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The concept of climatic refugia is an important idea in biogeography (Haffer, 1982). Traditionally, the term ‘refugium’ or ‘refugia’ was used to describe areas in the Northern Hemisphere into which species retracted and survived during Quaternary ice ages (Keppel et al., 2012). Recently, however, the term has also been used to refer to environments in non-glaciated landscapes in the Americas, Africa, Asia, Australia and the Pacific that have sheltered species from past climate change, or that may function similarly in the future (Keppel et al., 2012), thus allowing survival of species or ecosystems by maintaining climatic conditions similar to those prior to climatic change or by offering more stable habitat than surrounding areas (Hilbert et al., 2007; Bennett & Provan, 2008; Ohlemüller et al., 2008; VanDerWal et al., 2009). This broader definition has necessitated recognition of diverse refugial types varying along spatial, temporal and disturbance dimensions (Couper & Hoskin, 2008; Ashcroft, 2010; Stewart et al., 2010; Keppel et al., 2012). Here, we investigate rain forest refugia in eastern Australia associated with climatic change, focusing on larger areas that have remained suitable for rain forest through a number of glacial cycles (stable, mesic macrorefugia).
It is uncertain how refugia can best be defined and identified (Bennett & Provan, 2008; Ashcroft, 2010), but they are generally considered to have distinctive patterns of biodiversity, including: high species diversity (Médail & Diadema, 2009); high endemism; concentrations of climatic relict, poorly dispersed and vegetatively reproducing species; or other traits that reduce survival elsewhere including drought or fire intolerance (see Keppel et al., 2012, and references therein).
The persistence of palaeoendemic species and the accumulation of neoendemic species in refugia can generate areas with unusually high concentrations of endemic species (Keppel et al., 2012). While some endemism can be expected by chance as a result of differing species range sizes (Laffan & Crisp, 2003; Jetz et al., 2004), areas combining higher levels of endemism, concordant patterns between plant and animal distributions, and more stable climatic conditions are likely to have functioned as refugia, allowing the survival of relicts and evolution of new species (McGuigan et al., 1998; Moritz et al., 2001). Endemism (particularly of range-restricted taxa with low dispersal ability) is positively correlated with past climatic stability globally (Jansson, 2003; Sandel et al., 2011). There are numerous examples of concordance between climatic refugia and centres of endemism in the European Alps (Tribsch & Schönswetter, 2003; Casazza et al., 2008), Africa (Fjeldså & Lovett, 1997), and Australian Wet Tropics (Hilbert et al., 2007; Graham et al., 2010), although this notion is contested in some areas (Knapp & Mallet, 2003). Areas with high concentrations of range-restricted endemic species, termed ‘centres of endemism’ (Jetz et al., 2004) may be particularly indicative of refugia. This is because many range-restricted species also share ecological traits that reduce survival outside or expansion from refugia, such as dependency on relict climates or poor dispersal (Keppel et al., 2012). Although some endemism in centres of endemism may be attributed to recently evolved species from widespread linages, these areas may function as future refugia following species range expansion and contraction. An area can function as a refugium for lineages that were previously more widespread and contracted leading to evolution of neoendemic species in refugia. The majority of neoendemic Australian rain forest plants are likely to represent the latter case due to long-term contraction of rain forest.
Gondwanan rain forests were isolated when Australia was separated from Antarctica by sea-floor spreading and the continents became separated by ocean c. 49 Ma (Truswell, 1993). Rain forests were still widespread in Australia in the late Eocene (35 Ma) including in inland areas that are now arid (Macphail, 2007). Following collision with Asia during the Miocene numerous rain forest plant lineages migrated into northern Australia (Sniderman & Jordan, 2011). The loss of rain forests in central Australia around 10 Ma was a consequence of increasing aridity (Byrne, 2008). A further contraction of this community has occurred within the past 5 Myr (Crisp et al., 2004; Byrne, 2008). Drying climates resulted in rain forest becoming restricted to areas retaining suitable climate along the Great Dividing Range and east coast (Floyd, 1990; Crisp et al., 2004). Rain forests now cover < 1% of Australia (Webb & Tracey, 1981a) and are limited to areas with low susceptibility to bush fires (Bowman, 2000). Not all rain forest patches are stable over long time-scales (Graham et al., 2010) and many current rain forest areas represent expansions since the Last Glacial Maximum (LGM), as indicated by charcoal deposits, and are not considered refugia (Hopkins et al., 1993). Northern Australian rain forests have been enriched by many Asian plant lineages since the Miocene, yet those in subtropical Australia also retain significant numbers of Gondwanan lineages. Conversely, Asian lineages are comparatively rare in temperate Australian rain forests, including subtropical montane forests (Sniderman & Jordan, 2011). Persistence of these lineages indicates that some areas may be long-term refugia (Heads, 2009).
Phylogeographical, pollen and biodiversity studies focusing on fauna have provided evidence for a link between refugia and centres of endemism in the Wet Tropics (north-eastern Australia) (Williams & Pearson, 1997; Schneider & Moritz, 1999; Yeates et al., 2002; Graham et al., 2006; Hilbert et al., 2007; Moussalli et al., 2009). The link between endemism and refugia is less well understood for Australian subtropical rain forest flora. Past studies have been limited by state borders, relied on qualitative methods, focused on a single species group, or considered all flora without distinguishing processes specifically affecting rain forests (McGuigan et al., 1998; Queensland CRA/RFA Steering Committee, 1998; Crisp et al., 2001; National Land & Water Resources Audit, 2001). This is a notable gap in our knowledge given the high biodiversity values (Adam, 1987; Hunter, 2004) and projected vulnerability to future climate change of Australian subtropical rain forest flora (Australian National University, 2009; Laidlaw et al., 2011). The importance of subtropical and tropical forests of Australia as globally significant biodiversity hotspots has recently been recognized due to high levels of endemism and habitat loss (Williams et al., 2011).
Here, we evaluate spatial patterns of richness and endemism for rain forest flora in subtropical Australia and consider whether centres of endemism have functioned as refugia. Specifically, we test distribution patterns of endemic wet rain forest plants against random models and correlate patterns of endemism with a palaeoclimate model for rain forest habitat stability over 120,000 years (Graham et al., 2010). We hypothesize that centres of rain forest plant endemism should coincide with areas of high environmental stability, sheltering palaeoendemic species and favouring the evolution of new species through divergence of isolated populations (Ponniah & Hughes, 2004; Rossetto & Kooyman, 2005; Mast et al., 2008). We contrast the influence of historical habitat stability against other processes potentially governing patterns of richness, such as rain forest patch size, current environmental suitability and topographic heterogeneity. We also examine associations between dispersal ability of endemic plant taxa and modelled rain forest stability and hypothesize that large-seeded species are most likely to be restricted to historically stable habitat. Poor dispersal ability is likely to have reduced the range expansion of large-seeded species following the amelioration of climate since the LGM.