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Evidence for the Distinct Evolutionary Histories of Canopy and Understorey in the Eucalyptus Forest-Heath Alliance of Australia

1978; Wiley; Volume: 5; Issue: 4 Linguagem: Inglês

10.2307/3038029

1365-2699

Walter E. Westman,

Geology and Paleoclimatology Research

A hypothesis is proposed to explain the distinctive ecological adaptations of the heath understorey and eucalypt-dominated canopy where these co-occur on infertile soils of coastal Australia. Key members of the heath synusium were present in Australian Gondwanaland in the cool, wet period between the late Cretaceous and Eocene. During this period of 30-50 million years or more, they evolved adaptations to late spring-summer growth. During the warmer period of the Eocene-Miocene, heath species grew on the infertile coastal soils, while more fertile soils were occupied by a Cinnamomum-dominated rain forest flora which included members of Eucalyptus. When the Australian plate rafted northwards and collided with the Asian plate in the late Miocene, Australia entered drier horse latitudes, the Cinnamomum flora faced competition from new Laurasian elements, and members of Eucalyptus retreated to mesic, coastal, infertile heath sites. However, they retained their flexible pattern of growth, remaining capable of increased growth when warm temperatures, adequate moisture and higher fertility were available. Extensive opportunities for radiation in Eucalyptus occurred in the Pliocene and Quaternary. Pheno- logical patterns, nutrient tolerances and preferences, the quasi-independent distribution of floristic elements, and fossil evidence at present favour an inter- pretation of disparate evolutionary histories for the two synusiae over suggestions of a contemporaneous evolution of the Eucalyptus-heath alliance. Additional evidence is necessary, however, to distinguish the present scenario conclusively from alternatives.