The Impact of Climatic Changes on the Development of the Australian Flora
DAVID C. CHRISTOPHEL
University of Adelaide
Australia in the Tertiary provides an excellent opportunity to examine evolution in an isolated system because for almost 30 million years (m.y.) following the Eocene break with Antarctica, the Australian plate had no major contact with any other. On at least two occasions during that interval, global climatic events were reflected in the fossil plant record. The mid-Eocene cooling is demonstrated by two neighboring floras from either side of the event being dominated by totally different plants. The terminal Eocene cooling is clearly marked by both pollen and megafossil shifts. Thus, through the Tertiary we see a basically greenhouse Eocene Gondwanic flora respond to climatic deterioration and evolve into the sclerophyll and arid communities that dominate the continent today.
UNIQUENESS OF THE AUSTRALIAN SYSTEM
Although the geological and biological uniqueness of Australia is well known and documented, there are certain aspects of that uniqueness that are particularly relevant to the consideration of climate change and the effect on vegetation though the Tertiary Period. The first of these is related to Australia's geographical isolation during much of the Tertiary. It is generally agreed (e.g., Frakes et al., 1987) that from the Late Eocene (from roughly 38 m.y. ago (Ma) to 8 Ma—about 30 m.y.) the Australian Plate was isolated from all other continental plates. This has two important consequences. The first is that climatic changes, and their reflected vegetation patterns, are not significantly masked or diluted by events on other continents. Second, the consequences of change can be followed through time. As an example, a megafossil flora observed for the Early Miocene must have been based on a gene pool present in the Australian Oligocene since no credible external source is available. A corollary is that if a sudden floristic change is observed over a specific time period, the causal factors must be local (e.g., climatic change) because no outside influence can be seriously considered. Therefore, if one thinks of this time interval as an evolutionary experiment, the variables are far more limited than for any other such "experiments" on different continents.
A second important, unique feature of the Australian system is the nature of its plant fossil record. Australia has perhaps the only tropical to subtropical rain forest system in the world that has a well-documented macrofossil record; hence, its evolution can be traced through time. There are three factors contributing to this situation: first, a large number of Australia's macrofossil deposits are preserved as mummified leaves, allowing maximum taxonomic and physiognomic information to be gleaned from them (Christophel, 1981). Second, the isolation of the continent alluded to earlier means that there is a far better chance of actually identifying taxa and communities, and tracing them through time, without having to search for floras of other continents for matches. A consequence is that far greater confidence can be placed on labeling an unidentifiable fossil as extinct because the likelihood of an external taxonomic affinity is much reduced. Finally, a large number of deposits are known from the portion of the Eocene Epoch at or near the time of the early Tertiary plate separation, providing a better than average understanding of the gene pool from which later floristic elements must have been derived. A similar, though somewhat weaker case, can be made for the documentation of some of Australia's less mesic vegetation types, the qualifying feature here being the more recent evolution of these vegetation forms and their components, and hence the greater chance of external influence following Miocene collision with the Sundra Plate (Kemp, 1981).
GEOLOGICAL AND PLATE TECTONIC SETTING
The major events in the physical movements of the Australian Plate during the Tertiary are not contentious. There is general agreement that during the early Paleogene, Australia was attached to Antarctica via its southeastern corner and Tasmania (Figure 10.1) and that, although by mid-Miocene the shelves between components were likely submerged, they were still joined and oceanic circulation over that shelf was minimal. Near the end of the Eocene the rate of northward movement of the Australian Plate increased two- to threefold, and it continued at that rate until the leading, northern edge collided with the island arcs of the Sundra Plate in the Middle- to Late Miocene (Galloway and Kemp, 1981). Tectonic activity was minimal across most of the plate during this isolated rafting period, with the uplift of the eastern highlands likely occurring at an early stage of the Miocene (Ollier, 1986).
MODERN VEGETATION OF AUSTRALIA
In order to assess the impact of Tertiary climatic change on the makeup of the modern Australian flora, it is first necessary to categorize the floristic or vegetational elements in the modern-day flora. A somewhat simplified vegetation map of Australia is shown in Figure 10.2. By distilling that further, it is possible to identify four categories of vegetation: (1) the closed forest systems, (2) the open forest or woodland systems, (3) the heath scrub or mallee systems, and (4) the great arid and semiarid systems that occupy a high percentage of the continental mass. A more thorough treatment of specific vegetation types in Australia may be found in Specht (1981a,b).
