As a group, the eucalypts include an unusually wide range of growth forms, varying from the world’s tallest hardwood trees to prostrate shrubs and spanning a height range of more than two orders of magnitude (<1 m to >100 m). Individual species also exhibit exceptional flexibility in their growth responses and are capable of adopting a wide range of growth forms depending on habitat conditions. For example, the same species (e.g. Eucalyptus obliqua) may occur as a tall forest tree in favourable habitats, but also exist nearby as a low shrub in response to unfavourable habitat conditions. Ground-creeping or procumbent forms of some eucalypts also occur; for example, at the seaward edges of coastal communities in Western Australia (M.I.H. Brooker, CSIRO, personal communication).
Certain species of eucalypts can attain great size in response to the high rainfall conditions and the deep, relatively fertile soils of the continent’s most resource-rich environments. These exceptional species constitute the tall open eucalypt forests of Australia. They have been described as the “supreme expression of the genus Eucalyptus sensu lato.” (Ashton 1981a). One of the dominant species of the tall forests in the southeast of the continent, mountain ash (Eucalyptus regnans), has been documented as including the tallest hardwood trees in the world with recorded heights exceeding 100 metres (Penfold and Willis 1961, Mabberley 1997). For example, in the last century a specimen of mountain ash was recorded as reaching 110 metres (Hardy 1935). Other eucalypt species that attain exceptional heights include alpine ash (Eucalyptus delegatensis), manna gum (E. viminalis) which occurs in the southeast, and karri (E. diversicolor) which occurs in the southwest of the continent.
Under adverse conditions, many eucalypt species can adopt a low, multi-stemmed habit known as “mallee”. The mallee growth habit is characterised by multiple stems arising from a partly buried, woody organ called a “lignotuber” (Kerr 1925). In some species, the mallee habit is characteristically expressed as the adult life form, in others it is adopted opportunistically. Mallee eucalypts range in structure from larger forms with few stems and heights of up to 9 metres or more (bull mallees) to dwarf forms with many stems of only one to several metres in height (whipstick mallees) (see Parsons 1981, Wardell-Johnson et al. 1997).
In non-mallee species, the lignotuber is important during the seedling stages, but is not retained in the adult tree except under exceptionally adverse conditions. In the latter case, these non-mallee species adopt the mallee habit. Some eucalypt species do not have the capacity to develop a lignotuber; for example, some of the “ash” species in the subgenus Monocalyptus. Other species, known as “marlocks” (Burbidge 1952), occur as single-stemmed, low growth forms in which the lignotuber is absent or only poorly developed (Parsons 1981).
Eucalypts generally occupy the tallest stratum of the vegetation in which they occur, constituting the main canopy species. In many vegetation types, they dominate the canopy exclusively. Non-dominant eucalypt species may also occur occasionally as understorey species, although this is relatively rare. Usually, eucalypt species present in the understorey are also the canopy dominants of the community (Kirkpatrick 1997).
The wide structural variation that occurs in eucalypt-dominated vegetation can be interpreted as forming a continuum, reflecting the response of the vegetation to gradients of resource availability or environmental factors such as altitude. The major structural characteristics (height and canopy density) of eucalypt-dominated vegetation are determined by this response to environment. As a general rule, eucalypt-dominated vegetation becomes progressively shorter in stature and has a more open canopy structure as environmental conditions become less favourable, for example due to low rainfall, high altitude, high salinity, waterlogged soils.
The structural continuum of eucalypt-dominated vegetation in Australia therefore extends from the world’s tallest hardwood forests (see Ashton 1981a) to low, multi-stemmed mallee (see Parsons 1981). The tall open forests at one extreme of this structural continuum form a discontinuous arc extending from southern Queensland (latitude 25° S) to southern Tasmania (latitude 42° S) to southwest Western Australia (latitude 35° S) (Ashton 1981a). Shrublands dominated by mallee eucalypts at the other end of the structural continuum are typical of Australia’s semi-arid environments, particularly those regions with a Mediterranean-type climate on the southern, eastern and western margins of the arid centre of the continent. Mallees are also found in other regions of the continent, including throughout the arid zone. As well, there are outliers of mallee species in the higher rainfall zones; these usually occur as isolated patches interspersed amongst other vegetation types. These mallee outliers are often associated with infertile soils or other adverse environmental conditions. It has been suggested that the mallee outliers may be relictual, reflecting a former, wider distribution of this vegetation type (Hope 1994).
The jarrah forest in south-west Western Australia is a unique example of a tall open forest in a region with a Mediterranean-type climate (Dell and Havel 1989). Elsewhere in the world, these areas are dominated by sclerophyllous shrub-dominated communities.
Structural growth forms intermediate between the extremes of tall open forest and low mallee shrublands are defined on the basis of height and percentage foliage cover, and include various categories of open forest, woodland and shrubland (see Table 4, also Specht 1970, Carnahan 1976, AUSLIG 1990, National Forest Inventory 1998). These intermediate structural forms constitute the majority of Australia’s eucalypt-dominated vegetation, which occurs throughout the north, east, southeast and southwest regions of the continent (see AUSLIG 1990, map p. 14).
The structural continuum evident in eucalypt-dominated vegetation is most pronounced amongst the altitudinal sequences that run almost unbroken from near sea-level on the east coast to the alpine areas of the Great Divide in the south-east of the continent. The structural continuum is also starkly evident along the gradients of decreasing rainfall and increasing evapotranspiration that extend inland from the coastal and mountain regions at the margins of the continent towards the arid regions at its centre.
The eucalypts exhibit a globally-outstanding variety of growth forms, ranging from the world’s tallest hardwood trees to procumbent or ground-creeping shrubs. The existence of such a broad range of growth forms, spanning two orders of magnitude in height, is exceptional within a single phylogenetic group of woody taxa. The wide diversity of these growth forms is also expressed to a large extent within some individual species (e.g. Eucalyptus obliqua) as well as amongst the phylogenetic group as a whole.
Eucalypt forests include the world’s tallest angiosperms or flowering plants, Eucalyptus regnans, E. delegatensis and E. viminalis in the southeast and E. diversicolor in the southwest of the continent. Under adverse conditions, a low, multi-stemmed shrub habit called “mallee” is commonly adopted. Procumbent shrub forms can also occur in extreme alpine and coastal environments. A large range of growth forms occurs between these extremes of tall forests and low shrublands.
This unusual flexibility of growth form means that eucalypt-dominated vegetation forms a structural continuum of varying height and canopy density in response to environmental variation in many parts of the continent. This structural continuum is best expressed along the altitudinal gradients which extend from near sealevel to the alpine zone in the southeast of the continent, and along the rainfall gradients that extend from the wetter margins towards the arid centre of the continent.