As discussed above, the evolution of the eucalypts is believed to have been influenced by the environmental changes that took place on the Australian continent as it drifted slowly northwards. Factors such as declining soil fertility, increasing climatic variability, periods of aridity, and increases in the incidence and intensity of fires are thought to have had a major influence on the evolutionary development of the eucalypts (Florence 1981). The ecological and evolutionary responses of the eucalypts to these factors are likely to have been important in enabling them to exploit an increasingly-wide range of habitats and environments across the continent over time and, eventually, to dominate most of its woody vegetation communities. The ecology of eucalypt-dominated communities, which encompasses their adaptations to Australia’s environments and their interrelationships both amongst themselves and with the other biota that make up these communities is both unique and outstanding in the global context.
An unusual and important feature of the Australian continent is the widespread absence of steep environmental gradients. This feature derives, in part, from the exceptionally low relief of the ancient landforms that dominate much of the continent, and also from the gradually-changing nature of climatic gradients that commonly occur across the landscape. Although there are exceptions, environmental variation over much of Australia tends to occur gradually and over relatively large distances, resulting in large areas of relatively similar or gradually varying environments.
This gradual expression of environmental variation is also manifest in a very gradual transition from closed forest formations typical of the wetter areas to the grassland communities of the drier parts of the continent. The transitional zone between these extremes of the vegetation is particularly broad in Australia, extending over large distances and dominating large parts of the continent. The transitional zone occurs throughout the latitudinal extent of the continent, and also extends over a large part of the range of altitudes, rainfall zones and landscape and soil types. In contrast, landscapes such as those in Indonesia, Timor, Papua New Guinea, and South Africa tend to exhibit relatively sudden transitions between closed forest and grassland associated with much steeper environmental gradients. Keast (1981) has described continental Australia as characteristically “flat, open and dry” compared with New Guinea which is “tropical, largely rain forest covered and with high mountains” (p. 1589). The transitional zone therefore forms a much smaller component of these landscapes compared with Australia.
The eucalypts are the dominant vegetation of the transitional zone between closed forest and treeless vegetation in Australia. The environments that characterise this transitional zone generally have poor soils and are subject to periodic drought and recurrent disturbance, particularly due to fire. The eucalypts exhibit a wide range of ecological and morphological adaptations which have enabled them to persist and to dominate in these types of environments. These adaptations are discussed in the following sections.
Fire is an important ecological factor which affects the majority of ecosystems in Australia. Described as an ecological “agent of change” (Gill 1997), fire has a wide range of forms, intensities, frequencies and times of occurrence (Gill 1975). The impacts of fire may have a profound influence on the ecology of vegetation communities, both directly through effects on regeneration processes, plant growth and competition, and mortality, and also indirectly, for example, via effects on populations of predators, pathogens and pollinators.
Fires commonly occur in eucalypt-dominated ecosystems in Australia. Gill (1997) commented that “All eucalypt species are subject to fires but the nature, frequency and season of occurrence of those fires varies widely across Australia” (p. 152). This results in part from the exceptionally wide range of environments and habitats dominated by the eucalypts, spanning much of the rainfall gradient of the continent from the wetter coastal fringes to the drier interior, and extending from the tropical northern regions of the continent to the cool-temperate parts of the southeast and southwest. Climate, soil, terrain and vegetation are among the important determinants of fire regimes in eucalypt communities. The influence of these factors is particularly significant in terms of their effect on rates of production of biomass and also on the accumulation and breakdown of litter which acts as fuel for fires (Walker 1981, Gill 1997).
High intensity, summer fires which occur at intervals of up to one to several centuries are typical of the wetter forests in southern parts of the continent (Ashton 1981b). In contrast, dry season fires of low intensity occur every one to several years in the wet-dry tropical savannas of the north (Braithwaite and Estbergs 1985). Fires of high intensity, but with widely-varying intervals and seasonal incidence determined by factors such as productivity, rates of litter accumulation and ignition sources, are more typical of the eucalypt formations of the drier regions (Gill 1997); these include the jarrah forests of the southwest (Abbott and Loneragan 1986, Bell et al. 1989). The fire regimes in eucalypt-dominated communities in Australia therefore exhibit an enormous variation throughout the continent (Gill 1997).
Eucalypts produce large quantities of litter which includes leaves, woody fruits, twigs, small branches and bark. These dead tissues are produced as a result of the natural growth and canopy replacement processes of eucalypts (see Jacobs 1955). Eucalypts demonstrate continual growth of apical buds and also lack the terminal control and pre-determined growth shown by most other woody taxa, contributing to high rates of growth and litter production.
Eucalypt litter is highly flammable and is an important component of the fuels which carry fires in eucalypt-dominated communities. Other types of fuels are also important in eucalypt-dominated communities, including some live fuels associated with shrubs and small trees. The major types of fuel in a eucalypt-dominated community depends on a range of factors associated with its physical environment, particularly climate and soils, and also factors associated with the vegetation, including its structure, and species composition. Variation in the major types of fuel in eucalypt communities has been discussed by Gill (1997). These major fuel types are summarised in Table 5 below.
Table 5 Fuel types in eucalypt-dominated communities (from Gill 1997)
- coarse-leaved grasses
- eucalypt litter
southern wet forests
- eucalypt litter,
- living shrubs and small trees
southern open forests
- eucalypt litter
- bark on the stems of eucalypts
- fine-leaved grasses
- eucalypt litter,
- living shrubs and small trees
- eucalypt litter,
- living shrubs,
- hummock grasses
The dynamics of fuels in selected Australian vegetation types including eucalypt-dominated vegetation has been considered by Walker (1981). He noted that the components of fuel in eucalypt-dominated communities include mainly leaf, twig and branch litter, and also some bark material (Walker 1981). For example, the work of Ashton (1975) showed the litter standing crop in mature Eucalyptus regnans forest in southeast Australia was composed of leaves (approximately 23% by weight), bark (13%) and wood (60%) (see Walker 1981, Table 5, p. 110). Leaf fall measured in this E. regnans forest was approximately 56% by weight of the annual litter fall per unit area compared to 26% for the non-leafy woody components of the litter (Ashton 1975, Walker 1981)
The importance of branch shedding in the maintenance of the eucalypt crown has been described by Jacobs (1955) who outlined mechanisms that promote the rapidity of this process. These mechanisms are associated with the development of a brittle zone which ensures that lower branches are shed rapidly and at relatively small sizes (up to about 2.5 centimetres diameter). In contrast, for most trees of the world, branch shedding is mediated by indirect factors such as fungal attack and is a much slower process, requiring up to several years before branches become brittle enough to break off. Rapid shedding of smaller branches from the eucalypt canopy therefore is unusual and is an important source of the high proportion of woody material in eucalypt litter.