In examining the vegetation types one at a time, the first type to be considered is the closed forest system. As may be seen from Figure 10.3A, this system can also
basically be called a rain forest system. The forest shown in this figure is found in the tropical regions of northern Queensland and is known variously as a Complex Mesophyll Vine Forest (sensu Webb, 1959) or Megathermal Seasonal and Nonseasonal (sensu Nix, 1982). Both authors agree that there is a latitudinal/altitudinal gradient in these closed forests from the tropical in the north to the cool temperate southern beech forests in the south. Although currently covering less than 0.4% of the land mass, the closed forest is particularly important to the evolution of Australian vegetation systems because it contains some of the most ancient plant associations. In general, closed forests may be categorized by high diversity and biomass, a low subcanopy light regime, and constituent plants dominated by Gondwanic taxa.
The second major vegetation type is the open forest or woodland (Figure 10.3B). It is most prevalent in eastern and far southwestern Australia. This community is dominated by Eucalyptus species, and has a much lower diversity and biomass accumulation than the closed forest. A far greater amount of light reaches the subcanopy in these forests because of the vertical positioning of Eucalyptus leaves in general, and the majority of taxa in this community type are first reported in the Neogene.
The third vegetation system is the heath scrub or mallee vegetation (Figure 10.3C). It is characterized by an unexpectedly high species richness, with a flora of mixed origins but with reasonably low biomass accumulation. A family of shrubs found in this vegetation type is the Ericridaceae, the sister family of the Northern Hemisphere Ericaceae or heath family—hence the labeling of this vegetation system as "heath." The term mallee comes from a growth form of some Eucalyptus species as small, multistemmed trees growing from an underground lignotuber (Figure 10.3C). It is interesting to note that some Eucalyptus species (e.g., Eucalyptus baxteri) can be found growing as either a large tree or a mallee form, depending on the environment in which it is found. The mallee vegetation type is dominated by plants considered to be sclerophyllous—an environmental adaptation that is discussed later.
Finally, the arid and semiarid regions of the continent have a complicated system of vegetation types, of which two are most common. These are represented in Figures 10.3D and 10.3E and are Acacia shrublands and chenopod scrub, respectively. Although this vegetation type has exceptionally low biomass and diversity during much of its life, the bi- or triennial rains affecting the region can greatly increase the biomass production and the standing
species richness. This vegetation type has a marked resemblance to the sagebrush communities of North America as may be seen by comparing Figures 10.3E and 10.3F, although the taxa filling the various niches are totally unrelated.
It is interesting to note that the highly diverse and successful Eucalyptus is absent from the true arid, as it is from the closed forests. The presence and/or relative balance of these four systems can be examined as potential indicators of the climate changes through the Tertiary.
FACTORS AFFECTING VEGETATION AND TAXON-BASED CHANGES
Simplistically, individual green plants can be considered as oxygen-burning food factories, and plant communities as industrial complexes competing for resources and for the general market. The structure of each factory and the balance between individuals (species) can be examined as responses to basic supply parameters. The study of foliar physiognomy has recognized these taxon-independent responses, and early in the twentieth century, Bailey and Sinnott (1916) noted a pattern of response between major climatic factors and specific foliar features. Wolfe (1990), however, demonstrated that univariate comparisons between individual climatic factors and plant responses were likely to lead to oversimplified or even erroneous conclusions.
In Australia, Webb (1959) erected a rain forest classification based on foliar physiognomic features, and Christophel and Greenwood (1988, 1989) demonstrated a predictable relationship between the canopy signatures used by Webb and the signatures of leaf litter. Thus, foliar physiognomy provides a tool for assessing the environment of a plant community that is independent of taxonomic identities of the constituent taxa.
Two climatic factors, available water (usually in the form of precipitation) and temperature (either mean annual, range, or extreme exclusive value), have been most frequently considered as basic to determining physiognomic signatures of floras. Within the Australian system, a third factor, the edaphic feature of soil nutrient availability (particularly phosphate), has been shown to have great importance (Beadle, 1966).
