its color varies from yellowish-gray to grayish-green), useful as lumber (it tends to warp and split after cutting), or even thought well of as firewood (oak, nutwoods and fruitwoods are the preferred commercial firewoods in Northern California).
Their presence in the Oakland-Berkeley hills in California has led to periodic wildfires that have spread into dense urban areas, and since these rapidly regenerate from root sprouts the problem will continue until they are completely removed. Severe cold for a period of several weeks has killed off large stands, which had to be immediately removed to eliminate the consequent severe fire hazard. The shedding of bark creates an open and flammable forest litter that also snags on limbs and so provides a direct fire path from forest floor to tree crown.
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