Bark shedding is also important in contributing to the fuel of many eucalypt-dominated communities. In some species, the old bark is shed in small plates or patches. In other species such as Eucalyptus viminalis the bark is shed in long strips. Larger strips of bark in the litter can blow around considerably in high winds, and bark of this type that has been recently shed often represents a significant component of the fuel in some eucalypt communities (Jacobs 1955). Eucalypt bark often has a high phenolic content, contributing to litter accumulation by slower rates of break-down.
The amount of woody material in eucalypt fuel is also influenced by the different residual times of the components of the litter. For example, the half life of leaf litter may be less than one year compared with nearly five years for woody components (Walker 1981). Despite higher proportions of leaf material in annual litter fall, the relative proportion of the woody components of the litter standing crop in eucalypt-dominated communities therefore will tend to increase over time with respect to the non-woody components. As a result, the fuel of eucalypt-dominated vegetation has a relatively high proportion of woody material.
As well as providing a substantial proportion of the fuel in eucalypt communities, woody components of the litter, particularly the branches and larger twigs, also influence the extent of aeration of litter on the forest floor. The presence of branches tends to reduce the extent of packing of leaves and bark remnants, thereby contributing to the presence of many air spaces and a greater overall volume of the litter on the ground surface.
Rapid canopy growth, high rates of production of litter including woody tissues such as small branches, the accumulation of large quantities of litter on the ground and also the presence of dead twigs, woody fruits, bark etc in the canopy together constitute an unusual and characteristic feature of most eucalypt communities. The litter of eucalypt-dominated communities is unusual in the global context, particularly in relation to its high proportion of woody material. In comparison, the litter of most other woody ecosystems comprises most leaf material with some bark components, but fewer twigs and branches.
Long-distance propagation of fire
Fire spread in most cases can be seen as a process of continual ignition due to flame contact with unburnt fuel. The rapid spread of fires can be facilitated by ignitions occurring metres to kilometres ahead of the flame front as a result of burning brands being generated in the flames, lofted by convection, and cast downwind in a process which is called “spotting” (Cheney 1981). The fuel types of eucalypt-dominated communities provide an abundant supply of material for firebrands, especially bark. The eucalypts with loose fibrous bark or stringybark are regarded as having the greatest potential for short-distance spotting (Cheney 1981). In the case of stringybarks, Jacobs (1955) noted that fires tend to run up the fibrous strands of the outer bark and that pieces of the burning bark break off and can blow considerable distances.
Spotting over long distances has been described as a “characteristic unique to eucalypt fires” (Cheney 1981). Long-distance spotting greatly facilitates fire spread. It is typically associated with high intensity fires and commonly occurs in eucalypt communities with species that have bark decorticating in strips or ribbons. Jacobs (1955) described an example of spotting over distances of 24 kilometres (15 miles) due to burning Eucalyptus pauciflora (snow gum) bark blowing from the distant ranges into Canberra City during the 1939 fires. Cheney (1981) also noted that long-distance spotting can occur up to 30 kilometres ahead of the main fire in eucalypt vegetation. These characteristics of eucalypt-dominated vegetation that facilitate long-distance propagation of fire are both unique and outstanding in the global context.
Survival and reproduction
Recurrent disturbance is an important factor influencing the ecology of eucalypt-dominated communities. Disturbance may have a profound influence on the dynamics of eucalypt communities through effects on regeneration processes, growth and competition, and mortality. Fire is a common disturbance in eucalypt-dominated vegetation (Gill 1978) although there are other types of disturbance that periodically affect these communities, including insect predator outbreaks (Abbott 1992), drought (Pook 1967), severe frosts (O’Brien et al. 1986), and cyclones (Unwin et al. 1988).
The major ecological effects of fire on eucalypts are those that influence the survival of individuals, the survival of seeds, dispersal of seeds, production of seeds and the success of regeneration both from seed and also vegetative buds (Gill 1997). There are two basic types of ecological response to fire amongst the eucalypts associated with species that have relatively fire sensitive populations, and species that have relatively fire resistant populations (Gill 1981). Gill (1997) has termed these as “seeders” and “sprouters” respectively (p. 162). Fire sensitivity relates to the capacity of individuals to survive the effects of fire. It is largely determined by the extent to which the tissues essential for growth and vegetative regeneration are protected from fire. Seeders are killed by fires that kill all the leaves and small twigs in the crown, and regeneration is necessarily by seedling establishment. Sprouters are not killed by such fires; regeneration is largely from dormant buds, but sometimes also includes successful seedling establishment.
Eucalypts exhibit a range of characteristics that avoid or moderate the effects of fire, and therefore facilitate survival and reproduction under different fire regimes. In general terms, these characteristics afford protection to buds and other generative tissues, seeds, and stored food reserves. The combination of characteristics may vary markedly between eucalypt communities depending on factors such as the constituent species, type of environment and fire regime (Gill 1997). Characteristics of the eucalypts that are important for survival and reproduction are summarised in the following sections.
The majority of eucalypts develop a regenerative organ called a “lignotuber” (Kerr 1925). The lignotuber begins to develop during the early seedling stage and forms a swollen, woody structure located at the base of the stem. It contains reserves of dormant buds and food and may be partly buried. With these reserves and its relatively protected position, the lignotuber represents an important mechanism by which the eucalypts are able to persist in difficult environments and to survive disturbances such as those caused by fire (see Jacobs 1955). Lignotuberous species are sprouters (sensu Gill 1997); they occur widely in favourable as well as harsh environments.
In most eucalypt tree species, the lignotuber is present in the early, seedling life stages but ceases to develop in the later sapling and adult stages. In these species, the lignotuber is eventually incorporated into the main stem or trunk which continues to grow around it. This is not always the case. Adverse conditions may result in the lignotuber continuing to develop in the later life stages, ensuring its persistence in adult plants. For example, in the mallee growth form typical of eucalypts in adverse environments, the lignotuber reaches its maximum expression as a dominant component of plant form and may attain sizes of up to several metres in diameter.
Rhizomes or root suckers represent another means of vegetation regeneration in eucalypts although this is relatively uncommon. Rhizomatous regeneration has been documented for several eucalypt species in the tropical savanna woodlands of northern Australia (Lacey 1974, Lacey and Whelan 1976, Gillison 1994).