In a particularly important paper, Beadle (1966) concluded that a significant portion of the sclerophyll component of the Australian flora could likely have evolved its characteristic features in response to low nutrients rather than relative aridity. Sclerophyll plants are those with reduced, lignified leaves with short branch internodes and often thickened cuticles. Such features would commonly be thought of as having an adaptive advantage in a drying environment. Acceptance of Beadle's ideas allows a much more realistic mechanism for the floristic changes observed in the Tertiary of Australia. Beadle suggested that many sclerophyllous plants could have evolved in nutrient-poor soils around the margins of Paleogene rain forests. Thus, when climatic deterioration did occur, expansion of existing taxa from those pre-evolved low-nutrient pockets could occur much faster than if all responses to aridification had to be newly speciated. As can be seen later, Beadle's argument makes tenable the sometimes difficult to explain sclerophyllous (often thought xeromorphic) elements that crop up in otherwise mesic Paleogene rain forest plant fossil assemblages. Evidence in modern vegetation of the validity of Beadle's hypothesis comes from the Hawkebury Sandstone region of New South Wales, where in conditions of high rainfall and optimal temperatures a sclerophyll community thrives in the midst of a closed forest system. The only significant environmental difference is the very low nutrient levels of the soils supporting the sclerophyll vegetation.
MAJOR TERTIARY CLIMATIC CHANGES
The scale of climatic change being observed or monitored is related directly to the accuracy of calibration of the tools being used for the monitoring. Thus, when the palynology of late Tertiary or Quaternary deposits is being considered (e.g., Kershaw, 1976, 1981; Kershaw and Sluiter, 1982), it is possible to consider vegetation (and by inference climate) changes on a scale of thousands of years. As yet, however, it has not been possible to calibrate the Australian plant megafossil record to such accuracy.
It is still possible, however, to correlate some of the major climatic changes through the Tertiary with specific megafossil floras. If we examine a generalized chart of oxygen isotope curves for the Tertiary (Figure 10.4), some of the major climatic changes that affected many parts of the globe can be seen. Two are of particular interest. Although the Early Eocene is considered to be the most recent time at which a nearly greenhouse Earth was achieved, there was a significant cooling event at the beginning of the Middle Eocene. McGowran (1986, 1989) correlated this in Australia with an approximately 8-m.y. period during which almost no floral or faunal fossil record exists. This cool period was followed by a rapid rewarming in the late Middle and Late Eocene. However, there was a marked terminal Eocene event resulting in a rapid cooling. This Oligocene cooling is correlated in Australia with the initiation of the circum-Antarctic currents between Australia and Antarctica, and also with the glaciation of Antarctica. This cooling continued to the Middle Miocene, at which point there was a brief return to a warming cycle, followed by a cooling that has continued to the present. The two specific events to be considered relative to the Australian
megafossil record are therefore (1) the Middle Miocene fluctuation and (2) the terminal Eocene event.
PLANT MEGAFOSSIL EVIDENCE FOR CLIMATIC CHANGE
The first area for which to consider plant megafossil evidence is the cooling cycle within the Middle Eocene. As is seen in Figure 10.5, there are a large number of known Eocene localities in southern Australia. Four of them—Maslin's Bay (Christophel and Blackburn, 1978), Golden Grove (Christophel and Greenwood, 1987), Anglesea (Christophel et al., 1987), and Nelly Creek (Christophel et al., 1991)—are late Middle Eocene and represent the return to warmth shown on Figure 10.4. They include classical tropical rain forest taxa such as Elaeocarpus/Sloanea, Lauraceae, and Gymnostoma. These four floras show a physiognomic signature consistent with that of the litter from Webb's Complex Notophyll Vine Forest or his Complex Mesophyll Vine Forest (Christophel and Greenwood, 1988). A little-documented site at Dean's Marsh (Douglas and Ferguson, 1988) (Figure 10.5), approximately 80 km east of the Anglesea locality, displays a physiognomic signature almost identical to that of the Anglesea locality. Whereas the Anglesea fossils occur high in the Eastern View Formation and are considered late Middle Eocene, the Dean's Marsh locality is basal Eastern View Formation and is late Early Eocene (Douglas and Ferguson, 1988). An examination of the taxa collected from the two localities has shown no taxa held in common. Although the deposits of the late Middle Eocene scattered across southern Australia show similar mixes of families and in many cases genera, the Dean's Marsh locality shows no such correlation. One explanation that fits nicely with the known data is that although they occur in very similar environmental situations, and hence have similar physiognomic signatures, the two deposits occur on either side of the 8-m.y. cooling period discussed by McGowran (1989); hence, the evolution and natural selection that occurred during that time selected totally different taxa for the same niches.
