Vegetation pattern according to Groves (1981, 1994)
Groves (1994) provided a recent overview of vegetation in Australia at a continental-scale, based primarily on structural characteristics including canopy density and height. The eucalypt-dominated component of the vegetation is divided into tall open forests (Ashton and Attiwill 1994), open forests (Gill 1994), woodlands (Gillison 1994, Williams and Costin 1994) and scrubs and shrublands (Parsons 1994). A brief summary of aspects of biogeographic pattern amongst each of these vegetation types, derived primarily from respective chapters of Groves 1994, is included below.
The tall open forests are characterised by a height exceeding 30 metres, and a canopy cover of between 30 and 70 percent (Specht 1970). These are predominantly eucalypt forests which include examples of the tallest hardwood trees in the world. Occasionally, other genera may form tall open-forests, but these are now very restricted in distribution; they include Melaleuca, Acacia and Lophostemon (Ashton and Attiwill 1994).
Two main types of tall open-forests were identified by Ashton and Attiwill (1994). These included “wet sclerophyll forests” (Beadle and Costin 1952) which occur in high rainfall areas (1000 to 1500 mm per annum) with deep, fertile soils on both the east and the west sides of the continent, and river red gum forests which occur as occasional stands limited to optimal sites associated with the Murray River system (Ashton and Attiwill 1994).
In the east, the wet sclerophyll forests form a discontinuous arc extending from northern Queensland to Tasmania. In the west, they are confined to small areas in the far south-west of corner of Western Australia. Wet sclerophyll forests exhibit wide variation of both understorey and overstorey species, both across their latitudinal range in the east, and also between the east and west of the continent, where there is a marked discontinuity in species composition. In the east a gradual replacement of canopy dominant species occurs with latitude, and species with a primarily southern distribution may also extend further into northern regions associated with areas of higher altitude (Ashton and Attiwill 1994).
Ashton and Attiwill (1994) identified three broad species groups within wet sclerophyll forest. These include a northeast group primarily associated with central New South Wales and Southern Queensland (including Eucalyptuscloeziana, E. microcorys, E. pilularis, E. saligna and E. grandis), a south-east group primarily in Victoria and Tasmania (Eucalyptusregnans, E. viminalis, E. obliqua, E. globulus, E. fastigata, E. delegatensis, E. cypellocarpa, E. dalrympleana, and E. nitens), and a far south-west group in Western Australia (including Eucalyptusdiversicolor, E. calophylla, E. guilfoylei, and E. jacksonii) (Ashton and Attiwill 1994).
Open-forests of southern Australia
Open-forests are defined as having a height of between 10 and 30 metres and canopy cover of between 30 and 70 percent (Specht 1970). They are dominated almost exclusively by eucalypts, including species of Eucalyptus and Angophora. Open forests typically occur in areas with moderate rainfall and temperature and intermediate levels of available soil nutrients. They extend broadly from the margins of tall open-forest at the wetter end of their range, to woodland vegetation at the drier end of their range (Gill 1994).
In the south-east of the continent, open-forests are found throughout coastal and sub-coastal regions, extending from Brisbane to Adelaide, as well as in Tasmania. In the west, they occur predominantly in the south-west of Western Australia. Open-forests in the east tend to be dominated by mixtures of co-occurring species, whereas in the west, monospecific (pure) stands are more common. Co-occurring species are often, but not always, from different eucalypt subgenera. The understorey of open-forests varies considerably across its range, and includes many species with disjunct distributions. Many understorey genera are common to open-forest in both the south-eastern and south-western parts of the continent. The south-west region also includes a large number of endemic understorey taxa (species and genera) (Gill 1994).
Woodlands are an intermediate structural form between the open-forest of wetter areas and the shrubland vegetation typical of extreme environments associated with low rainfall, high altitude, low soil fertility, etc. Most eucalypt woodlands are evergreen, although deciduous woodlands with some eucalypt species occur in parts of northern Australia. More than 400 eucalypt species have been recorded in woodland formations. These are mostly from the subgenera Blakella, Corymbia, Eudesmia, Idiogenes and Symphyomyrtus, although some Monocalyptus species occur in high altitude woodlands (Gillison 1994).
Eucalypt woodland vegetation varies considerably in structure, depending on environmental factors that influence plant growth. The canopy dominants are predominantly single-stemmed. Gillison (1994) described several broad woodland types based primarily on height. These included: very tall woodland, tall woodland, medium-height woodland, low woodland and very low woodland. Woodland vegetation also varies widely in species diversity (Gillison 1994)
Very tall woodland occurs on deeper, weathered sandy soils in the tropical northern regions. The understorey vegetation includes both shrub and grass components. Tall woodlands extend eastwards from the semi-arid regions of eastern Australia. Eucalyptus microcarpa is a common canopy dominant in the drier areas whereas Eucalyptus moluccana dominates in wetter areas, sometimes extending into open-forest formations. Overstorey taxa of tall woodlands in eastern Australia also include the ironbarks Eucalyptuscrebra, E. drepanophylla, E. melanophloia and E. sideroxylon and the bloodwoods Eucalyptusintermedia and E. polycarpa. In northern areas, the dominants also include species of woollybuts and stringybarks. The understorey of the tall woodlands is highly variable, with grassy understoreys being more common in northern regions (Gillison 1994).
