Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
THE IMPACT OF CLIMATIC CHANGES ON THE DEVELOPMENT OF THE AUSTRALIAN FLORA 178 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