Some eucalypt species do not develop lignotubers or rhizomes at all. These non-lignotuberous species are typically seeders (sensu Gill 1997) and have populations that are killed by fires which burn the crown. The majority of these, such as some species in the Ash group of eucalypts, occur in environments characterised by relatively high levels of resource availability and where the probabilities of successful seedling regeneration after fire are likely to be high (see Jacobs 1955). Some non-lignotuberous species also occur in less benign environments; for example, the marlocks and the mallets which occur in dry environments in Western Australia that more commonly support mallee eucalypts.
Lignotubers have been recorded in other plant taxa. For example, Gardner (1957) noted the occurrence of lignotubers in taxa of the families Casuarinaceae, Dilleniaceae, Leguminosae, Proteaceae, Sterculiaceae and Tremandraceae in Australia, but the incidence of lignotubers within these families is much less common than in the eucalypts. Estimates indicate that lignotubers occur in all but forty or so (Pryor and Johnson 1981) of the more than 700 species of eucalypts (Brooker and Kleinig 1994). The development of lignotubers in such a large number of species within one phylogenetic group is highly unusual.
The eucalypts have four principal ways of producing leafy shoots, and all are significant in their growth and survival (Jacobs 1955). These include: shoots from naked buds in the leaf axils, shoots from accessory buds, shoots from dormant buds, and shoots from lignotubers. The first two provide the mechanisms for normal canopy growth processes in which the naked buds develop into new growing tips, and the accessory buds provide a backup if the naked buds are lost; for example, due to insect attack or frost damage (Jacobs 1955). As discussed above, the lignotuberous buds also provide an important mechanism for vegetative regeneration available to most eucalypts during the early seedling stage and, for some species growing in adverse environments, in the later sapling and adult stages.
Many species of eucalypts have numerous dormant buds, also called “epicormic” buds (Jacobs 1936). Each dormant bud develops from the accessory bud associated with a leaf. Large numbers of dormant buds occur in each eucalypt tree, usually with one bud for each leaf that has developed during the growth of the tree. After a leaf is lost, its dormant bud persists and continues to grow radially outwards at the same rate as the diameter growth of the stem (Jacobs 1955). The dormant buds are distributed throughout the trunk and branches. They are protected at least to some extent from damage, for example due to fire, by the outer bark layers which are either relatively thick but dead, or thinner, but living and moist. Any death or damage to part of the crown may result in the release of dormancy of a large number of these buds, producing a flush of “epicormic growth”. The buds therefore can provide new stems and leaves to replace the old canopy. Release of dormancy and subsequent growth of the epicormic buds is an important mechanism by which eucalypts are able to re-establish their crown following a disturbance which damages the growing tips.
Under low nutrient conditions, epicormic growth may also provide a mechanism for increased turnover of canopy tissues through the continuous processes of canopy growth and dieback, allowing the rapid recycling of nutrients (Florence 1981).
Dormant buds as a mechanism for vegetative regeneration are not uncommon amongst other woody taxa throughout the world. However, when considered in conjunction with lignotuberous buds, the presence of these mechanisms in the majority of the estimated 700 species of eucalypts is both highly significant and unusual. Their combination enables the persistence by vegetative regeneration of the eucalypts in an exceptionally wide range of habitats throughout the continent.
Regeneration from seed
With the possible exception of some woodland communities, eucalypt regeneration from seed is almost always cued by a disturbance, particularly by fire. In the case of seeders, regeneration from seed is an obligate response to fires which kill the canopy since the population becomes dependent on seedlings for its continuing survival (see Gill 1997). For sprouters, regeneration from seed mostly follows a fire, but does not necessarily follow all fires or other disturbances. In many Australian environments, regeneration of eucalypts from seed is a relatively infrequent event, requiring a particular combination of pre- and post-disturbance conditions for its success.
A reliable seed supply and conditions suitable for seedling establishment are prerequisites for regeneration from seed. Eucalypts in southern parts of the continent retain their seeds in woody capsules in the canopy for up to several years. This characteristic protects the seed and ensures a continuous supply of canopy seed store is available for regeneration. In contrast, many eucalypts of the tropical savannas of northern regions, notably the ghost gums, release their seed soon after it is mature (Setterfield and Williams 1996). The wet-dry tropical environments of northern regions are typically subject to fires, often on an annual basis, and also have reliable wet seasons each year, both of which may be important in relation to this phenomenon.
Eucalypt seed is relatively short-lived once dispersed (see Gill 1997). Rapid removal of newly-released seed by predators occurs widely in eucalypt-dominated communities, particularly by seed harvesting ants (see Jacobs 1955, Ashton 1979, Andersen 1982, O’Dowd and Gill 1984, Wellington and Noble 1985), and also by vertebrate predators (Stoneman and Dell 1994). In the case of mallee eucalypts, the half-life of isolated seeds on the soil surface has been estimated to be only about 5 days (Wellington and Noble 1985). Eucalypt seed that escapes predators and becomes stored in the soil apparently does not persist for long periods, with estimated longevities of one year or less (e.g. Grose 1960, Wellington 1989).
A disturbance such as a fire in a eucalypt-dominated community may stimulate the relatively sudden release of all eucalypt canopy seed stores over a period of a few weeks. This phenomenon of mass seed release following fire has been demonstrated for some species of eucalypts in the forests of the south-west of the continent (Christensen 1971, Burrows et al. 1990), in the tall eucalypt forests in the wetter parts of the southeast (O’Dowd and Gill 1984) and in mallee shrubland in the drier regions of the southeast (Wellington, unpublished data).
Sudden, large changes in seed availability have been shown to occur in other plant taxa around the world; for example, associated with “mast” flowering years, and the effects of these sudden changes in seed availability in overcoming seed predators has been termed “predator satiation” (Janzen 1971). Mass release of canopy seed stores in response to fire is believed to be an important mechanism enabling some species of eucalypts to overcome the effects of seed predators. This mechanism provides an outstanding example of the way in which plant seed dynamics may interact with disturbance regimes to overcome the effects of predators and thereby maximise opportunities for reproduction from seed.
Seedling establishment of eucalypts most commonly follows fire (see Gill 1997) although for some species, such as Eucalyptus camaldulensis (river red gum) which occurs in riverine environments, seedling establishment typically follows a flood (Jacobs 1955). Post-fire seedling establishment has been demonstrated for a range of eucalypt-dominated communities in Australia, including eucalypt forests of the southeast (Grose 1960, Ashton 1976, O’Dowd and Gill 1984) and southwest (Abbott and Loneragan 1986), eucalypt woodlands (Withers 1978) and mallee (Wellington 1989).
