There is a continuing debate on how plants and animals have survived through past major climatic changes, enabling the assembly of temperate communities during the current interglacial. Many authors, particularly using fossil pollen data from the northern hemisphere, have proposed that temperate forest species migrated very rapidly from a few distinct refugia located in regions that escaped the major climatic changes of the glaciations (Huntley & Birks, 1983; Hewitt, 1996). In this model, current temperate tree populations outside known glacial refugia are thought to have been established during the postglacial period via migration over thousands of kilometres (Jackson & Overpeck, 2000), and across both land and sea barriers (Davis et al., 1986; Webb, 1987; Bennett, 1995). More recently, macrofossil and phylogeographic evidence has been used to argue for more complex histories involving expansion from multiple refugia (Stewart & Lister, 2001; McLachlan et al., 2005; Petit et al., 2008). This has resulted in the identification of refugia in locations unexpectedly close to regions where glacial climates had major effects on the environment. The locations of these refugia often conflict with present knowledge of the tolerance range and/or adaptive abilities of species; models of glacial climatic conditions (e.g. temperature, aridity, permafrost and ice cover); interpretations of pollen evidence; and predictions of biogeographical histories of species from current distribution patterns. Such refugia are sometimes referred to as ‘cryptic’ (Stewart & Lister, 2001; Provan & Bennett, 2008). The southern hemisphere provides opportunities to better understand how important large-scale postglacial migrations and/or expansions from multiple glacial refugia may have been in shaping the current forests of the temperate zone.
Temperate rainforests of the southern hemisphere occur from latitudes 28°S to 55°S in South America, New Zealand and Australia. In each of these regions, palaeoecologists have proposed that these forests recovered from glacial climates by expansion from multiple refugia without extensive range shifts (Macphail & Colhoun, 1985; McGlone, 1985; Markgraf et al., 1995). In southeastern Australia, most areas of cool temperate rainforest, as defined by Webb (1959), are dominated by Nothofagus cunninghamii (Nothofagaceae). These rainforests have a widespread but discontinuous distribution in the wettest and most fire-protected regions (Hill et al., 1988) and are surrounded by more extensive sclerophyll forests.
Many authors have argued that arid conditions during Pleistocene glaciations would have made almost all of southeastern Australia inhospitable for cool temperate rainforest (Hope, 1994; Hill, 2004). The available pollen evidence in southeastern Australia from the Last Glacial Maximum (LGM) 18 000 yr ago indicates a more or less treeless landscape dominated by glacial steppe vegetation probably to present sea level. Pollen evidence identifies rainforest tree LGM survival in only two places: the western half of Tasmania, where the coastal plains exposed by depressed LGM sea levels may have provided suitable habitat (Kiernan et al., 1983; Colhoun, 2000) and the central highlands of Victoria (McKenzie, 1997).
There has been particular controversy about whether two regions of southeastern Australia contained glacial refugia: the highlands of northeast Tasmania and southern Victoria (Fig. 1). Depauperate cool temperate rainforest communities are reasonably extensive in these regions (Busby, 1984) but they receive < 50% of the precipitation of the wetter parts of western and southern Tasmania (Nunez, 1978) where the most geographically extensive cool temperate rainforests exist. The available geomorphological and pollen-based evidence indicates extensive glacial aridity during the LGM in northeast Tasmania and southern Victoria (Galloway, 1965; Bowler, 1982; Colhoun, 2002), with the Australian desert thought to have extended to within 100 km of current cool temperate rainforest populations (Bowden, 1983; Hill & Bowler, 1995). In northeast Tasmania, palaeoclimatic modelling can reconstruct conditions favourable for LGM survival of cool temperate rainforest only by invoking eastern Tasmanian climates with similar rainfall to the present (Kirkpatrick & Fowler, 1998), contrary to the evidence for much higher aridity. Even this modelling could only identify refugia in the wettest part of eastern Tasmania, Blue Tier. An alternative explanation involving dispersal rather than glacial refugia for the occurrence of rainforest in northeast Tasmania must invoke Holocene dispersal across > 150 km from the nearest documented refugia in western Tasmania, a scenario that has been considered unlikely because of the low dispersal capacity of many rainforest species (Dodson & Ono, 1997; Kirkpatrick & Fowler, 1998), particularly N. cunninghamii. Herein lies the conundrum: the areas where cool temperate rainforest currently occur can only have arisen from in situ glacial refugia if our understanding of glacial climates is wrong, and/or the ecological tolerance range of species during past climatic changes was greater than would be predicted from their modern distribution. However, if rainforest did not survive in multiple regions, extensive range shifts must be invoked (Jordan, 2003). Therefore, the cool temperate rainforest system of southeastern Australia provides an opportunity to test the relative roles of multiple refugial survival and postglacial dispersal.
This study aims to address this conundrum by investigating the chloroplast DNA phylogeography of the dominant cool temperate rainforest tree in southeastern Australia, N. cunninghamii. The current distribution of chloroplast DNA (cpDNA) haplotype variation across a species’ range can provide independent evidence for the history of genetic exchange by seed and isolation of populations (Schaal et al., 1998). Isolated populations may differentiate over time and, through genetic drift, form distinct genetic lineages. Dispersal can result in the territorial expansion of genetic lineages (Avise, 1994) and the sharing of lineages between populations. Chloroplast DNA phylogeographic studies have been used to investigate the location of glacial refugia and migration histories of temperate forest mostly in the northern hemisphere (Soltis et al., 1997; Okaura & Harada, 2002; Petit et al., 2003). Although range-wide cpDNA phylogeographic studies have been completed in some southeastern Australian sclerophyll forest Eucalyptus species (Byrne & Moran, 1994; Freeman et al., 2001), this study is the first cpDNA phylogeography of a widespread cool temperate rainforest species in Australia. This study assesses the contributions of multiple glacial refugia to postglacial recovery of N. cunninghamii. Specifically we address whether glacial survival occurred outside putative refugia in coastal western Tasmania and the central highlands of Victoria.