Forest Trees of Australia

By DJ Boland, MIH Brooker, GM Chippendale and 6 more

Forest Trees of Australia is the essential reference for observing, identifying and obtaining information on the native trees in this country. It describes and illustrates over 300 of our most important indigenous trees, which have been carefully selected for their environmental significance, their importance to the timber industry, or their prominence in our landscape.

This new and thoroughly revised edition has been fully updated throughout and includes treatments of 72 additional species. New maps and photographs show us a wonderfully diverse range of forests, from mangrove swamps, tropical regions and deserts, to alpine areas and majestic stands of temperate forests. A colour section illustrates some of the major forest types of Australia and bark from a diverse range of species.

Forest Trees of Australia is an unsurpassed guide to identification for horticulturists, botanists, foresters, students, farmers, environmentalists and all those who are interested in our native trees.

• Language: English
• Publisher: CSIRO PUBLISHING
• Release date: Dec 1, 2006
• ISBN: 9780643098947

Book preview

Introduction

Origin of Australian Trees

A visitor to Australia can only be impressed by the fact that the vegetation is strikingly different from that of countries in the northern hemisphere. In particular forests and woodland dominated by eucalypts and acacias are unique to this country. But further observations show that some formations do have affinities outside Australia. For example, there are strong similarities between the tropical rainforests of Australia and Malaysia, and between the cool temperate rainforest of Tasmania and New Zealand.

In the past, Australia was joined to Antarctica as part of Gondwana during the early Cretaceous period (Fig. 1). This is the period when the flowering plants were evolving, and there is considerable evidence from fossils that, as well as gymnosperms, many elements of the developing angiosperm flora were present on Gondwana by the beginning of the Tertiary, some 65 million years ago.

Because of the breakup of Gondwana, direct migration between South America, Africa and Australia for warm-temperate and subtropical plants was last possible about 100 million years ago. More or less direct migration between Australia and South America via Antarctica may have been possible via elevated areas on the South Tasman Rise until about 85 million years ago (Fig. 2), but is assumed to have been limited to cold-tolerant plants.

Figure 1. Gondwana in early Cretaceous times (135 million years ago): Gymnosperms are well developed and widespread. Development of angiosperm families has begun and migration amongst all the southern continents is possible. Land surfaces of Australia are shown stippled.

Figure 2. Southern continents in Cretaceous times (100 million years ago): Africa, India, New Zealand and South America have separated and Australia is about to separate from Antarctica. This is the last opportunity for the overland entry of Gondwana plants into Australia.

By the mid Miocene (15 million years ago) the Australian plate had moved away from Antarctica and collided with the Asian and Pacific plates in the north (Fig. 3), establishing the first direct path for the migration of plants between Asia and Australia.

The fossil evidence indicates that the flora of southern Australia during the late Cretaceous and early Tertiary was composed largely of forms corresponding to the Antarctic and Indo-Malayan elements mentioned above (Table 1). This flora extended at least to central Australia but unfortunately there is only a sparse fossil record of the Tertiary flora over tropical Australia and the greater part of Western Australia. Pollen and macrofossils of Banksia and Casuarinaceae, which were considered to be part of the autochthonous element, have been identified from the lower Tertiary (Oligocene) in Victoria, and fossils of Eucalyptus, Casuarina and Banksia plus fruits like those of Santalum and various Proteaceae have been found in inland South Australia, in rocks which have been very tentatively attributed to the lower Oligocene. Other groups, including Acacia, do not appear until the late Oligocene–early Miocene.

On the basis of continental movements and the fossil records of plant distribution, it can be postulated that the ancestors of the southern-hemisphere gymnosperm were present in Australia in the Middle Jurassic and that the ancestors of many of our angiosperms spread to Australia from other parts of Gondwana in the Cretaceous and early Tertiary. This influx included constituents of all three major elements, but additional taxa have entered from Asia since the Miocene period, and other more-diverse immigrants have come from various sources.

From a consideration of present distribution and degrees of differentiation shown, estimates have been made of the probable time of entry of various families or their ancestors. Those families which are endemic to Australia, or which have a few species in adjacent regions, are believed to have evolved here from ancestors which were present in the Cretaceous but have since disappeared. Examples are Casuarinaceae, Myoporaceae, Pittosporaceae, Epacridaceae. In Proteaceae the subfamily Persoonioideae is about equally divided between South Africa and southern Australia (with one species of Persoonia in northern Australia and one in New Zealand) but the subfamily Grevilleoideae, with 14 Australian endemic genera, is also represented in South-East Asia and South America, and this is interpreted as showing a family origin in Australia or in Antarctica close to Australia, before the separation of Africa from other parts of Gondwana; the spread to South-East Asia is presumed to be post-Miocene. In the family Myrtaceae the dry-fruited subfamilies Leptospermoideae and Chamelaucoideae are centred in Australia—the latter is wholly Australian—but the fleshy-fruited Myrtoideae have major centres in South America and tropical Asia as well as Australia. Early development in Gondwana would explain the South American–Australian distribution. The Asian occurrence may have originated from Australia.

Figure 3. Australia’s neighbours in Miocene times (15 million years ago): The Australian plate has moved away from Antarctica and collided with the Asian plate. Migration of plants between Australia and Asia, across a narrow sea barrier, is possible.

Other families, while not restricted to Australia, may have one or more endemic genera and, if the number of these is taken to indicate the length of time they have been in the region, then Cunoniaceae, Rutaceae, Santalaceae, Escalloniaceae, Monimiaceae and Sterculiaceae were early arrivals. Fossil evidence supports this postulate in the case of the Cunoniaceae and Santalaceae.

In contrast, there are about 30 families that are represented in Australia by a few genera, none of which are endemic, although their Australian species may be. These are apparently relatively recent immigrants; many are confined to northern or north-eastern areas. Examples are Burseraceae, Annonaceae and Myrtisticaceae.

Other families, with a few genera endemic to Australia or with non-endemic genera represented by many species, probably occupy an intermediate position. Special mention should be made of mangroves and strand-line trees such as Terminalia catappa and Hibiscus tiliaceus. These species are evidently dispersed easily between countries and could have entered more than once, independently of other representatives of their families.

Variation and Adaptation of Australian Trees

About one-third of the seed-bearing plant genera in Australia (about 570 genera out of 1700) are endemic and, of these, over 100 are restricted to the south-western province, 86 to the arid zone, 75 to the tropics and 14 to Tasmania.

At the species level this pattern is accentuated. No precise estimates are available but over 90 per cent of Australian species are endemic. In addition to the 570 endemic genera, 270 genera are represented in Australia by endemic species only.