Woodlands of medium height are the most widespread of all woodland vegetation. They include a wide range of dominant canopy species. In the north, canopy dominants include species of bloodwoods, ironbarks and gum-barked eucalypts. In eastern Australia, the dominants largely comprise boxes, including poplar box (Eucalyptus populnea), and ironbarks. In the west, the canopy dominants include Eucalyptusptychocarpa and Eucalyptussalmonophloia. The understoreys are variable, and may include varying proportions of grasses and shrubs depending on a range of factors including soil type (Gillison 1994).
Low eucalypt woodlands typically occur in the semi-arid areas of northern Australia characterised by summer rainfall and winter drought. The formation also includes the paperbark (Melaleuca) woodlands typical of the extensive swamp areas of far north-eastern Australia (Gillison 1994).
Low woodlands are also characteristic of the sub-alpine regions of eastern Australia where snow gum (Eucalyptus pauciflora) constitutes the main canopy dominant. The understorey of these high altitude woodlands may be grassy or heathy or include shrubs up to 5 metres in height (Williams and Costin 1994).
Very low woodlands typically occur in extreme environments characterised by periods of severe drought in northern Australia. The canopy is low (from 2 to 4 metres in height) and usually sparse (2 to 60 percent crown cover). The understorey comprises predominantly grasses, including hummock grasses (Gillison 1994).
Scrubs and shrublands
Eucalypt scrubs and shrublands are characteristically dominated by multi-stemmed shrub forms of Eucalyptus. Canopy height varies between 2 and 10 metres. Scrubs have a denser canopy cover (>30%) compared to shrublands. The dominant eucalypts include lignotuberous mallee growth forms, and non-lignotuberous marlocks (Parsons 1994).
Marlock vegetation occurs in the south-west of Western Australia where it has a relatively restricted distribution. In contrast, mallee vegetation is widespread, particularly in semi-arid areas (200 to 550 mm mean annual rainfall) in the south of the continent where it extends from New South Wales to Western Australia. Isolated patches of mallee also occur scattered throughout the arid areas of the continent (Parsons 1994).
Mallee represents the most arid of the eucalypt-dominated vegetation types in Australia. In wetter areas, it is replaced by woodland, and in more arid areas, by Acacia-dominated shrubland. As discussed previously, mallee vegetation also occurs in other areas in response to extremes of temperature, soil infertility, salt exposure etc (Parsons 1994).
An estimated 200-210 eucalypt taxa occur as canopy dominants in mallee vegetation. All eucalypt subgenera are represented amongst these. Some species such as Eucalyptus diversifolia, E. incrassata and E. eremophila exhibit widespread distributions in both eastern and western areas of the continent. Many others have restricted distributions. Up to 75 percent of mallee eucalypt species are endemic to Western Australia. Mallee areas of greatest species diversity also occur in south-west Western Australia. Marlock vegetation occurs in this region as well, with canopy dominants including Eucalyptus annulata, E. platypus, and E. spathulata (Parsons 1994).
Mallee communities include a wide diversity of understorey vegetation. The main types of understorey comprise scleromorphic shrubs, semi-succulent shrubs, and hummock grasses. A large complement of annual species is also present in some communities. The distribution of understorey types is broadly correlated with climate and soil factors. Infertile soils in areas with high and intermediate rainfall tend to support dense, rich understoreys dominated by scleromorphic shrubs. In the drier areas, infertile soils tend to support understoreys of hummock grasses, with varying complements of shrubs. Fertile soils display a variety of understorey types with varying proportions of grasses and shrubs. Generally, semi-succulent shrubs form an increasingly dominant component of mallee understoreys on fertile soils with decreasing rainfall (Parsons 1994).
Vegetation pattern according to Bridgewater (1987)
Another interpretation of vegetation pattern in Australia has been put forward by Bridgewater (1987). He used a combination of the natural regions defined by Barlow (1985), broad climate types based on those of UNESCO (1973), structural characteristics of the vegetation and the distribution of a few key taxa, including the eucalypt subgenera, to delineate major patterns in the distribution of vegetation at the continental scale.
Bridgewater (1987) used three broad classes of mapping units for the vegetation, including: Tropical/Subtropical evergreen wooded vegetation; Semi-arid grassland and shrublands; and Temperate forests, grasslands and shrubland. Within these classes, 34 vegetation types were recognised, including 16 eucalypt-dominated vegetation types. A map of these may be found in Bridgewater (1987, pp 78, 79). The eucalypt vegetation types described by Bridgewater (1987) and their mapping unit numbers are listed below:
These mapping units for eucalypt vegetation can be interpreted within the eight broad biogeographic regions of Beadle (1981, Figure 6.2, p. 125) as follows. Mapping units are included in square brackets.
1. Wetter Tropics, which extends across large parts of the tropical north of the continent [1, 2, 5];
2. Eastern Coastal Lowlands, which extends the length of the east coast
[6, 8, 9, 21];
3. Eastern Inland Lowlands, which includes the inland slopes of the Great Divide together with the northern parts of the Divide [1, 7, 9, 23, 24, 28];
4. Eastern Highlands, comprising the southern parts Great Divide in New South Wales and Victoria [26, 28];
5. Tasmania [27, 28];
6. the Mallee, which extends across the southern part of the continent and is divided by the Nullarbor Plain [18, 22, 28];
7. South-western Australia, which includes higher rainfall areas in the south-west of the continent [18, 22, 29, 30]; and
8. the Semi-arid and Arid Areas, which comprise a large part of the centre of the continent, extending to the coast in some areas [5, 8, 18].