OFR 151.pdf - CRC LEME

southeastern Australia than elsewhere in Australia (M.K. Macphail and A.D. Partridge
unpubl. data). Exceptions are mainly taxa whose NLRs are confined to cool temperate
rainforest, e.g. Dacrydium and Nothofagus, and one now widespread family of herbs
(Stylidiaceae), which first occurs in Tasmania during the Eocene-Oligocene transition
(Macphail and Hill 1994). Whether the north-west bias implies that northern Australia was a
main point of entry, or the region was a centre of speciation during the Late Cretaceous and
Tertiary is uncertain. More generally, times of first occurrence are likely to reflect the
vagility of the parent plants as well as climatic forcing.
7.3.4 Plant competition and climatic change
Factors contributing to the competitive success of the major plant taxa inhabiting the
continent included climatic change, the impact of deep leaching on soil fertility, disturbance
due to tectonism and volcanism, and (Late Tertiary) herbivory. Because of northward drift of
the continent, low light intensities during winter are unlikely to have been a significant
limiting factor except possibly in Tasmania during the Danian, whilst the amount and
seasonal distribution of rainfall replaced temperature as the major environmental forcing
factor during the Late Palaeogene and Neogene except at high elevations.
During the Palaeogene, angiosperms largely replaced gymnosperms as the canopy dominants
in rainforest over much of the continent. Only the Podocarpaceae with functionally broad
leaves were able to compete successfully with angiosperms in the low-light environment
within the forest canopy (Brodribb and Hill 1997, Hill and Brodribb 1999). Araucariaceae
and Cupressaceae, which lack flattened foliage, remained fairly common only in less shaded
habitats. Hill (1990a, 1998a, 1998b) has proposed that sclerophyll species evolved originally
in response to low fertility soils in rainforest environments during the Palaeogene, and the
morphological expressions of sclerophylly subsequently pre-adapted these plants to reduced
or increasingly seasonal rainfall. As aridity became more severe during the Neogene,
sclerophyllous plants evolved xeromorphic adaptations to prevent water loss; predominantlymesic
biomes such as rainforest gave way to more xeric/open biomes such as sclerophyll
forests, woodlands and savannah. The same xeromorphic adaptations may have adapted
sclerophyllous plants to withstand the cool-cold (microtherm) conditions that began to
develop at high elevations during the Late Oligocene or Early Miocene, and also may have
contributed to the probable increased incidence of wildfires at lower elevations during the
Late Neogene.
7.3.5 Climatic indicators
The Tertiary differs from the Cretaceous in that a significant number of macrofossil and
microfossil species can be assigned to extant genera with a moderate to high degree of
confidence. As for the Late Cretaceous, key climatic indicator taxa are araucarians,
podocarps, palms (Palmae), Nothofagus, Anacolosa and other mesotherm-megatherm taxa
although it is recognised that the ecological preferences and tolerances of these and most taxa
will have changed (possibly narrowed) to a lesser or greater degree during the Tertiary. For
example, Nothofagus (Brassospora)-dominated microfloras are more likely to represent
rainforest growing under warm (lower mesotherm) conditions than the cool (microtherm)
conditions preferred by most living Nothofagus spp. Typically megathermal taxa such as
palms are able to survive under mean annual temperatures as low as 14 ° C (lower mesotherm)
as long as the annual temperature range is very low (Wolfe 1987). Accordingly, where other
data indicate a more extreme temperature range, fossil palm pollen such as Dicolpopollis,
Longapertites and Spinizonocolpites is reliable evidence for very warm to hot (upper
mesotherm-megatherm) conditions.
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