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Effects of Past Global Change on Life
FIGURE 10.2 Simplified vegetation map of Australia. (Modified from Christophel and Greenwood, 1989.)
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