Effects of fire that are important for seedling establishment include removal of litter which acts as an impediment to seedling growth, particularly in relation to the development of the root system (Jacobs 1955), and the creation of an ashbed (Mott and Groves 1981). The “ashbed effect” (Renbuss et al. 1973) promotes rapid germination and seedling growth and has been associated with the effects of heating and the burning of litter and plant tissues. Factors believed to be significant in the ashbed effect include: an increase in nutrient availability due to the addition of ash to the soil (Humphries and Craig 1981), changes in soil conditions that may inhibit seedling growth (Florence and Crocker 1962), changes in the presence of allelopathic substances (e.g. May and Ash 1990), increases in the wettability of the soil (Wellington 1981), and microbiological changes (Renbuss et al. 1973,) which affect populations of pathogens and also of organisms beneficial to seedling growth such as mycorrhizal fungi (see Warcup 1981).
Other important effects of fire in facilitating seedling regeneration may result from changes to the physical and competitive environments faced by seedlings in the early stages. These changes can include reduced competition for water (e.g. Wellington 1984), and increased light levels (Jacobs 1955) and elevated temperatures (Raison et al. 1986) at the forest floor associated with the loss of leaves from the canopies of the adult trees.
Adaptation to a wide range of soil nutrient availability and moisture regimes
The wide distribution of the eucalypts in Australia encompasses a diverse range of conditions of soil type, nutrient availability and moisture regimes. The eucalypts demonstrate an extraordinary ability to cope with this range of environmental variation, persisting and growing in severe environments as well as benign, despite limiting nutrient supplies and recurrent periods of high soil and atmospheric water deficit.
A classification of the major Australian soil types has been made by Stace et al. (1968). Brief descriptive summaries of the 44 Great Soil Groups for the continent may also be found in Beadle (1981, Table 1.4, pp. 24-29). Some of these Great Soil Groups are very widely distributed and occur over a range of different climatic zones; others are more limited in extent. For example, the distribution of the Red Earths extends from the arid centre to the wet tropics (Beadle 1981). The wetter regions which support eucalypt forests in the southeast, southwest, east and north of the continent encompass a broad range of different soil types reflecting the wide variation in parent materials in these areas. The majority of the soils are highly leached. Soils of the drier areas associated with eucalypt woodlands and shrubland are also very diverse. The majority of these are formed from fine to medium textured alluvial parent materials, and many show incomplete leaching. Soils of the mallee regions also include large areas of aeolian dunes, often with a high calcareous content (see Keith 1997).
Australian soils generally have a low to very low nutrient status on a world scale, especially in terms of their concentrations of organic carbon (C), nitrogen (N) and phosphorus (P) (Beadle 1981). Areas with soils of moderate to high nutrient status also occur in Australia but these are of relatively limited extent. They are usually associated with areas of recent geological activity, such as the basaltic soils derived from volcanic activity in the east of the continent, particularly on the Great Divide (see Beadle 1981, Keith 1997). The very low nutrient status of Australian soils is associated with the great age of the majority of the landscapes, a long-term absence of geological activity leading to soil rejuvenation (see Keith 1997), and the extensive leaching and laterization (Wild 1958, Prider 1966) and repeated cycles of weathering and erosion (Stewart 1959) that have affected many soils. Phosphorus, which has an average concentration of about 0.03% (Keith 1997) but ranges down to 0.003% (e.g. Mulligan and Patrick 1985), was regarded by Beadle (1981) as the most important limiting element in Australian soils. These concentrations are low in comparison to other soils in the world; for example, North American soils exhibit a range for P of 0.04 to 0.09% (Wild 1958).
The ability to cope with wide variations in soil properties is an unusual feature of the eucalypts (see Beadle 1981, Keith 1997). Individual species vary considerably in their response to soil nutrient status; some are limited to relatively fertile soils but many demonstrate remarkable tolerances to low nutrients. Under conditions of high nutrient supply associated with fertile soils, many eucalypt species demonstrate high growth rates. Reduction in growth rate is an important mechanism allowing eucalypts to reduce nutrient requirements and therefore to persist on relatively infertile soils. Most eucalypts show a capacity to respond to low nutrient conditions by reducing their growth rate, even among species which are potentially fast growing (Mulligan and Patrick 1985).
Extreme soil properties such as high calcareous content, acidity, large concentrations of salt or amounts of certain elements such as aluminium (Al), molybdenum (Mo) and manganese (Mn), or the presence of a lateritic duricrust are also important factors limiting the distribution of many eucalypts. In most situations, however, there are species able to tolerate even these conditions and therefore exploit these more extreme environments. Examples include: Eucalyptus saligna which can tolerate highly acidic soils and Mn concentrations toxic to many other eucalypt species (Winterhalder 1963); species such as E. microtheca which are able to grow on highly calcareous soils that few other species can tolerate (Eldridge et al. 1993); E. camaldulensis which includes ecotypes that have an unusual tolerance to high salinity; and E. marginata which is able to thrive on duricrusts, persisting in a lignotuberous form with a minimum of above-ground development (Dell and Havel 1989) until its roots are able to penetrate to the deeper soils via channels in the duricrust (Abbott et al. 1989, Schofield et al. 1989), contrasting markedly with species such as E. diversicolor which is unable to persist in areas where the duricrust is intact (McArthur and Clifton 1975) (also see Keith 1997).
Moisture regimes across the continent vary widely. A large part of the continent (over one third), including the arid central regions, receives less than 250 mm mean annual rainfall. The extensive semi-arid regions surrounding the arid centre receive up to 500 mm mean annual rainfall. The humid areas towards the northern, eastern and southern margins of the continent receive in excess of 500 mm. Moisture regimes are also influenced by rainfall seasonality, particularly the extreme seasonality in the wetter areas (Fitzpatrick and Nix 1970). In general, seasonal rainfall patterns vary with latitude, with summer rainfall predominating throughout the northern regions and winter rainfall in southern regions. Average moisture index values mapped for the continent for both summer and winter clearly demonstrate this strong seasonality (see Fitzpatrick and Nix 1970, Figure 1.7, p. 13 and Figure 1.8, p. 14). Coastal and montane areas tend to exhibit a more even seasonal distribution of rainfall (see Beadle 1981). Rainfall variability is very high for most areas of the continent and tends to increase as mean annual rainfall decreases towards the centre of the continent (Beadle 1981).
