Keeping up with climate change: assessing the vulnerability of Eucalyptus species to a changing climate in the south-west of Western Australia

Under the influence of global climate change, the area experiencing a Mediterranean climate in the south-west of Western Australia – of which a large portion is classified as a biodiversity hotspot – is predicted to decrease by up to 51% by the year 2100. The increase in temperature and decrease in precipitation in this region has already caused large scale health decline in Eucalyptus species in a number of woodlands in more mesic climates. As eucalypts are the dominant overstorey species in many habitat types in the region and provide many ecosystem services, the large scale loss of the species would likely result in significant changes to community composition. Thus, it is necessary to increase our understanding of the vulnerability of Eucalyptus species to a changing climate so that we can better conserve these species and the flora and fauna that depend on them. In this thesis, vulnerability is defined as the combination of a species’ exposure to climate change, its resilience to change and its adaptive capacity. These components of vulnerability are explored in the context of species’ current distribution patterns, as understanding how a species is currently adapted to its current climate can allow us to predict what future distributions may look like. Exposure to climate change was quantified using species distribution modelling techniques to predict the percentage of species’ current distribution that will no longer be suitable for species’ survival. Above and below ground plant functional traits that contribute to both species’ resilience to change and adaptive capacity were then quantified to improve our understanding as to how these species have adapted across an existing aridity gradient.

Results suggest that species in more arid conditions will have a greater proportion of their habitat exposed to unfavourable conditions compared to mesic species, with some predicted to lose all suitable habitat. Though the shallow climate gradients of inland Western Australia result in large distribution ranges for these species, it also means that, in comparison to more mesic species, a given change in climate will result in a larger proportion of their distribution becoming unsuitable. As these species are unlikely to be able to migrate fast enough to keep up with the changing climate, the high level of exposure faced by many species means that species’ sensitivity and adaptive capacity are very important for long term survival. In terms of above ground traits, species in more arid conditions appear to be well adapted to current climate conditions, with important traits showing some degree of plasticity within species. This was observed as a decrease in vessel diameter, increase to sapwood density and leaf mass per area and a decrease in leaf size across an aridity gradient, all of which confer a reduced sensitivity to hydraulic failure in water-limited environments. Though the combination of the above traits resulted in more arid species having lower xylem conductivity, this was balanced by an increase in sapwood area to leaf area ratio, maintaining total stomatal conductance to xylem conductivity ratios across the aridity gradient. However, in a rapidly changing climate, species may not have enough time to adapt and change their average trait values to those of species that replace them along the aridity gradient. Even if species could migrate to new locations, seedling root architecture of species typically growing in finer-textured soil (such as many arid Eucalyptus species) may not be suitable for establishment in a coarser-textured soil (more typical of the regions to which they would likely need to migrate). This is because species typically growing in fine-textured soil tend to invest most resources into producing a taproot, with minimal lateral roots produced. This is likely to be a disadvantage in a coarser-textured (a soil with lower-water holding capacity), for which more lateral roots are likely to be needed to be able to explore a large enough soil volume for water. Thus, due to the combination of a high exposure to climate change and local root system adaptation to predominantly fine textured soils, Eucalyptus species in more arid environments are likely to be at greater risk to a changing climate than those living in a more mesic climate. These findings suggest that our research, restoration and conservation efforts should not focus disproportionally on more coastal higher rainfall species which occur conveniently close to human population centres; rather, consideration should be given to those species further inland where the greater impacts may actually occur.