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THE IMPACT OF CLIMATIC CHANGES ON THE DEVELOPMENT OF THE AUSTRALIAN FLORA 175 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 Figure 10.1 Reconstruction of Australia in the Eocene showing location and paleolatitude of several Eocene megafossil localities referred to in the text. The estimated altitudes and inferred forest type of each Eocene flora are shown graphically to the right of the map. MMF is microphyll mossy forest, SNVF is simple notophyll vine forest, CNVF is complex notophyll vine forest, and CMVF is complex mesophyll vine forest. (Modified from Christophel and Greenwood, 1989.)
THE IMPACT OF CLIMATIC CHANGES ON THE DEVELOPMENT OF THE AUSTRALIAN FLORA 176 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. Figure 10.2 Simplified vegetation map of Australia. (Modified from Christophel and Greenwood, 1989.) 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 familyâhence 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 sclerophyllousâan 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
THE IMPACT OF CLIMATIC CHANGES ON THE DEVELOPMENT OF THE AUSTRALIAN FLORA 177 Figure 10.3 Illustrations of major Australian vegetation types shown in Figure 10.2: (A) tropical rain forest near Noah Creek in northern Queensland; (B) Eucalyptus woodland near Adelaide, South Australia; (C) mallee or heath scrub in southeastern South Australia (shown is a typical multistemmed mallee form Eucalypt); (D) arid zone vegetation featuring Acacia near Alice Springs in Northern Territory; (E) semiarid chenopod scrub featuring blue bush (Maireana) and salt bush (Atriplex) in northern South Australia; (F) sagebrush habitat from Kansas showing similar vegetation form to Figure 10.3E.
