Five Decades of Hydrological, Vegetation and Biodiversity Responses to Climate Change in a Southwestern Australian Jarrah ( Eucalyptus marginata) forest (1972-2024)
Water is a key driver for many ecosystem processes. The integration of various sets of data for the 2000 ha 31 Mile Brook catchment allows an holistic evaluation of the responses by this ecosystem to substantial changes in hydrology. Excellent baselines for hydrology and vegetation are available from 1972. From the late 1960’s there has been a major reduction in rainfall in the South-West of Western Australia and an even greater decline in stream flow. Modelling simulations closely matched the observed drying trends in stream flow, flow days and groundwater depth. The model was then used to estimate the impact of thinning treatments on these values. Between 2004 and 2010, the estimated evapotranspiration matched the average rainfall and soil moisture storage fell. Estimates of Leaf Area (LAI) show steady canopy growth after good rainfall years (to 1.5) and then a substantial decline, as a result of the very dry 2010 winter. Crown scorch and some tree deaths on shallow soils were observed in autumn 2011 and again in 2024. Detailed mapping of species composition in 1972 and remapping in 2012 showed only a slight xeric shift. In 2024, the tree stratum was healthy, but small changes in composition were observed, some as the result of a prescribed burn, others due to drought. Aquatic biodiversity between 1984 and 2010 reveal shifts in faunal assemblages, species richness and abundance, but only for a few species that have longer life cycles. A range of terrestrial biota show little change, as the understorey vegetation on which they depend for food and shelter is still healthy. So far, this ecosystem has shown remarkable resilience to a changing climate. Modelling suggests that thinning the tree stratum to a basal area of 14 would benefit soil moisture and increase stream flow. Thinning for ecosystem health is now approved in the Forest Management Plan (DBCA 2024). Recommendations for ongoing management and research are made.
