Diversity and Distribution of Eucalyptus
Eucalyptus is a genus of hardwood evergreen forest trees, and is the most conspicuous element of Australian vegetation. Its members constitute 95 percent of the continent's forests and are the dominant trees of Australia's woodlands (Kelly 1969). It is an extremely diverse group, with approximately 500 named species and subspecies and nearly 200 described hybrid varieties (Blakely 1965). The genus is overwhelmingly endemic to the Australian mainland and Tasmania. Only seven species occur naturally outside the continent, mostly on the islands of Papua/New Guinea and the Indonisian island of Timor, and only two of these species are not represented on the Australian continent. Eucalyptus is classified in the family Myrtaceae and is a member of the mostly endemic, 26-genera subfamily Leptospermoideae (Beadle 1981).
The foundation of this genus' great diversity is simply its ability to adapt and to produce species suitable for various environmental conditions. Eucalypts are quite hardy and have successfully adapted to a wide range of conditions. For example, the snow gums (Eucalyptus niphophila, E. pauciflora, and others) inhabit altitudes of 5-6000 feet and can therefore withstand exposure to intense cold, heavy snowfalls, and high winds characteristic of the shallow-soiled Australian Alps. At the other extreme, in the arid, parched deserts of the interior, eucalypts are restricted to watercourses and sheltered depressions where sufficient moisture allows survival during long periods of drought. Amidst these two extremes, eucalypts occur in both tropical summer-rainfall and cool temperate winter-rainfall regions. They occupy dry and wet sites, including swamps. They can be found on both exposed slopes and plains and in sheltered valleys. They grow in shallow infertile sands, rich mellow loams, and intractable clays (Hall, Johnston, and Chippendale 1970). Thus, in response to this great diversity in habitat, Eucalyptus has, through many generations, adapted and speciated to become one of the most diverse living groups on earth. The diversity of the genus lies not only in its shear number of species, but also in the various forms, features, and morphological characters it displays.
Because of its great range of habitation, the widespread Eucalyptus displays a variety of shapes and sizes. The Snow Gum (E. pauciflora), which must deal with the harsh conditions of high altitudes, takes the form of twisted, windswept shrubs, often barely reaching three feet in height (Kelly 1969). However, the Mountain Ash (E. regnans) of the wet, rich-loamed Victorian and Tasmanian forests, often exceeds heights of 320 feet, making it the tallest Australian species and the tallest hardwood in the world. Most forest species rarely attain heights of 150 feet, while woodland species generally do not exceed 80 feet but have a short, thick trunk with heavy limbs to support a broad, spreading crown....