File(s) under permanent embargo
Eucalyptus tree species, commonly referred to as eucalypts, are among the most planted hardwoods in the world (Doughty 2000). They are generally long-lived, evergreen species belonging to the predominantly southern-hemisphere, angiosperm family Myrtaceae (Ladiges et al. 2003). They are native to Australia and islands to its north (Potts and Pederick 2000; Ladiges et al. 2003), where they occur naturally from sea level to the alpine tree line, from high rainfall to semiarid zones, and from the tropics to latitudes as high as 43° south (Williams and Woinarski 1997). Eucalypts are the dominant or codominant species of virtually all vegetation types in Australia except rainforest, the vegetation of the central arid zone, and higher montane regions (Wiltshire 2004). They are generally sclerophyllous and adapted to low nutrient soils (Eldridge et al. 1993; Florence 1996; Specht 1996) and fire (Pryor 1976; Ashton 2000; Burrows 2002).
The eucalypt lineage is old, possibly extending back to the Late Cretaceous -ca. 70 million years ago (Hill et al. 1999; Ladiges et al. 2003; Crisp et al. 2004). Their ancestors were likely to have been widely dispersed on the supercontinent of Gondwana, as there are macrofossils ascribed to eucalypts of Eocene (55 to 34 Mya) age from northeastern Australia (Rozefelds 1996) and possibly Patagonia (Hill et al. 1999) and of Miocene (27 to 10 Mya) age from New Zealand (Pole et al. 1993) and Australia (Hill et al. 1999). The tectonic isolation of Australia (ca. 32 Mya) led to cooler, drier, and more seasonal climates and consequently a transition from a rainforest-dominated flora to Australia's unique sclerophyll flora (Hill et al. 1999; Ladiges et al. 2003; Crisp et al. 2004; Hill 2004). There is little doubt that the current dominance of the Australian continent by eucalypts is relatively recent and linked with the onset of severe aridity during the Late Miocene (10 to 7 Mya) and the present climatic system of extreme wet-dry glacial cycles that commenced around 2.9 Mya (Crisp et al. 2004). The increasing prevalence of fire played a significant role in the transformation of the Australian biota over this drying period (Kershaw et al. 1994), with eucalypts believed to have expanded from drier, central regions of the continent into more coastal environments climatically suitable for fire-sensitive rainforest taxa (Hill et al. 1999). The arrival of Aborigines on the Australian continent at least 55,000 years ago and the instigation of "fire-stick farming" would have continued this shift (Kershaw et al. 1994; Bowman 1998).
The latest molecular dating (Crisp et al. 2004) argues that divergence of the eucalypt genera and subgenera predated the final development of an ocean between Australia and Antarctica ca. 32 Mya. While molecular dating is contentious (Ladiges and Udovicic 2005), it is suggested that diversification proceeded steadily for at least 30 millions years before Australia was isolated and continued thereafter. However, despite over 100eucalypt species having been sequenced for ITS (Steane et al. 2002), there is still insufficient sampling to test for more recent rapid radiation (Crisp et al. 2004).
Publication titleGenome Mapping and Molecular Breeding in Plants: Forest Trees
Department/SchoolSchool of Natural Sciences
Place of publicationBerlin
Rights statementCopyright 2007 Springer