The large number of endemic species and the restriction of many of these to particular regions are evidence of continuing evolution but the pattern of distribution of species is complex. Some have evidently developed from genera, which migrated from Asia after the Miocene. Terminalia might be considered a good example; of the 29 Australian species only T. catappa occurs in Indonesia, although there are 50 species in the region. Other genera, such as Casuarina, undoubtedly have had a much longer period of speciation, but there is no indication of the age of individual species. On the other hand, there is evidence that new species are evolving at present in several other genera.

Although much work has been done to define morphological differences between species, there has been only limited research on the variation within species on a quantitative basis. Within the genus Eucalyptus, various studies of individual species (e.g. E. camaldulensis) or groups (e.g. E. globulus and relatives) have assessed morphological, chemical or physiological attributes. Similar work on a smaller scale has been done for Acacia. Fossil evidence for occurrence of Eucalyptus earlier than Miocene is inconclusive, but on other indications it is an old genus, differentiated into several subgenera and many species. Many Australian tree species exhibit considerable variation in morphological, chemical and physiological attributes, often in patterns that show correlation with geographic patterns of occurrence. The geographic patterns have been partly formalised by the recognition of provenances, but the delimitation of provenances is not usually explicit. Pattern analysis has shown that for a particular attribute or combination of attributes there may be core areas connected by gradients. This has led to problems in taxonomy, particularly in the definition of species; in some cases the solution appears to be to reduce the status of the intergrading species to subspecies.

Table 1. A geological time scale for the first appearance of some important Australian plant families and genera and the relationship of these events to general changes in Australian climates and landscapes* (Key: K = thousands of years; M = millions of years; NW = north-western Australia; S = southern Australia; SE = south-eastern Australia; NG = New Guinea; NSW = New South Wales; NZ = New Zealand; Qld = Queensland; SA = South Australia; Tas = Tasmania; Vic = Victoria; WA = Western Australia).

*Information for this table was collated from numerous sources, including Hill et al. (1999), McPhail and Hill (2001) and Johnson (2004).

From an ecological point of view, the recognition of variations within species leads to the question whether each variant is adapted to its particular site or whether variants are only partly adapted over a range of sites which themselves vary both in space and time.

Genetic studies in some species of Eucalyptus show that, as well as differences between species and between populations of single species, there is a high degree of genetic variation within populations. This variation in Eucalyptus appears to result from high levels of outbreeding (approximately 70–90 per cent) plus lower viability of selfed seed. In spite of this high level of outbreeding, differences between adjacent stands may be maintained by differential selection pressures. For example, in E. urnigera in Tasmania there is evidence that correlated gradients in leaf wax, leaf shape, bark texture and timing of growth are maintained over an altitudinal range of about 100 m and a horizontal distance of 2–3 km, by strong selection pressures operating in opposing directions.

Similar studies in many other widespread tree genera such as Melaleuca, Banksia and Callitris have shown comparable patterns of variation. In most genera there is usually a clear demarcation between species and more variation within populations than between populations. The implication is that these species are adapted to particular niches and have high competitive ability in niches that are relatively stable. The high variability within populations produces some progeny suitable for each of a range of sites and can take advantage of changing situations.

Adaptation is, of course, related to all aspects of a plant’s environment, but some particular attributes are related to particular features. For example, mangroves are adapted to brief periodic inundation with seawater and have developed mechanisms for excluding salts from their tissues or for excreting them. Because they grow in silty, badly aerated soils with low structural strength, many mangroves have also developed shallow, spreading root systems with emergent pneumatophores, or prop roots. Combinations of morphological and physiological features needed to grow on tidal sites have been developed in species of several unrelated families. In some genera all species are mangroves (e.g. Rhizophora) but in others related species occur in rainforests, suggesting that similar adaptations have occurred at different times.

On drier sites, many species show adaptations to periods of drought stress. Seasonal drought in the tropical region is often associated with the deciduous habit, particularly on sites of moderate fertility, where semi-evergreen or deciduous vine thickets occur. Many of the dry-season-deciduous species appear to be post-Miocene immigrants from the Indo-Malayan region, but Brachychiton is an endemic Australian genus, with species in rainforests, deciduous vine thickets, open forests and woodlands, one species occurring in low open woodlands in the arid region. All are more or less deciduous, suggesting that this feature may be a relic of past conditions. Several dry-season-deciduous species also have swollen taproots in seedling stages and swollen stems in adult forms, which have been interpreted as adaptations for survival in seasonal drought.

With more extended periods of drought stress in the semi-arid and arid zones, leaves become smaller and thicker, often linear in shape and erect or pendulous rather than horizontal. Phyllode-bearing acacias become the dominant trees over large areas, the eucalypts being relegated to specific sites such as watercourses.

Adaptation to temperature stresses is more common in physiological than morphological attributes, although the narrow, vertically-arranged foliage so common in inland species reduces solar heating and increases convectional cooling. Most resistance appears to be physiological but there is evidence that wax coating of leaves and stems is in some cases an adaptation to frost. The low altitude of the tree line in Australia and the poor survival of Australian tree species in northern hemisphere winters indicate that adaptation to cold is not well developed. This is to be expected in a country with a generally mild climate. Low light intensities throughout northern hemisphere winters probably also inhibit the growth of most Australian trees.

Similarly, few Australian trees show adaptation to wind; most species are deformed by wind-shearing in exposed alpine or coastal situations. Of course, some of the deformation on coastal sites must be the effect of salt spray. Araucaria cunninghamii, Banksia integrifolia var. integrifolia and Casuarina equisetifolia are examples of species that are able to grow as upright, symmetrical trees on windswept coasts.

On the other hand, most Australian trees outside the rainforests are well adapted to fire, with attributes such as thick bark, epicormic buds, lignotubers, rhizomes and root-shoots. Many species have woody fruits that protect the seeds from fire and are dependent upon, or stimulated by, fire to release their seed. Others have hard seeds that are stimulated to germinate by heat. In some of these, such as acacias, the seeds are collected and buried by ants, surviving for many years and germinating only after hot fires.

Few rainforest trees are fire-resistant and the boundaries between rainforest and open forest are often determined by fire frequency; rainforests are rarely dry enough to burn but trees on the margins may be killed by scorching. Callitris glaucophylla on the other hand, has evolved in a semi-arid climate, where fires are frequent, but it has limited ability to recover from fire injury. Callitris has been able to survive in a fire-prone region because it forms relatively dense stands on sandy soils, where the sparse ground cover will not normally carry fire.

