Multi-scale controls on water availability shape the distribution of karri (Eucalyptus diversicolor) in southwestern Australia

Tolerance of tree species to changing water availability plays a critical role in determining forest resilience to climate change, particularly in drought prone warm temperate regions. However, the specific environmental drivers and water availability thresholds governing current distribution of many southern hemisphere tree species remain largely unknown. We investigated how climate, landscape position, and soil properties influence water availability and the distribution of karri (Eucalyptus diversicolor), a tall forest tree species endemic to southwestern Australia. While predominantly found in areas with rainfall above 800 mm/year, some outlying karri stands also occur in lower rainfall zones. Using a binomial generalised linear model (GLM) with a logit link, we modelled the probability of karri presence across the southwest range as a function of mean annual rainfall (bio12), mean temperature of the warmest quarter (bio10), and the topographical wetness index (TWI). On a regional scale, we found this simple model to predict karri occurrence with high accuracy, showing the importance of high rainfall, mild summer temperatures, and well drained upland landscapes. Second, we analysed a comprehensive reference dataset of soil physical and chemical properties across southwest Australia to better characterise local scale conditions associated with karri growth. We also investigated soil depth using passive seismic and contrasts in soil electrical resistivity between karri stands and neighbouring forest types. Our preliminary results indicate that karri preferentially occurs on deep, well drained, clay rich soils, but also persists in landscape positions where shallow soils or soil types with poor water-holding capacity (e.g. sands) maintain higher water-availability, owing to deeper water storage (e.g. karstic systems) or regular rapid recharge (e.g. runoff from adjacent rocky outcrops). Our research confirms that karri distribution is likely determined primarily by water availability, but this is moderated at different scales by interacting climatic, topographic, and soil controls. This study provides a more nuanced foundation for predicting vulnerability of particular populations to future drought stress. Ongoing studies are quantifying water use across the distribution range under varying site conditions.