Predicting post-fire changes to streamflow using forest self-thinning parameters

Stand-replacing wildfire in Eucalyptus forested catchments alters streamflow, however these changes can be highly variable, ranging from moderate increases through to large reductions (greater than to 50%) over decades with serious consequences for the water supplies of dependent communities.  Predicting change has proven difficult, with the most widespread models relying on time- based proxies for hydrologic changes caused by post-fire tree mortality and regrowth dynamics (e.g. Kuczera 1987).  The aim of this research is to replace these time-based proxies, which have been found to be sometimes inconsitent with more recent catchment data, with a forest-stand self-thinning model of post-fire forest dynamics, such that streamflow is an explicit function of forest density, tree size, and growth rates, which are in turn functions of the initial condition of the stand (early regeneration), and the site conditions the stand is exposed to (i.e. climate and soil conditions). This presentation will; i) use a 42 year streamflow timeseries across 18 fire-affected catchments to illustrate the limitations of time-based proxies (Benyon et al 2023), ii) outline our initial analytic work to couple the forest self-thinning model with a hydrologic model (Inbar et al 2022), iii) present the results of forest inventories with age ranges from 1 to 80 years to quantify suitable self-thinning line parameter values (Harrison 2024), and lastly, iv) present the results from studies to identify abiotic and biotic (inter-species competition with Acacia) controls on self-thinning dynamics that could plausibly explain the large variation in post-fire streamflow responses.  It is hoped that this research will enable the earliest possible identification of expected decadal-scale post-fire streamflow reductions so that water supply policy makers can respond appropriately and minimize water supply disruptions to dependent communities.

References

Kuczera, G. (1987). Prediction of water yield reductions following a bushfire in ash-mixed species eucalypt forest. Journal of Hydrology, 94(3-4), 215-236.

Inbar, A., Trouvé, R., Benyon, R. G., Lane, P. N., & Sheridan, G. J. (2022). Long-term hydrological response emerges from forest self-thinning behaviour and tree
sapwood allometry. Science of the Total Environment, 852, 158410.

Benyon, R. G., Inbar, A., Sheridan, G. J., Lyell, C. S., & Lane, P. N. (2023). Variable self-thinning explains hydrological responses to stand replacement in even-aged forests. Journal of Hydrology, 618, 129157.

Harrison, M., 2024. Could Eucalyptus regnans stocking density explain post-fire streamflow responses in Melbourne’s water catchments? (Master of Environmental Science). The University of Melbourne, Melbourne, Australia.