Fossil pollen of Eucalyptus diversicolor, E. marginata, and E. calophylla has been
identified in peat deposits from south-western Australia, where the species are prominent
and economically important forest trees. The extant distribution of each species has been
surveyed and the presence or absence of each, within and beyond the margins of their
ranges, shows a close relationship with the mean rainfall of the wettest and driest
months of the year. No such relationship was found with temperature data, and it is
evident that the availability of water is a major factor influencing the distribution of these
three species.
Past changes in the relative eucalypt pollen frequencies have been dated by
radiocarbon assay, and the dates appear to cluster around 3000,1200, and 500 B.C. and
A.D. 400 and 1200. Charcoal is common in the peat and it is evident that fires have
frequently occurred around the sites investigated, for at least the past 5000 years. Many
of these fires have severely burnt and truncated the peat deposits. However, charcoal is
not always present at levels of substantial change in the pollen frequencies, from which
it is concluded that equilibrium between these forest eucalypts and fire has existed for
at least the past 7000 years. Fossil evidence of the presence of prehistoric man in
Australia predates the period under investigation, but the impact of man on the
vegetation was probably limited to his use of fire.
From what is known of the moisture requirements of the two species, it seems
probable that a climate which favoured a relative increase of the E. diversicolor/E.
Calophylla ratio would be much wetter than that which would favour a high
E. calophylla/E. diversicolor ratio. This being the case, it is evident from the prehistoric
changes in the ratios of these two species that the climate from at least 4000 until
3000 B.C. was wetter than at present, and thereafter it became increasingly dry (maximum
dryness at c. 1200 B.C.) until about 500 B.C., when conditions once more became
wetter and continued so until A.D. 500; after this a period of rapid drying out is evident
until A.D. 1200-1500, when conditions became wetter up to the present day.