The vast majority of Australian soils are very infertile, either because the parent rocks (e.g., siliceous sandstones) are low in nutrient minerals or because the present soils are the products of very long periods of leaching, wind-sorting and erosion. In many areas, phosphate availability is further reduced by the high concentrations of aluminium and iron. Consequently, much of the flora has become adapted to low nutrient levels; nitrogen-fixing plants such as legumes and casuarinas are common and nutrient-conserving strategies such as internal recycling have been developed. Some species have also developed growth habits which appear to enable the seedlings to build up a large root system and accumulate nutrient stocks before the aerial shoot is extended, e.g. Eucalyptus marginata and Acacia peuce.

There are fairly extensive areas of calcareous soils in Western Australia and South Australia and the tolerance of particular species to high soil calcium has been recognised for some time. Differences in calcium tolerance have been demonstrated between provenances of Eucalyptus camaldulensis; field occurrences of other species suggest similar differences but these have not yet been shown experimentally.

Kinds of Trees—Some Important Australian Families

This section describes features of some important Australian plant families of which representative species are treated in this book. The descriptions include two additional families which are not represented by species, but which nevertheless warrant discussion, namely Euphorbiaceae and Pittosporaceae. The families within the major groups gymnosperms and angiosperms are arranged alphabetically with the exception of the three closely related families Caesalpiniaceae, Fabaceae and Mimosaceae, which are treated together in the following discussion.

Gymnosperms

The name gymnosperm is derived from the Greek and means naked seed. The gymnosperms consist of conifers and their allies. They are plants whose ovules and seeds are unprotected and open to the atmosphere rather than enclosed in fruits. Ovules are borne on the surface of megasporophylls. Both the micro- and megasporophylls (stamens and carpels) are usually arranged in cones.

In number of species, the gymnosperms are poorly represented in Australia. Only two of four orders, Cycadales and Coniferales, are present and, in Coniferales, only families Podocarpaceae, Araucariaceae, Cupressaceae (includes Taxodiaceae in the broad sense). The Podocarpaceae produce their seeds singly, in clusters or small cones, each seed with a fleshy aril, bract or swollen stem (the shrubby Microstrobos is exceptional in producing cones with four to eight thin scales). Podocarpus is the most widespread genus in the family; there are nine Australian species, with the greatest concentration in northern Queensland but with an extension to Western Australian open forests and to alpine sites in Tasmania and the south-eastern part of the mainland. The leaves of Podocarpus species are linear to oblong, from 1 to 25 cm long. Seeds are borne singly above a swollen fleshy stalk, which is usually brightly coloured. Phyllocladus is represented in Australia by a single species, P. aspleniifolius, restricted to Tasmania; its leaves are reduced to scales but the twigs are flattened to form rhomboidal cladodes. Seeds are borne singly or in clusters, on the edges of the cladodes, each with white aril on a pink fleshy bract. Lagarostrobos franklinii (syn. Dacrydium franklinii) is the sole Australian representative of the genus and is also restricted to Tasmania. It has tiny scale-like leaves and tiny cones with four to eight fleshy scales, with one or more seeds each in a fleshy aril.

Podocarpaceae (Podocarpus elatus)

Podocarpaceae (Phyllocladus aspleniifolius)

Araucariaceae produce large woody cones which generally disinte-grate when the seeds are ripe—Araucaria bidwillii cones fall whole and can be rather dangerous. Araucaria has awl-shaped or lanceolate, sharply pointed leaves; its seeds are united with the cone scale. There are three Australian species: A. bidwillii has sharply pointed lanceolate leaves and large cones (up to 30 cm long) with large, wingless seeds; A. cunninghamii has narrow leaves, cones about 10 cm long and seeds with two approximately equal wings; and the recently discovered ‘living fossil’ Wollemia nobilis has decurrent, rounded leaves in 4 rows and globular to broadly ellipsoidal cone to 12.5 cm long and circumferentially winged seeds. A. bidwillii is confined to Queensland but A. cunninghamii extends from northern New South Wales to New Guinea, while W. nobilis is known from only one gorge in the Wollemi National Park. Each of the former two species occupies a position in each of the two major groups of araucarias: A. bidwillii in section Colymbea, where the adult leaves are flat and the cotyledons remain below ground level; and A. cunninghamii in section Eutassa, where the adult leaves are awl-shaped and the cotyledons are projected above ground level upon germination. Agathis also occurs in Queensland; there are three Australian species, all with elliptical leaves with blunt tips and one-winged seeds, which are free from the cone scales. Wollemia nobilis has characters that exclude it from either of the two major extant groups of araucarias.

Araucariaceae (Araucaria bidwillii)

Cupressaceae (Athrotaxis selaginoides)

The Cupressaceae and the Taxodiaceae are lumped together by some authors. These groups are well represented in the northern hemisphere, with such spectacular trees as the redwoods (Sequoia and Sequoiadendron) and swamp cypresses (Taxodium). The only Australian species considered to belong to the Taxodiaceae by some botanists are from the genus Athrotaxis, which has three species in Tasmania. They have spirally arranged leaves, scale-like and closely pressed to the twig in A. cupressoides but longer and spreading in A. selaginoides, while A. laxifolia is intermediate in shape. The cones of this genus consist of 12–25 spirally arranged scales on a short axis, with up to 6 seeds per scale. Other Australian Cupressaceae genera comprise species that have small cones with up to eight woody scales or valves in one or two whorls. The valves spread to release the seeds but the cones remain on the tree for some years. Callitris, the most important Australian genus in this family, has two whorls of usually three valves of unequal size (except C. macleayana where all valves are of equal length). Most species have small, scale-like leaves but C. macleayana produces needle-shaped juvenile leaves on parts of the mature crown. Callitris is widespread in Australia except in alpine areas, usually on sandy soils of low fertility. All species are susceptible to fire injury and regenerate from seed. Actinostrobus and Diselma are shrubby genera; the fruits of Actinostrobus, with two species in Western Australia, have a single whorl of six valves of equal size. Tasmanian Diselma archeri, the only species in this genus, has cones made up of two pairs of scales, the upper pair with two three-winged seeds.

Cupressaceae (Callitris preissii)

Cupressaceae (Callitris macleayana)

Angiosperms

The name angiosperm is derived from the Greek and means seed in a vessel. The angiosperms are plants in which the ovules are enclosed within an ovary, and the micro- and megasporophylls are borne in flowers. Plants belonging to angiosperms are divided into two groups on the basis of their having one (the monocotyledons) or two (the dicotyledons) cotyledons per seedling.

