A predominantly southern distribution conceals a northern reservoir of diversity in a wet sclerophyll tree

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The wet forests of south-east Australia can be coarsely divided into cool-temperate communities characterised by relictual Gondwanan lineages with a history of persistence in scattered refugia, and subtropical communities showing stronger Malesian floristic influences, with post-glacial expansion dynamics. Insights into the assembly and dynamics of these two biomes can be gained by studying taxa that inhabit both, such as the tree species Elaeocarpus reticulatus Sm. (blueberry ash). Here we use data generated from eight microsatellite loci to investigate population structure and eco-evolutionary dynamics across the core of this species’ range. Our nested sampling design allowed us to test for population differentiation along both axes of latitude and altitude, with the aim of detecting post-glacial colonisation dynamics. Our results show that the strongest genetic differentiation was associated with latitude zones, and population differentiation obeyed a typical pattern of isolation by distance. We did not find strong support for differentiation being driven by altitude. A trend towards higher diversity in the north of the range reveals that E. reticulatus conforms more to the post-glacial colonisation dynamics typifying subtropical, rather than cool-temperate lineages.