Eucalypts are found in most rainfall environments throughout the continent except for extreme environments, which include the driest parts of the arid regions of the centre and also areas of permanent inundation such as swamps. Many eucalypts are able to persist and grow in areas with pronounced seasonal drought and also in areas with low mean annual rainfall. The majority of eucalypts also have to cope with drought periods associated with the high variability of annual rainfall experienced in many parts of the continent.
The ability of the eucalypts to cope with soil water deficits that develop in these situations is directly dependent on their capacity either to tolerate or avoid the onset of high plant water deficits (Stoneman 1994). In general, the eucalypts postpone development of high plant water deficits via a range of mechanisms to maintain turgor (the normal state of the cells resulting from pressure of the cell contents against the cell wall). These involve either maintaining water uptake through changes to the root system, reducing water losses by changes in leaf function or form, or slowing water losses by increasing solute concentrations in a process known as osmotic adjustment. A recent review of these mechanisms may be found in Stoneman (1994).
Important adaptations exhibited by the eucalypts that enable them to maintain water uptake by the root system include: the capacity to develop deep root systems (Gibson et al. 1994) which is particularly pronounced in species from drier environments; development of higher root/shoot ratios in drier environments (Zimmer and Grose 1958); and flexibility of growth response resulting in the capacity to reduce shoot growth in response to increasing water deficits, effectively increasing the root/shoot ratio (Pereira and Kozlowski 1976, Stoneman 1994). The exceptional ability of the eucalypts as water scavengers is evident in some overseas plantings, for example in India, where they have been observed to lower the water table, causing wells, springs and marshes to dry out.
The eucalypts also exhibit a range of mechanisms to reduce leaf water losses associated with photosynthesis and leaf temperature regulation. These mechanisms may involve variations in physiological response including reduction in stomatal conductance in response to increasing plant water deficit or to high vapour pressure deficits, and diurnal patterns of midday depression or decreasing stomatal conductance (see Stoneman 1994). Leaf orientation may also be important in avoiding temperature effects due to insolation in the hotter periods of the middle of the day, with the majority of eucalypts having a predominantly vertical orientation of adult leaves (Jacobs 1955), and some also demonstrating passive leaf movement associated with increased verticality due to leaf wilting (Stoneman 1994).
Other mechanisms of the eucalypts to reduce water losses involve reducing leaf area either through flexibility of growth response and the slowing of growth rates, or by producing smaller leaves. Leaf shed is a common mechanism which rapidly reduces leaf area at the onset of higher plant water deficits. Osmotic adjustment is also an important mechanism shown to occur commonly amongst the eucalypts which enables them to maintain the turgor necessary for normal physiological function despite increased plant water deficits. As well, some eucalypts have been found to be able to increase the elasticity of cell walls formed during drier conditions, effectively postponing dehydration by decreasing the amount of water that will be lost per unit plant water deficit for those cells (see Stoneman 1994).
In summary, the eucalypts display an extraordinary capacity to survive and grow under a wide range of soil types, soil nutrient conditions and moisture regimes. This very wide ecological amplitude within a single phylogenetic group of vegetation dominants is outstanding in the global context.
Adaptation to low nutrients
The eucalypts also demonstrate an exceptional ability to maintain high growth rates under a wide range of soil nutrient conditions, and to develop large biomass even under the low nutrient conditions associated with of many Australian soils (see Bowen 1981, Hingston et al. 1989, Keith 1997). Adaptations that enable the eucalypts to maintain high growth rates despite conditions of low nutrient availability rely primarily on enhancement of nutrient uptake or efficient nutrient recycling. In extreme situations, however, nutrient limitations also have a marked influence on eucalypt productivity and species distribution despite these adaptations.
Mechanisms that enable eucalypts to increase nutrient uptake involve the development of root characteristics that increase efficiency of uptake. As discussed in relation to moisture regimes, most eucalypts develop large root systems. As well, the eucalypt root system includes a much higher proportion of finer roots and well-developed root hairs compared with many other species, maximizing the surface area available for nutrient uptake per unit investment of root biomass (e.g. Barrow 1977). Symbiotic associations between eucalypt roots and mycorrhizal fungi are a major mechanism enabling many eucalypts to enhance absorption of nutrients from the soil (Bowen 1981). Some eucalypt species also have adaptations enabling them to extract nutrients that are generally unavailable to non-eucalypts, for example, by extracting P from insoluble aluminium and iron phosphates (Mullette et al. 1974).
Flexible response of the eucalypts to temporal variation in environmental conditions is important in allowing eucalypts to grow in situations of limiting nutrient availability. Many species show a capacity to maintain themselves with little growth for considerable periods, responding to the onset of favourable conditions with a rapid growth flush (see Keith 1997). This type of response is particularly important in drier environments where the availability of soil water is a major factor influencing nutrient availability. Many species of eucalypts are also able to maintain high rates of water usage; this may be associated with their capacity for high growth rates under appropriate conditions.
Mobilization of nutrients within the plant and the re-translocation of these nutrients from older tissues to sites of active growth is an important mechanism in eucalypts for reducing nutrient requirements while maintaining growth. For example, redistribution of nutrients from heartwood to sapwood is higher in eucalypts than in other trees growing on more fertile soils (Keith 1997). However, re-translocation of nutrients from senescing leaves in eucalypts is generally similar to other species, with values for eucalypts ranging up to 80% for P and up to 66% for N (Keith 1997).
Differential allocation of nutrients amongst tissues is another mechanism whereby the eucalypts are able to increase their relative efficiency of nutrient use. The maintenance of low nutrient concentrations in certain tissues means that smaller quantities of nutrients need to be tied up in biomass. For example, eucalypts maintain an overall high ratio of non-living to living biomass associated with large amounts of dead heartwood in the trunk; this reduces the overall requirement for nutrients while retaining a high biomass in organs important for structural support. As well, eucalypt leaves have relatively low nutrient concentrations per unit weight; this is associated with sclerophylly and also reflects a lowered metabolic requirement for nutrients (see Keith 1997).
The eucalypts also show a pronounced capacity for opportunistic uptake of nutrients at times of higher availability and storing them if necessary until conditions favourable for growth occur. This phenomenon occurs in response to seasonal variation in soil moisture availability, but can also be important in environments with high variability of annual rainfall. The presence of nitrogen-fixing species including legumes in eucalypt understoreys is another important factor that may influence nutrient availability. Nutrient storage is typically very well developed in eucalypts growing on low nutrient soils (Mulligan and Sands 1988). Important nutrient storage sites in eucalypts include sapwood, twigs, lignotubers and roots (Grove 1988).
