Most species of the genus Eucalyptus, belonging to the family Myrtaceae, occur naturally only in Australia. Eucalypts, which are evergreens, constitute about 95 per cent of Australia's forest trees and a large portion of the woodlands. Eucalypts are found throughout the continent and Tasmania, from the coast to the alpine treeline and from the edge of the rainforests to the desert fringes. They vary greatly in size. The smallest species, Mallees, grow to about 1 m in height although some reach 12 m. At the other end of the scale are the coastal forest giants, the tallest hardwoods in the world, which grow to a height of more than 90 m. Most eucalypts are forest trees which range between 30 m and 50 m in height or woodland trees of 10 m to 25 m in height.
The identification of Eucalyptus species is extremely difficult and classification depends mainly on the shape of the operculum (the covering flap on the seeds) and the fruit. For practical purposes eucalypts are usually placed into groups based on bark types. The five principal bark types are: smooth (the various gums), scaly (bloodwoods), scaly to subfibrous (boxes), fibrous (stringybarks and peppermints) and furrowed (ironbarks).
Some gums take two or more years to shed their bark. This process of gradual shedding gives the trunks a patterned approach as the bark changes colour due to weathering. Colours may range from yellow in newly exposed wood to silver, dark grey and brown. The spotted gum, Eucalyptus maculata usually has an attractive mosaic bark and the brittle gum, E. mannifera maculosa, has a powdery white trunk which is red in summer just before the ark is shed. The river red gum, E. camaldulensis, is often the most widely spread of the gums. Its bark is often red with patches of white and red.
The ribbon gum, E. viminalis, is so called because the bark is shed in long, ribbon-like strips which hang from the branches and trunk. The scribbly gum, E. haemastoma, has distinctive bark patterned by scribbly lines caused by the larvae of a small moth. Trees such as the woolly butt, E. longifolia, and blackbutt, E. pilularis, are so called because of the persistent bark around the trunk which becomes dark-coloured with age.