Arecaceae (Livistona australis)

Monocotyledons

Relatively few of the significant forest trees of Australia are monocotyledons. Palms represent a relatively unusual group of plants because they are tree-like monocotyledons. Because of differences in anatomy, particularly the lack of a cambium layer to produce secondary thickening of the stem and the limited potential for branching, monocotyledons have markedly different architecture from either dicotyledons or gymnosperms. Arecaceae (formerly Palmae) is the most notable palm family in Australia that has tree size species. Palms characteristically have a single slender cylindrical stem surmounted by a crown of large feather- or fan-shaped leaves, large inflorescences of small cream or pale coloured flowers and fruits varying from small, single-seeded, fleshy fruits (e.g. dates) to large nuts such as the coconut. They are important commercially and for domestic use overseas but have scarcely been used in Australia. Bamboo and Pandanus are other monocotyledon species may also reach tree size. Bamboo is very important in Asia, but there are few species native to Australia and these are limited in natural distribution to the tropical regions.

Dicotyledons

The vast majority of Australian forest trees are dicotyledons, representing a wide range of families. Apocynaceae is a widespread family, well represented in tropical and subtropical regions, with a few temperate species. Several genera of lianes occur in Australia, including species in rainforests, open forests and woodlands, with one species in Tasmania (as well as a coastal shrub, Alyxia buxifolia) and a few species in southwestern Australia. Several genera are represented by shrubs, while Ochrosia and Alstonia have tree species in tropical regions, with Alstonia constricta extending to western New South Wales. Many species of Apocynaceae have milky sap. The leaves are opposite or whorled, simple, entire, without stipules or with small, gland-like stipules on the stem or petiole. The flowers are regular, with five sepals, five corollalobes and five stamens attached to the tube of the corolla. The ovary normally consists of two separate carpels, which in the fruit form a pair of follicles opening along the inner edge. These follicles may be 30 cm long in Alstonia. In other genera, including Ochrosia, the carpels are united to form a brightly coloured drupe.

Apocynaceae (Alstonia scholaris)

Bombacaceae (Adansonia gregorii)

The family Bombacaceae is strongly represented in Africa and South America, but in Australia we have only one species of Bombax, in deciduous vine forests, and one of Adansonia, in open woodlands of the Kimberley region. Both species are deciduous; they have digitate leaves, large flowers and fruits and, in Adansonia, grotesquely swollen stems.

Caesalpiniaceae (Erythrophleum chlorostachys)

Fabaceae (Castanospermum australe)

The families Caesalpiniaceae, Fabaceae and Mimosaceae (together called the Leguminosae in some classifications) are a large group of species containing plants that produce fruits in the form of legumes (monocarpellate pods which normally split along both edges, although some forms are fleshy or break into segments).

Caesalpiniaceae is a mainly tropical family, represented in Australia by Lysiphyllum, Erythrophleum and Senna. The Lysiphyllum species and Erythrophleum chlorostachys occur in tropical open forests, woodlands and savannah woodlands, while Senna has many species ranging from desert shrubs to rainforest trees.

Fabaceae is very large, widely distributed and contains the pea-flowered plants, which are mainly herbaceous. Many Australian species are shrubs, which form an important component of the vegetation (with acacias), with ability to fix atmospheric nitrogen. The main Australian tree genera in Fabaceae are: Castanospermum, with a single rainforest species, C. australe; Erythrina which is represented in Australia by E. vespertilio, widespread in the tropical and subtropical regions; and Barklya, with B. syringifolia in the rainforests of Queensland. A few other genera such as Jacksonia and Sesbania have species, which reach small tree size.

Mimosaceae (Acacia salicina)

Mimosaceae is widespread particularly in species of Acacia. The acacias are generally smaller than the eucalypts and, where they occur together, usually form a lower layer of trees or shrubs in the vegetation. Acacia species are found in rainforests as well as in open forests, but are common in the drier inland regions of Australia where they are often the dominant trees. Trees in the family Mimosaceae have numerous showy stamens, which are free in Acacia but are united basally in Albizia and Pithecellobium, forming a tube around the ovary. Albizia is a genus of mainly African and Asian trees and shrubs; many of them are deciduous. The Australian species, occurring in Western Australia, Queensland and the Northern Territory, include valuable timber trees. Archidendron species occur in the tropical and subtropical rainforests and produce very showy flowers.

Casuarinaceae (Casuarina glauca)

Casuarinaceae is a ubiquitous Australian family with representatives as well in the tropical region from Malaysia to Fiji. Without close relatives although possibly related to Hamamelidaceae, it has three genera, Allocasuarina, Casuarina and Gymnostoma, which show reduction of the leaves to scales or toothed sheaths surrounding the twigs, male flowers to single stamens and female flowers to single carpels clustered in cone-like heads. Species of Casuarina occur from the seashore to mountains and deserts and from the tropics to southern Tasmania. This is one of Australia’s few families of wind-pollinated trees other than conifers.

Cochlospermaceae (Cochlosperm gillivraei)

Cochlospermaceae is a tropical family with three genera of trees and shrubs that have lobed leaves, deciduous in the dry season, and large showy flowers. The Australian species of Cochlospermum occur on seasonally dry sites in the tropical regions, from northern Queensland to the Kimberley region of Western Australia. The bright yellow flowers are produced while the small trees are leafless. Flowers are regular, with five sepals and petals, numerous stamens and an ovary of five fused carpels which develops into a relatively large capsule, enclosing numerous black seeds with a mass of white cotton-like fibres. The bark of Cochlospermum contains tough fibres; the wood is soft and brittle.

Combretaceae (Terminalia platyphylla)

The family Combretaceae is distributed throughout the tropics but most genera are confined to specific regions, only Combretum and Terminalia being pantropical. Terminalia is well represented in tropical Australia, while Lumnitzera has two species of mangroves, both of which are found in Australia, and Macropteranthes is an Australian genus of evergreen shrubs or small trees with four species, each of restricted occurrence in Queensland or the Northern Territory. Leaves of the Australian species are spirally arranged, often crowded at the ends of branchlets, simple, entire, often with glands on petioles, leaf bases or laminae and, in Terminalia, deciduous. In some species of Terminalia, sympodial branching leads to a tiered arrangement of the crown. Flowers are bisexual, or male flowers are produced on the upper parts of spikes only; the perianth is united at the base forming a calyx tube, which extends above the ovary as a shallow cup with five triangular lobes. Petals are absent (Terminalia) or fall early; there are 10 stamens in most species. The fruits of Terminalia are dry, two-winged nuts or succulent, usually globular drupes; those of Lumnitzera and Macropteranthes are corky or woody, elongated or conic, with persistent calyx lobes and bracteoles.

The family Cunoniaceae is strongly represented in Australia, five of the 13 genera being endemic. They have opposite, simple, trifoliolate or pinnate toothed leaves, often with conspicuous stipules. Flowers have four or five sepals usually the same number of petals, but no petals in Geissois, Pseudoweinmannia or Ceratopetalum apetalum and twice as many stamens. The fruits are two to four-valved capsules except in Ceratopetalum and Schizomeria. Apart from Ceratopetalum, with eight species, the Australian genera are represented by one or two species, mostly occurring in rainforests or on stream banks.