The majority of the eucalypts are capable of relatively high growth rates under appropriate environmental conditions, and can also show a rapid growth in response to the onset of good conditions. This phenomenon is associated with their unusual feature of having naked buds (Carey 1930) in the leaf axils of the canopy (Jacobs 1955). The naked buds provide a mechanism for rapid growth response, allowing the eucalypts to build up a large crown very quickly, with the trunk growing in proportion to produce a high biomass (Jacobs 1955). In most situations in Australia, new growth is quickly regulated by insect attack of new bud and leaf tissues. In situations where insect attack is reduced or eliminated, including in overseas countries, the eucalypts show an extraordinary growth potential.
The high growth potential of the eucalypts and their exceptional ability to optimise productivity and to develop large biomass under conditions of low soil nutrient availability in Australia results from various combinations of these mechanisms for nutrient acquisition and recycling. In general, the relative importance of different mechanisms tends to vary at different life stages, with seedlings and saplings being more reliant on uptake mechanisms, and mature eucalypts on recycling (Keith 1997).
Formation of hollows
Eucalypts have a striking capacity to produce hollows in their larger branches and trunks. These tree hollows provide an important ecological resource utilised by a wide range of the fauna of eucalypt-dominated ecosystems including both vertebrates and invertebrates (Gibbons 1994).
Hollow formation is a normal part of the processes of crown development and senescence in eucalypts. The large, dead branches that occur in the healthy, mature eucalypt crown as it ages (Jacobs 1955) are important sites for hollow formation. The breakage of these large branches in mature individuals, or other damage including by fire, exposes the heartwood and provides entry points for the organisms responsible for hollow formation. The shedding of epicormic branches also provides sites for invasion by these organisms (see Jacobs 1955).
Hollows may be present over a relatively long period in the life span of a eucalypt. Formation of hollows most commonly occurs in older individuals in a eucalypt population (Gibbons 1994), although hollows may develop in eucalypt secondary tissues at any stage associated with damage and invasion by organisms that break down wood, such as termites and wood-rotting fungi.
Hollows are a particularly prominent feature of the “over-mature stage” of the eucalypt crown. Branch breakage leading to hollow formation is accelerated by the fungal attack which is present in most older eucalypts and which progressively weakens both the trunk and branches in senescent individuals. The more horizontal orientation of older shaping branches in the crown also increases their tendency to break. Several cycles of branch break and replacement commonly occur in senescent individuals leading to the formation of many hollows (Jacobs 1955).
A wide range of the vertebrate fauna of eucalypt-dominated communities use tree hollows in eucalypts, for example for shelter and nesting. Vertebrates recorded using tree hollows in eucalypts include birds (parrots, cockatoos, owls, treecreepers, pardalotes), mammals (possums, dasyurids, rodents, bats), arboreal reptiles and frogs (see Woinarski et al. 1997). In some cases, tree hollows represent an important resource for a significant proportion of the species in these fauna groups of eucalypt-dominated ecosystems. For example, Wardell-Johnson and Nichols (1991) estimated 20% of the bird fauna of eucalypt forests in southwest Western Australia use tree hollows.
Hollow-dependent vertebrate fauna in Australia rely on the availability of natural hollows rather than creating their own (see Woinarski et al. 1997). Suitable habitat for these taxa is therefore dependent of the presence of hollows which may be an important factor limiting these fauna, for example in eucalypt populations dominated by young individuals (Saunders et al. 1982). The availability of hollows has been shown to have a significant influence on the richness and species composition of the bird fauna of wet eucalypt forests in the southeast of the continent (Loyn 1985). In the case of some species of possums, the presence of suitable hollows is confined to very old trees (Inions et al. 1989) or to particular seral stages in a fire regeneration cycle (Lindenmayer et al. 1993) and is a significant factor influencing their populations.
The exceptional capacity of the eucalypts to produce hollows in their larger branches and trunks and the importance of these in eucalypt-dominated ecosystems is both unusual and outstanding in the global context. Eucalypt hollows are of primary importance for a significant proportion of the fauna of eucalypt-dominated vegetation which includes a large number of hollow-dependent species. Trees other than eucalypts that occur in these ecosystems seldom form hollows, placing an even greater emphasis on eucalypt hollows in the ecology of these fauna.
Other morphological and structural adaptations
In addition to the characteristics already discussed, the eucalypts possess other morphological and structural adaptations that play a significant role in the ecology of eucalypt-dominated communities, including an open canopy structure and differences between juvenile and adult leaf forms.
Canopy structure and light characteristics
The structure of the canopy of the majority of eucalypt species is much more open compared with the canopies of other similar vegetation types, such as the hardwood forests and woodlands of the northern hemisphere (Jacobs 1955). There are some dense canopied eucalypts with a predominantly horizontal leaf orientation (Eucalyptus torelliana, E. grandis, E. botryoides, E. calophylla) but these are relatively few in number compared with open-canopied species.
Factors that are important in contributing to this open structure of eucalypt canopies include the predominantly vertical orientation of adult leaves in most eucalypt species, described by Jacobs (1955) as a “hanging habit” (p. 96), relatively long internodes between successive leaves, and the concentration of the majority of the leaves in the last two orders of branches in the outer reaches of the crown (Jacobs 1955). In contrast, northern hemisphere trees provide dense shade associated with their more horizontal leaf orientation, the arrangement of leaves close together, short shoots and the distribution of leaves throughout the crown as well as at its periphery (Jacobs 1955).
An important effect of this hanging habit of eucalypt leaves is a decrease in light interception in the middle of the day when the sun is closer to the vertical, and increased interception of light in the early morning and late afternoon, when the sun is near the horizon. These interception characteristics allow greatly increased penetration of light to the ground below the canopy during the middle of the day. For example, Turton and Duff (1992) recorded about 45% light penetration to the understorey in Eucalyptus intermedia open forest in north Queensland, compared with 35% penetration for tall forest of E.grandis and only about 5% penetration for mature rainforest. These figures compare with averages of between 10 and 20% penetration of incident radiation to the understorey herbaceous flora for both deciduous broad-leaved forests and coniferous forests in the temperate zone of the northern hemisphere, and as little as 2% penetration below boreal birch-spruce mixed forest (see Larcher 1983).
