Phylogenetic patterns in snow gum eucalypts as indicators of past cold-climate refugia in south-east Australia

A project undertaken at the School of Botany, University of Melbourne, and supervised by Dr Michael Bayly

Background

In the mountains of southeastern Australia the Snow Gums (Eucalyptus pauciflora and segregate species; Fig. 1) generally dominate subalpine woodlands at the altitudinal limit to tree growth. In Victoria and Tasmania Snow Gums also occur at a range of lowland sites, even close to sea level (Fig. 2). It is thought that the distribution of Snow Gums was dramatically affected by climatic fluctuations of the Quaternary glacial/interglacial cycles and is likely to be strongly influenced by impending human-induced climate change.
Aims


This research aims to document genetic variation and diversity in Snow Gums to assess: present and past connectivity between populations/geographic areas; the extent of geneflow between related co-occurring species; the influence of past climate shifts on patterns of distribution; outstanding issues surrounding Snow Gums taxonomy.

Results

Chloroplast genetic variation in snow gums shows clear geographic patterns.  The most substantial divergence between populations is that of the uplands of eastern Victoria and New South Wales from all others (Fig. 1), and upland populations represent a large proportion of total snow gum genetic diversity.  This is consistent with long term persistence of snow gums forests in or around the mountains of eastern Victoria and Southern NSW.  Expansion of the Snow Gums from mountains of eastern Victoria into lowland areas during glacial periods is consistent with distribution of some cpDNA groups on the Gippsland Plains.  Divergence of eastern mountain populations from those of western Victoria and Tasmania is inferred to be an earlier event.  Presence of a distinct Tasmanian genetic group suggests a history of isolation from the mainland, but some connectivity with Wilson's Promontory is consistent with recent seed-mediated geneflow in that area.   Patterns of cpDNA variation are incongruent with current infrasepecific classifications of snow gums and there is genetic evidence of cpDNA introgression between snow gums and other species of Eucalyptus.  

Figure 1.Snow gum, Eucalyptus pauciflora subsp. acerina, Mt Baw Baw, Victoria

Figure 2. Lowland Snow Gum, E. pauciflora subsp. pauciflora, at a coastal site near Wonthaggi, Victoria.

Figure 3. Distribution and relationships of chloroplast genetic groups in Snow Gums.  Colour coding of groups on the phylogenetic network matches that on the distribution map.