Cunoniaceae (Geissois benthamiana)

Elaeocarpaceae is strongly represented in South-East Asia and South America; in Australia Elaeocarpus has about 20 species, occurring mainly in the rainforests of Queensland but extending from the Northern Territory to Tasmania. They have alternate, toothed leaves and bear their flowers in showy racemes; the flowers have five sepals and five fringed petals (except E. holopetalus), numerous stamens usually covered by short hairs and a two to five-celled ovary. The fruit is blue or black, with a fleshy outer layer covering a hard stone. Sloanea fruits are hard and dry, covered with rigid bristles and opening in two-four valves to expose the seeds, which are covered by a scarlet pulp or reddish coating. Sloanea flowers have numerous stamens, as in Elaeocarpus, but petals are undivided, or absent. The Sloanea species and some species of Elaeocarpus have prominent buttresses, which may extend up the trunk to a considerable height.

Elaeocarpaceae (Sloanea woollsii)

The family Eucryphiaceae, with a single genus Eucryphia, was earlier included in Saxifragaceae by some botanists and in Rosaceae by others. There are four species in Australia and three in South America. Of the two Australian tree species, the Tasmanian E. lucida has simple leaves while E. moorei of New South Wales has pinnate leaves, the leaflets of which are oblong and paler on the undersurface, like the leaves of E. lucida, but which have a fine extension at the tip. The young leaves, stipules and buds of Eucryphia are covered by a resinous substance; stipules of the pairs of opposite leaves are joined between the petioles. The four sepals of the flower are shed as a unit as the flower opens. Eucryphiaceae is evidently a family of the Gondwana flora, now restricted to cool moist climates.

Eucryphiaceae (Eucryphia lucida)

The family Euphorbiaceae consists mainly of herbs but there are several genera with tree species in Australia, including the mangrove Excoecaria agallocha. Trees in this family have mostly alternate leaves, which are simple, usually entire and frequently have two small glands at the top of the leafstalk. Some species have milky or coloured sap or clear sap which becomes coloured on exposure to the air; it is usually poisonous. Flowers are unisexual, often lack petals and commonly have a three-celled ovary which develops into a three-lobed capsule, although, amongst the Australian species, Drypetes deplanchei has red single-seeded fleshy fruit and Bridelia exaltata has yellow two-seeded fruit, while Austrobuxus swainii has two-celled capsules, each cell containing a single yellow-plumed seed, and Glochidion ferdinandii has five to seven-celled capsules with one to two seeds per cell.

Fagaceae (Nothofagus cunninghamiii)

Nothofagus is the only genus of the family Fagaceae occurring naturally in the southern hemisphere. It is well represented in South America, New Zealand, New Guinea and New Caledonia, and fossil (pollen) evidence shows that it was widespread in Australia in the early Tertiary period. At present there and three Australian species occurring in cool moist regions of Tasmania, eastern and southern Victoria, northern New South Wales and south-eastern Queensland. Nothofagus gunnii is the only known cold-deciduous tree in Australia.

Lauraceae (Cryptocarya erythroxylon)

Lauraceae is a family that is widespread in tropical and subtropical regions of the world, but in Australia the native trees in this family are restricted to the rainforests and gully vegetation, where they may be a conspicuous element. Species of Beilschmiedia, Cryptocarya, Endiandra and Litsea, with alternate, simple, entire leaves, often having very small oil glands, are very similar in appearance. Cinnamomum species usually have opposite leaves, while those of Neolitsea are clustered. In Litsea and Neolitsea the flowers are borne in clusters, surrounded by deciduous bracts, and the fruit is held in a cup-shaped receptacle. In Cryptocarya the perianth tube encloses the fruit, so that the lobes of the perianth are carried at the top of the fruit, while both Endiandra and Beilschmiedia carry the remains of the perianth at the base of the fruit. The last two genera are distinguished by Endiandra having three stamens and Beilschmiedia having nine. Many of the northern Queensland trees in this family are known as walnuts (Beilschmiedia and Endiandra) or laurels (Cryptocarya).

Meliaceae (Dysoxylum fraserianum)

Meliaceae (Toona ciliata)

Meliaceae is a family of tropical and subtropical trees including some which produce excellent timbers. They have large simple, pinnate or bipinnate alternate leaves, without stipules; many species are deciduous. Flowers are borne in large panicles and have four to six sepals and petals, usually twice as many stamens, mostly united into a tube. The fruit is a two to five-celled capsule or berry with one to several seeds. Many of the species occur in rainforests but species of Xylocarpus occur as mangroves, and some Owenia species are found in deciduous vine thickets and semi-arid woodlands. Toona ciliata, the Australian red cedar, is probably the best-known species but white cedar, Melia azedarach, is widely planted as a hardy ornamental tree. Dysoxylum with 15 species, has the largest Australian representation. Of the major Australia genera, Melia is distinct in having bipinnate leaves with toothed leaflets, pale lilac-coloured flowers and yellow berries. Toona, of which the pinnate leaves have asymmetrical leaflets with drawn-out tips and blunt bases, has fragrant white flowers with four to six free stamens, and light-brown five-valved capsules containing up to 25 winged seeds. Dysoxylum also has pinnate leaves, variable between species. All have small, cream or mauve flowers with the anthers borne on the inner side of a staminal tube, and capsular fruits up to 5 cm in diameter and two to five-celled, with one or two seeds in each cell. Owenia has pinnate leaves, often blunt-tipped, and anthers borne on a staminal tube, but the fruit is a drupe with a fleshy outer coat surrounding a hard, woody stone containing two to four single-seeded cells.

Monimiaceae (Doryphora sassafras)

Monimiaceae (Atherosperma moschatum)

Monimiaceae appears to be an ancient family with representatives in South America, Australasia and Malagasy Republic, and fossils in Europe. Leaves are opposite, often toothed, and various parts of the tree are strongly aromatic, giving rise to the common name of sassafras. Flowers are usually unisexual, with a lobed or toothed perianth, often cup-shaped, enclosing six or many anthers or carpels, the latter developing into achenes covered with fine hairs (or smooth and succulent in Wilkiea). The major Australian genera are Atherosperma, Daphnandra and Doryphora. Atherosperma has acutely pointed, toothed or entire leaves, paler on the underside, and occurs in cool temperate rainforests of Tasmania and Victoria (with minor occurrences in New South Wales). Daphnandra also has acutely pointed, toothed leaves but these are green on the underside; there are two species in New South Wales rainforests and two in tropical Queensland. Doryphora, with two species, is distinguished by having long extensions to the stamens; its leaves are bluntly pointed, coarsely toothed and green on the undersurface. Doryphora sassafras occurs in temperate rainforests from the Macpherson Range to southern New South Wales, while D. aromatica occurs in the rainforests of northern Queensland.