A major ecological effect of this open canopy structure is a relatively large increase in light available to understorey plants during the day. In some situations, this has been associated with increased species richness in the understorey, possibly due to an increased presence of species that are shade intolerant (e.g. Bowman and Kirkpatrick 1984). As discussed previously, the vertical orientation of eucalypt leaves also provides a means of minimising temperature increases due to incident radiation in the middle of the day, and also of maximising light interception in the early morning when photosynthetic rates may be higher and stomatal function less influenced by plant water deficits.
The leaves produced by most species of eucalypts in their seedling and early sapling stages tend to be markedly different from the leaves produced in the adult stage. For example, the juvenile leaves may be short, not stalked [ie sessile], opposite, largely horizontal in orientation and covered with wax, in contrast to adult leaves which may be very long, stalked [petiolate], alternate, largely vertical in orientation and glossy rather than waxy (see Jacobs 1955). Often a period of transition also occurs when leaves intermediate between juvenile and adult forms may be produced.
This unusual phenomenon of leaf difference, known as heterophylly (e.g. Bell and Williams 1997), also tends to be manifest in the juvenile leaves that develop on the first shoots from dormant buds in both the lignotuber and the trunk and main branches during vegetative regeneration (Jacobs 1955). In some species, including Eucalyptus perriniana and E. pulverulenta in southeastern Australia and E. gamphylla, E. peltata and others in northern Australia, the juvenile leaves are maintained throughout the life cycle (neoteny), with no development of adult leaves occurring (Chippendale 1988, Brooker and Kleinig 1994). In other species, a general trend of retention of juvenile leaf forms has been observed in extreme environments associated with low temperatures or drought (e.g. Potts and Jackson 1986). The reasons for and the physiological or ecological advantages associated with heterophylly in eucalypts are not fully understood (see Bell and Williams 1997). Jacobs (1955) has suggested that the phenomenon may reflect a recapitulation within the growth sequence of the individual involving leaf structures that were manifest during the evolutionary development of the eucalypts.
Diversity of invertebrate groups
Eucalypt-dominated vegetation supports high species diversities within the major groups of invertebrate fauna. There is evidence that co-evolution between invertebrates and the eucalypts has taken place over a very long time period (Matthews 1976), and this is likely to have contributed to the high diversity shown by particular groups of eucalypt specialists. Amongst the major insect groups, co-evolutionary relationships with the eucalypts have been documented in the following: bugs (Hemiptera), particularly the psyllids (Psylloidea) and leafhoppers (Cicadellidae), the beetles (Coleoptera) and especially the chrysomelid beetles (Chrysomelinae), the moths and butterflies (Lepidoptera), the ants and wasps (Hymenoptera), and the bees (Colletidae and Halictidae) (Matthews 1976, Majer et al. 1997). Particular invertebrate groups that have undergone evolutionary diversification in association with the eucalypts include the psyllids, coccid bugs (Eriococcidae), eurymelid bugs (Eurymelidae), cicadas (Cicadellidae and Cicadidae), squashbugs (Amorbini, Coreidae) and shieldbugs (Pentatomidae) (Carver et al. 1991).
An important example is the Oecophoridae or the mallee moths, a group which occurs in Australia mostly in communities dominated by the eucalypts (see Common 1990, Majer et al. 1997). The mallee moths currently include 2650 described species, although estimates of the number of undescribed species take this total to 5500 which represents a significant proportion (about 25%) of Australia’s moth fauna (Majer et al. 1997). The genera of mallee moths are mostly endemic to Australia. The two largest genera (Philobota and Eulechria) occur in the central parts of the east coast (southeast Queensland and northeast New South Wales) (Common 1990) in areas which also support exceptional diversities of eucalypt species (Chippendale 1981).
The psyllid fauna or Psyllidae is another invertebrate group with a high diversity of taxa in eucalypt-dominated vegetation in Australia. An estimated 67% of the total 379 psyllid species described for Australia use eucalypts as host plants (see Majer et al. 1997). The subfamily Spondyliaspidinae in particular includes large numbers of psyllid species associated with the eucalypts (Matthews 1976).
Outstanding universal value: Unique ecology of eucalypt-dominated communities
The ecology of the eucalypts and of eucalypt-dominated vegetation has been described as “highly unusual on the global scale” (Kirkpatrick 1994). The eucalypts and the communities which they dominate encompass a range of adaptations associated with Australia’s unique environments. These ecological adaptations are thought to have been important in the evolutionary development of the eucalypts in Australia as well as being pivotal to their success in dominating the woody vegetation over an exceptionally wide range of habitats, throughout the continent.
An outstanding feature of the ecology of the eucalypts is their capacity to persist in and to dominate the transitional zone between the extremes of closed forest in the wetter areas and grassland in the drier areas. Australia is distinguished by an unusually-wide expression of this transitional zone. In nearby countries, the transitional zone is often very narrow, representing a rapid change between closed forest and grassland. In Australia, the unusually low relief derived from the ancient landforms that typify most of the continent, and a relatively slow rate of change of environmental gradients across large areas have contributed to the very wide expression of this transitional zone.
The environments of the transitional zone in Australia usually include poor soils and are subject to periodic drought. Recurrent disturbance due to fire is another important feature of these environments. The transitional zone can also vary widely in terms of other environmental factors since it encompasses much of the environmental variation of the continent and extends across the entire range of latitudes.
Eucalypt-dominated vegetation is the principal vegetation type that occurs in this transitional zone in Australia. The eucalypts possess a suite of characteristics and adaptations that are fundamental to their ecology and which, in combination, provide the basis for their exceptional ability to persist in and to dominate the environments characteristic of this transitional zone.