The major cooling event at the end of the Eocene is much easier to document. Palynological studies have highlighted it for many years (e.g., Kemp, 1978)—the most obvious signal being the shift from the tropical Nothofagus brassii pollen type to the Fusca and Menzesii types that
represent the small toothed-leaf Antarctic beeches known from the modern cool temperate floras in New Zealand, southern South America, and Tasmania. The only megafossil record of leaves and cupules of the N. brassii type comes from the Eocene of Tasmania (Hill, 1987), whereas the small-leafed types are prevalent in sediments from then onward. Hill and Carpenter (1991) have also provided data suggesting that some of the smaller-leafed conifers may also show a leaf size reduction across this crucial boundary.
In general the late Middle and Upper Eocene floras mentioned above all show physiognomic signatures indicating warm, wet climates, whereas those known from the Oligocene-Miocene show much reduced, sclerophyllous signatures. Localities displaying these features include Kiandra in New South Wales as well as Berwick and Bacchus Marsh in Victoria (Figure 10.6).
Not only does the physiognomic signature change, but so does the taxonomic composition of the flora. As can be seen in Table 10.1, the domination by the Gondwanic Proteaceae, Lauraceae, and Gymnostoma evident in the Eocene floras has been lost by Oligocene time and is replaced by elements of the flora now found in different modern communities—namely, Eucalyptus, Acacia, and Epacridaceae. The Proteaceae and Casuarinaceae are still prevalent, but the Proteaceae is now dominated by Banksia and sclerophyllous forms, and Allocasuarina and Casuarina (sensu Johnson, 1982) have now replaced the more mesic Gymnostoma. All of the above changes are well documented in the pollen record, but frustratingly, very few occurrences of Eucalyptus and Acacia are known from the megafossil record. Thus, two distinct significant global climatic events (Figure 10.4) can be seen to be reflected in the Australian megafossil record and, when considered floristically, appear to have had a major effect on the vegetation development of the continent.
TABLE 10.1 Main Plant Groups Represented in Australian Megafossil Deposits
If the four major types currently found in Australia are examined once more, they can be viewed in the light of major Tertiary climatic changes to the continent. The closed forests of the north and east of the continent clearly have their affinities with the greenhouse and near-greenhouse phases of Eocene Australia. Gondwanic families dominate, and in many cases the relationships are reflected at the generic level. Although there is no Eocene evidence for Eucalyptus, Acacia, or other taxa listed as Neogene in Table 10.1, Beadles's hypothesis suggests that they nonetheless could have evolved under depauperate soil conditions, but in quantities too small to be observed in the fragmentary fossil record. Then too, they may well have been initiated or survived in the mid-Eocene cooling represented by McGowran's (in Frakes et al., 1987) 8-m.y. ''hole" in Australia's fossil record.
Whatever their Eocene status, the sclerophyllous plant elements that dominate the open forest and the heath scrub today either evolved or spread during the Oligocene-Miocene refrigeration. This of course was aided by the inability of the Gondwanic closed forest components to survive over large areas during this climatic deterioration. The mid-Miocene warming suggested by Frakes et al. (1987) may well have guaranteed the survival of some of those Gondwanic elements that struggled through the refrigeration, and may also be reflected in the mixture of floral provinces in some vegetation types such as the forests at Wilson's Promontory near the southern tip of Victoria.
As might be expected, the macrofossil record for arid floras is poor, although Chenopodiaceae and Mimosaceae pollen is well documented from the late Miocene and Pliocene of several localities (Martin, 1981). Although for Charles Darwin the flowering plants represented the "abominable mystery," for Australian researchers it is perhaps Acacia. Although the genus is one of the few to occur in all major Australian habitats, and contains more than 650 species in Australia (Morley and Tolkein, 1983), there is only one confirmed report of fossil leaves from the late Miocene (Christophel, 1989), and pollen is not common. Thus, the explanation for the origin and spread of a genus whose distribution suggests it to be Gondwanic, and hence ancient, remains shrouded but is almost certain to be related, when once unraveled, to the changing Tertiary climates.
Much of the research for this project was supported by grants from the Australian Research Council, Alcoa of Australia, and the Adelaide University/CSIRO Granting Scheme. The figures for this chapter were prepared by Linda Allen and Leonie Jane Scriven.
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