Moraceae (Ficus macrophylla)

Moraceae, with simple, entire or toothed leaves, milky sap and unisexual flowers in compound inflorescences, is a family which is widely distributed in the tropics and subtropics. The most conspicuous genus is Ficus, which has fruits aggregated inside a fleshy receptacle. Each leaf is enclosed during development in a single, broad-based stipule which falls to leave a ring-like scar. Figs are common in Australian tropical and subtropical rainforests, but occur also on rocky sites in monsoon and arid zones. Streblus, also widespread but less common than Ficus, has elliptical leaves with drawn-out points, toothed margins and rough surfaces. Its fruits are small, ovoid and fleshy, with a single seed.

Myrtaceae, subfamily Leptospermoideae (Eucalyptus pellita)

The most conspicuous and characteristic group of Australian trees is the family Myrtaceae, and particularly the subfamily Leptospermoideae which includes dry-fruited forms such as Eucalyptus, Angophora, Syncarpia, Lophostemon and Melaleuca. These have simple, entire leaves, usually with oil glands but without stipules, and bisexual flowers with the ovary at least partly enclosed in a hypanthium so that the four or five sepals, four or five petals (where present) and numerous stamens are borne above or in a ring surrounding the ovary, which has several cells and numerous ovules.

Myrtaceae, subfamily Leptospermoideae (Melaleuca quinquenervia)

The eucalypts, the largest genus in the family, occur over a wide range of climates and sites in Australia (plus a few in islands to the north), but not in mangroves, rarely in rainforests and only sparsely in arid areas. Eucalypts are distinguished within the family by the possession of opercula (single or double) covering the floral buds, and the lack of petals. The genus is treated more comprehensively later (see p. 200).

Myrtaceae, subfamily Leptospermoideae (Syncarpia glomulifera)

Myrtaceae, subfamily Leptospermoideae (Angophora floribunda)

Angophora and Syncarpia have opposite leaves—often clustered at the end of each growth flush in Syncarpia—but while Angophora has separate, stalked fruits, usually ribbed and bearing small calyx teeth on the rim, in Syncarpia the fruits are united to form a woody mass with seven openings. Melaleuca and Lophostemon have their stamens united into five bundles; in Melaleuca the flowers are stalkless and arranged in heads or spikes, while in Lophostemon each flower is stalked. Angophora species occur in dry sclerophyll forests or woodlands, over a range of sites; Lophostemon and Syncarpia are typical of wetter sites, stream banks or rainforest margins, but Melaleuca species occur over a wide variety of conditions, from freshwater swamps and seasonal flood-plains to deep sandplains, which have strongly impeded drainage.

The fleshy-fruited Myrtaceae (subfamily Myrtoideae) include many trees of tropical to temperate rainforests—one species, Syzygium smithii, extends to Wilsons Promontory in Victoria—the capsular fruited Backhousia and Lophostemon forming links between these and the genera of the drier forests and woodlands, e.g. Eucalyptus. Leaves in this subfamily are simple, opposite, entire, usually tapering to a slender point and usually with translucent oil glands. Flowers are coloured in a few species but generally white or cream, with four or five petals, numerous free stamens and the ovary immersed in the hypanthium. Fruits vary in diameter from 0.5 to 7 cm, in colour from white through yellow or red to blue, purple or black, and contain one (Acmena and Syzygium) to many (Rhodomyrtus) seeds. The subfamily includes both small and large trees with bark types ranging from smooth, mottled brown and green, through papery-scaly to fibrous.

In Pittosporaceae seven of the 10 genera are endemic to Australia. Hymenosporum occurs as well in New Guinea, and Citriobatus in Java and the Philippines, but Pittosporum has a wide distribution, from the Atlantic island of Madeira through Africa and Asia to the Pacific as far as New Zealand and Hawaii. The Australian species of Hymenosporum and Pittosporum are small to medium-sized trees of tropical and temperate rainforests or stream banks. One species (P. phillyreoides) occurs widely in arid low open woodlands. Trees in Pittosporaceae have alternate, simple, entire leaves, sometimes clustered at the ends of the twigs, rather large, fragrant white or yellow flowers with five sepals, petals and stamens. The fruits are two- or three-valved capsules containing winged (Hymenosporum) or sometimes sticky (Pittosporum) seeds.

Myrtaceae, subfamily Myrtoideae (Syzygium suborbiculare)

Another characteristically Australian tree family is Proteaceae. This family, and particularly subfamily Grevilleoideae, is strongly represented in north-eastern Australia by numerous endemic genera. The most widespread genera, Banksia and Grevillea, include many shrubs as well as trees, but several genera have one or a few species and occur only in rainforests. The Proteaceae are characterised by usually bisexual flowers with a four-lobed petaloid perianth (tepals), four stamens which are often attached to the tepals and a one- or two-celled ovary, often borne on a stalk (gynophore). Proteaceous leaves are usually alternate and without stipules, simple (occasionally compound) and entire or variously lobed and divided, and sometimes with apparently primitive or reduced venation. Many of the tropical species have large, deeply divided juvenile leaves.

Proteaceae (Banksia integrifolia)

The most distinctive genus in Proteaceae is Banksia, which is characterised by producing its flowers in dense cylindrical or globular spikes with very numerous projecting styles. The fruits become embedded in the swollen axis of the spike, to form a woody cone-like mass with horizontal lip-like valves. Banksia is widespread in eastern and southern but particularly south-western Australia.

Proteaceae (Grevillea robusta)

Grevillea is also widespread but few species reach tree size; two wellknown species are Grevillea robusta (silky oak) and G. striata (beef-wood). Many of the Grevillea species have showy bright-coloured flowers in large racemes and leathery fruits, one- or two-seeded. Leaves may be entire or variously dissected.

The pan-tropical family Rhizophoraceae consists of about 20 genera of mangroves and rainforest trees, centred in South-East Asia, with four genera represented in Australia. The Rhizophoraceae have opposite leaves with conspicuous stipules which sheathe the growing point somewhat like the stipules of figs. In many species there are small reddish-brown spots formed by groups of corky cells on the undersurface of the leaves. Inflorescences are axillary, normally in cymes or reduced cymes. In the flower, the calyx has 3–16 persistent lobes, the same number of petals (which fall early and are commonly lobed or fringed) and two or three times as many stamens. Fruits are fleshy, with the persistent calyx lobes either at the top or, by differential growth of the fruit, towards the base and reflexed. In the mangrove genera the seeds germinate on the parent tree.