These exceptional characteristics and adaptations of the eucalypts include:
the production of large quantities of fuel comprising flammable litter of leaves and bark with an unusually high proportion of woody material including twigs and branches;
the production of litter suitable for long distance propagation of fire by spotting; especially bark ribbons that can act as firebrands and be transported during a fire up to 30 kilometres ahead of the main fire front;
the development in most species of lignotubers which, by virtue of a relatively protected position at the base of the stem and reserves of dormant buds and food, provide an important means for vegetative recovery from fire or damage;
the capacity for vegetative regeneration from dormant buds that occur beneath the bark throughout the trunk and main branches and which provide a means of replacing part or all of the canopy following its damage by fire, insect attack or other cause;
the cuing of mass seed release from canopy seed stores in some species of eucalypts by fire occurrence, allowing seeds to escape seed harvesters by satiating predator populations;
post-fire release of seed into an ash bed in which environmental conditions are conducive to higher rates of germination and seedling establishment, given appropriate rainfall;
the capacity to cope with a wide range of soil types and soil nutrient availability, including acidic soils and calcareous soils and soils of exceptionally low nutrient status, via mechanisms to overcome or cope with adverse chemical and physical soil characteristics, and by lowering plant nutrient requirements through reduced growth rates;
the capacity to maintain high growth rates and to develop large biomass on soils of very low nutrient status via root system structures and other mechanisms that enhance nutrient acquisition, by flexible and opportunistic growth responses to periods of increased nutrient availability, by continuous internal recycling and translocation of nutrients from older tissues to sites of active growth, and by maintaining a high ratio of non-living to living biomass;
the capacity to persist in environments subject to low rainfall, to highly variable rainfall, or to seasonal rainfall characterised particularly by summer drought in southern latitudes and winter drought in tropical latitudes;
the ability to postpone the onset of high water deficits as a result of changes to shoot and root systems that effectively increase the ratio of water uptake to water loss tissues, by changes in leaf structure and function to reduce water losses, and by changes to tissue solute concentrations to slow water losses;
the vertical leaf orientation of adult leaves and a relatively open canopy structure in most species that allows a high proportion of incident light to penetrate to the ground layer flora, especially during the middle of the day, and which may be important in promoting high diversity amongst understorey vegetation communities;
the capacity to produce tree hollows which provide an important ecological resource used for shelter and nesting habitat by a wide range of vertebrate fauna and also by invertebrate fauna;
the high diversity of particular invertebrate groups associated with the eucalypts, reflecting important co-evolutionary relationships within eucalypt-dominated communities.
These ecological characteristics and adaptations may be viewed in combination as a “package” that underpins the exceptional success of the eucalypts in dominating the vegetation of the Australian continent. The versatility and resilience of the eucalypts and their resultant wide ecological amplitude is unparalleled amongst other comparable phylogenetic groups of dominant woody taxa in the world.
The traditional occupation and use of eucalypt-dominated landscapes by Aboriginal people has continued over millenia (e.g. Hope 1994). Aboriginal use of these landscapes has also been suggested as an important factor influencing eucalypt vegetation and its associated biota (Flannery 1994, Hope 1994), although the extent of the impacts of Aboriginal burning remains the subject of debate (see Norton 1997). For example, the use of fire as a management “tool” (Nicholson 1981) may have resulted in an artificial increase in ignition sources (Hill 1994). It has also been proposed that Aboriginal burning may have contributed to the eucalypts replacing other taxa such as Allocasuarina as the dominant vegetation type (e.g. Kershaw 1986).
Aboriginal cultural values associated with eucalypt-dominated vegetation may be related to its significance in providing living sites and a wide range of other resources, its use for hunting and gathering activities and the location of sites of religious and spiritual importance (see Thomson et al. 1987).
The expert workshop recognised that the cultural values of eucalypt-dominated vegetation may also have outstanding universal significance, including within the context of globally-outstanding cultural landscapes, and considered that this possible significance should be further investigated and assessed.
The most important aesthetic qualities of eucalypt-dominated vegetation are uniquely associated with Australia (see Kirkpatrick 1994). These qualities may include an exceptional natural beauty associated with the unusual shapes and colours of the eucalypts and eucalypt-dominated vegetation, including the rounded form and characteristic blue tints that eucalypt vegetation imparts to Australian landscapes, the outstanding aesthetic qualities of the tall eucalypt forests which are dominated by the tallest flowering plants in the world, the extraordinary range of form including exceptional structural and floral qualitites associated with the understorey vegetation, and the important contribution that eucalypt-dominated vegetation has made to the visual arts.
The aesthetic significance of eucalypt-dominated vegetation was noted by the expert workshop as also requiring further investigation in relation to its possible contribution to the outstanding universal values of the sub-theme.
Eucalypt-dominated vegetation represents an important economic resource in Australia. For example, a large proportion of the wood harvested in Australia comes from eucalypts, with recent estimates of about 12 million cubic metres annual hardwood log production and, in 1995, a total of 6.8 million tonnes of eucalypt woodchips approved for export (see Dargavel 1995, Norton 1997). As well as wood, eucalypt-dominated vegetation provides a large range of other important products including seeds, cut flowers, honey, pollen, essential oils, tannins, medicinal plants and bark products (Resource Assessment Commission 1992).
Other uses and amenity values of eucalypt-dominated vegetation that are of major economic significance in Australia include in water catchments, for conservation including wilderness areas and heritage areas, recreation and tourism, mining, grazing, defence, hunting and wild foods, education and research, and as infrastructure corridors (Resource Assessment Commission 1992).
Eucalypts are also widely planted throughout the world with estimates of over 200 species having been introduced into other countries (Mabberley 1997). The eucalypts are capable of exceptionally high growth rates when planted outside of their natural Australian habitats and they have been described as the most important plantation trees worldwide (Mabberley 1997). Ten eucalypt species contribute to most overseas plantings: Eucalyptus grandis, E. saligna, E. globulus, E. camaldulensis, E. tereticornis, E. urophylla, E. robusta, E. maculata, E. paniculata and E. viminalis (Brown and Hillis 1978). An estimated several million hectares of eucalypt plantations have been established in other countries, particularly in Brazil, Chile, China and South Africa. Their principal uses in other countries include wood production for building, fibre and fuels including charcoal (Clark 1995, Eldridge et al. 1993) and also for lowering the water table (Brooker and Slee 1996). They are also used for a wide variety of other purposes such as a source of oils and tannins, including medicinal and industrial chemical products, as ornamentals and for cut foliage (Boland et al. 1991, Mabberley 1997).
The economic importance of the eucalypts was identified by the expert workshop as requiring further investigation in relation to its possible contribution to the outstanding universal values of the sub-theme.
Eucalypt-dominated vegetation in Australia constitutes a major repository of genetic material. This genetic resource is important from the point of view of conservation of natural values associated with eucalypt-dominated ecosystems, including the diversity of the eucalypts and associated plants and animals in the ecosystems they dominate. Eucalypt vegetation also represents a significant genetic resource in relation to the unique properties that provide the broad range of utilitarian values and products derived from eucalypts and eucalypt-dominated vegetation both in Australia and throughout the world. Natural populations of eucalypts in Australia provide the base genetic resource for domestication programmes being undertaken for eucalypts in countries around the world (Eldridge et al. 1993).
The significance of eucalypt-dominated vegetation as a genetic resource in the global context was recognised by the expert workshop and identified as requiring further investigation in relation to its possible contribution to the outstanding universal values of the sub-theme.