Phylogenetics and drought response of Australian alpine grasses (Poa spp.)

© 2011 Dr. Philippa Clare GriffinClimate change will affect every ecosystem to some extent, but Australia’s alpine region is particularly threatened. We currently have little knowledge of how resilient our unique alpine ecosystems will be to climate change, or what form ecosystem changes will take. Poa tussock grasses dominate much of alpine Australia and are structurally and ecologically important, so their climate change response will influence that of the entire Australian alpine region. To predict the response of a species to environmental change, we require a solid understanding of species structure and gene flow between/within species. Therefore, in this project, I first investigated species relationships in the alpine Poa tussock grasses, using microsatellite markers and chloroplast and nuclear DNA sequence. As part of this study, I developed a novel method for sequencing a mixture of multiple gene regions (some multiple-copy) from multiple barcoded individuals. All taxa were allotetraploid and closely related to American Poa species. Despite high microsatellite variation and functional heterozygosity, little sequence variation was detected. Current species structure was not supported in a multiple-gene species tree, indicating a recent, rapid radiation with possible ongoing hybridisation. The Tasmanian endemic P. gunnii was distinct from the rest of the Australian species, however, and may have resulted from a separate colonisation event. Another requirement for predicting environmental change response is to identify the stress(es) that will most influence the organism of interest. I identified drought as a stress projected to increase under climate change that showed signs of impacting these grasses. I investigated the environmental factors of elevation, plant-available water and soil depth, all of which affected the spatially-variable drought mortality observed in the field. Drought was identified as the cause of mortality because grass survival was significantly predicted by dry-season plant-available water. I then used common-garden experiments and field observations to compare drought responses in Poa hiemata, P. phillipsiana and P. hothamensis. Poa hothamensis was less drought tolerant than either of the other species, which demonstrates that species concepts in this group still have ecological relevance despite the lack of genetic support. P. hothamensis may therefore become less common under anthropogenic climate change. These grasses exhibit drought tolerance variation, although heritability and therefore adaptive potential for this trait remain unknown. This, in combination with their polyploidy and apparent propensity to hybridise should allow them to endure some degree of climate change