Rhizophoraceae (Rhizophora stylosa)

Rhizophora is probably the most easily recognised mangrove genus, with looping pneumatophores developed from the prop roots, and aerial roots from the branches. Bruguiera has stilt roots from the base of the trunk, and knobby pneumatophores, while Ceriops, normally only a shrub, has small prop roots and a club-like stem. Carallia brachiata, the only Australian non-mangrove representative of this family, occurs in the vine forests of the tropics, but occasionally forms aerial roots from the lower part of the trunk.

Rosaceae is a very large and diverse family, mostly occurring in temperate regions and with many species in cultivation. There are herbs, shrubs and trees in the family; leaves are usually alternate, with stipules, often pinnate and toothed, and often deciduous. The flowers are basically bisexual and regular, with four or five calyx lobes, the same number of petals and many stamens in whorls of five, borne on the edge of a hypanthium. There is a progression from a single pistil, producing a fleshy fruit such as the cherry, through multiple separate pistils producing aggregate fruits, like blackberry, or enclosed in the hypanthium, as in the rose, to usually five carpels fused with the hypanthium as in pome fruit. Australia has very few native members of this family and these are mostly herbs or scrambling shrubs.

Rubiaceae (Nauclea orientalis)

Rubiaceae is a very large family of mostly tropical plants, but representation in Australia is not great. The major Australian tree species is Nauclea orientalis which occurs mainly on river levees across the northern part of the continent. The Rubiaceae have opposite or whorled leaves, often with stipules joining the petioles of each pair of leaves and leaving a scar on the stem when they fall. In Nauclea the stipules are large and circular; the uppermost pair of stipules encloses the growing point.

Rutaceae (Flindersia brayleyana)

Rutaceae also has oil glands in the leaves, which are usually opposite and may be simple, trifoliolate or pinnate and occasionally toothed. Flowers have four or five sepals and petals, and as many or twice as many stamens as petals. The ovary consists of four or five (occasionally up to nine) carpels which may be more or less separate. Fruit varies from fleshy and citrus-like to one to five-celled capsules. In Flindersia the five carpels are united in a capsule which splits into boat-shaped segments. Flindersia is one of the most important sources of Australian cabinet timbers, including Queensland maple, silkwood and silver ash, although several species in Rutaceae have hard, tough wood; however, they are generally available in only small quantities. Most trees in Rutaceae occur in rainforests, but some species of Geijera and Flindersia are found in deciduous vine thickets and semi-arid woodlands.

Santalaceae (Santalum spicatum)

The family Santalaceae consists of shrubs and trees which are usually parasitic on the roots of other plants, but they do have some chlorophyll-bearing tissues and might therefore be considered as hemi-parasites. They are distinguished from Loranthaceae by the fact that the ovules are differentiated from the placenta, although without integuments. (The Western Australian Christmas tree, Nuytsia floribunda, belongs in Loranthaceae.) The two main tree genera of Santalaceae in Australia are Exocarpos and Santalum. The native sandalwood, Santalum spicatum is a shrub of semi-arid regions, the wood of which is used as incense. Santalum has lanceolate or linear leaves; Exocarpos has scale-like leaves on green twigs; the fruits are small nuts borne above swollen and succulent, often coloured, pedicels. Exocarpos species are common in open forests, as understorey trees with dense dark green crowns; they commonly produce root-shoots when the ground is disturbed.

Sapindaceae (Diploglottis australis)

Although the family Sapindaceae is widespread in the tropics and subtropics and well represented in Australia many of the species are small trees, occurring mostly in dry or littoral rainforests from northern Queensland to southern New South Wales, with one species, Alectryon subcinereus, reaching East Gippsland. Several tree species occur in the monsoon forests of the Northern Territory; Atalaya hemiglauca is widespread in woodlands of the semi-arid zone, from northern New South Wales to the Hamersley Range in Western Australia. The leaves of sapindaceous trees are alternate and compound, consisting of 2–12 leaflets (up to 18 in Jagera pseudorhus) often irregularly arranged and without a terminal leaflet. Flowers are usually small and numerous, in panicles or racemes, and fruits are commonly two- or three-lobed capsules, winged in Atalaya, splitting to reveal one or two (in Harpullia) dark seeds, commonly with a red or yellow aril. The wood is very hard and the surface of the sapwood is corrugated.

Sterculiaceae (Brachychiton populneus)

Sterculiaceae includes large, prominently buttressed trees of the rainforests (Argyrodendron) and non-buttressed but often swollen-stemmed Brachychiton species, some of which grow in rainforests while others occur in much drier formations ranging from sclerophyll forests to shrublands of the arid zone. One species of Heritiera is a tropical mangrove. In this family the leaves are alternate and may be simple, often deeply lobed (Brachychiton) or digitately compound (Argyrodendron). Species of Brachychiton, including those of the rainforests, are briefly deciduous and flower usually in the leafless state. In the major tree genera the flowers are often unisexual, lack petals and have their stamens united into a cup, tube or globular mass, while the three to five, more or less separate carpels of the ovary develop into separate, one-seeded, winged fruits or several-seeded boat-shaped follicles.

Sterculiaceae (Argyrodendron actunophyllum)

The family Verbenaceae includes many genera of trees, shrubs, herbs and lianes, mostly in the tropics and subtropics. Rainforest trees in the genera Gmelina, Vitex and Premna. This family includes some excellent timber trees, the best known being teak, native to South-East Asia, and, in Australia, white beech, Gmelina leichhardtii. In Verbenaceae the leaves are opposite, usually simple and entire, and sometimes deciduous. The inflorescence is compound, often large and showy, with blue, yellow or purple flowers. Fruits are usually fleshy, with a hard inner part divided into four one-seeded cells, and blue or pink in Gmelina, red or black in Vitex and Premna. Gmelina has large ovate leaves with conspicuous venation, and fine brown hairs on the undersurface; Premna has smaller lanceolate leaves, glossy on the upper surface; Vitex usually has digitate leaves with three to five narrow leaflets.

Verbenaceae (Gmelina leichhardtii)

Factors Controlling the Distribution of Australian Tree Species

The present distribution of tree species in Australia is the result of the interaction of several factors on successive generations of individual trees, going back at least to late Tertiary times.

Past climates and the changing Australian landscape

Fossil pollen data indicate a continued dominance of rainforest in the east and south-east during the Miocene, although elements of open forest are present—Acacia, Casuarina, Myrtaceae, Proteaceae, Compositae and Gramineae. In central Australia there is evidence of more open grassland but some rainforest persisted, apparently along the watercourses, and there were increasingly arid conditions in the north-west of the continent.

During the Miocene considerable areas of southern Australia were submerged, destroying the existing vegetation, imposing barriers to east-west migration and, particularly in the Great Australian Bight region, resulting in the formation of extensive limestone deposits which later formed calcareous soils, thus perpetuating the barriers to migration.

In the late Miocene, earth movements in eastern Australia initiated the uplift of the Great Dividing Range and rejuvenated the erosion cycle. Somewhat later, probably during the Pliocene, block-faulting in South Australia elevated the Mt Lofty–Flinders Range system and produced the St Vincent and Spencer Gulfs, simultaneously increasing the range of sites available and imposing additional barriers to migration. The Pliocene also appears to have been a period of generally wet climate with formation of extensive laterites, possibly drier than the Miocene but without strong seasonal contrasts.

Information about the Pliocene flora is limited but there are indications that Myrtaceae and Casuarina were common in inland New South Wales. Nothofagus pollen disappeared from this area in the early Pliocene, but reappeared briefly in the middle Pliocene, without representation of N. brassii type, which is now found in Nothofagus species in New Guinea.

During the early Pleistocene an east-west fold in the central part of South Australia, associated with down-warping on its northern side, cut off the flow of the central Australian rivers and formed an extensive lake, including the present Lake Eyre and possibly Lakes Blanche, Gregory, Callabonna and Frome. Large lakes were also formed in the Murray basin near the present border between Victoria and South Australia and in the Fortescue basin in Western Australia.

The Pleistocene saw a series of glacial and interglacial periods, which were arid and humid respectively, so that during the glacial periods the more mesic flora would be forced by temperatures to migrate northwards and by water availability to move towards the wetter eastern and southern coastal regions. Evidence from buried soils is that there were four arid periods. During the interglacials the climate of much of northern Australia may have been suitable for rainforest; it is possible that species such as the palms Livistona mariae in central Australia and L. alfredii in the Fortescue River area of Western Australia became established in this period although they may well have entered the areas much earlier, during the late Miocene or Pliocene, and persisted through the arid periods.

The alternating cold-dry and warm-moist periods would have resulted in movement of climatic zones and the loss or fragmentation of plant populations. This would have been followed by replacement, either by expansion of remnants of the earlier flora or by invasion of new species, and the Pleistocene must have been an important period in the evolution and distribution of Australia’s flora. This is particularly true of the most recent glacial period, which commenced about 40 000 years ago, reached its maximum between 20 000 and 17 000 years ago, and ended about 10 000 years ago. During this period, glacial and periglacial conditions in Tasmania and the highlands of the south-eastern part of the mainland, lowered the tree line. Increased aridity in central Australia caused loss of vegetation, extensive wind erosion and formation or extension of sand dunes, and lower sea levels extended the shorelines so that the mainland was joined to Tasmania and New Guinea.

Between 8000 and 3000 years ago the climate became warmer and wetter than at present, allowing re-vegetation of inland sand dunes and replacement of eucalypt forest with rainforest in parts of north-eastern Queensland, and alpine herbfields with woodland at high altitudes.

From 3000 years to the present there appear to have been minor fluctuations but generally slightly drier conditions, affecting the size of lakes, the boundaries of vegetation and the proportional composition of flora rather than producing sweeping changes in the vegetation.

The effects of Pleistocene fluctuations in climate are seen in the occurrence of closely related species in eastern and western regions and the disjunct distribution of species, either across south-eastern Australia (e.g. Eucalyptus macrorhyncha) or along the eastern mountain chain (e.g. Nothofagus moorei).

As a result of these historic influences, a suite of species is potentially available for each site. Actual occurrence depends on the effects of climate, topography, soil, fire, insect or other animal predation, disease and competition.

These factors have been considered for each of the species treated in this book, although not all are specifically mentioned for each species.

Present climates

Because of Australia’s geographic position, the mainland lying between 10° and 39°S latitude, with Tasmania extending almost to 44°S, and generally low altitude, it is a warm to hot, dry country (Figs. 4, 5). In summer, daily maximum temperatures of over 30°C are common, and periods of several days when the maximum is over 38°C occur each summer over much of the inland region. There is little indication that these high temperatures directly control the distribution of native plants, however, because there is always an association with water balance.

Figure 4. Average daily minimum temperature for July (isotherms in C°).

Figure 5. Average daily maximum temperature for January (isotherms in C°).

Low temperatures, on the other hand, do appear to control the distribution of many species. The height of the tree line is about 2000 m in the Kosciusko area and 1300 m in Tasmania; at these altitudes the temperature range of the hottest month is about 5 to 14°C and for the coldest month about–5 to +2°C. Both of these parameters may be significant: the summer temperatures indicating the potential for sufficient net photosynthesis to allow growth of woody tissues; the winter temperatures showing the possibility of frost injury. Frost injury may also inhibit tree growth in highland valleys, where temperature inversions may lead to winter temperatures as cold as those at the upper tree line (Fig. 6).

Solar radiation is important both directly for the promotion of photosynthesis and indirectly for its effect on soil and plant temperatures. On a broad scale, radiation (expressed here in units of cal cm–2 day–1) is more or less constant at about 450 across the northern part of Australia; in January there is an increase to about 650 over the central part of the continent, then a decrease to about 500 in Tasmania (Figs. 7, 8). In July there is a fairly uniform gradient from 450 in the north to 150 in Tasmania. This broad pattern is modified by the presence of cloud and atmospheric moisture so that, in general, coastal localities have lower radiation values. Topographic shading is also important in the mountainous areas.

Annual rainfall is a first approximation of the supply of water for plants but the effectiveness of rainfall is modified by seasonal distribution, intensity, soil storage, evaporation, run-off and run-on. Mists and fogs may further add to water supply. Demands for water or potential water losses must also be taken into account. These are due to direct evaporation and transpiration, both of which are related to solar radiation, air temperature and wind. The distribution of species is a function of, on the one hand, the relative ability to grow rapidly when water is not limiting and, on the other, the ability to survive periods of drought stress.

Australia is often cited as the driest continent; 37 per cent of its area receives less than 250 mm of rain annually, 57 per cent gets less than 375 mm and 68 per cent gets less than 500 mm. The mean annual rainfall shows a broad pattern of concentric zones, from less than 125 mm in the northern part of South Australia to over 4000 mm in north-eastern Queensland and about 3700 mm in western Tasmania (Fig. 9). Small areas in the southern part of Western Australia, on the highlands of Victoria and near Darwin receive over 1500 mm.

Figure 6. Frost period—median annual length (days per year). A frost day has a minimum temperature ≤ 2°C.

Figure 7. Global radiation for January (average daily amount in milliwatt-hours per square centimetre; based on Eppley pyranometer data 1968–74).

Figure 8. Global radiation for July (average daily amount