Long-term hydrological response emerges from forest self-thinning behaviour and tree sapwood allometry

Fires in forested catchments are of great concern to catchment managers due to their potential effect on water yield. Among other factors such as meteorological conditions and topography, dominant vegetation and its regeneration traits can play a key role in controlling the variability in the type and recovery-time of the hydrological response between forested catchments after stand-replacing fires. In temperate South-Eastern Australia, a long-term reduction in streamflow from catchments dominated by regenerating tall-wet Eucalyptus obligate seeder forests was observed, which has substantial implications for Melbourne's water supply. While the unusual hydrological response has been attributed to the higher water-use of the regrowth forests, the dominant underlying mechanism has not yet been identified. Here we show analytically and with a closed-form solution that this streamflow pattern can emerge from forest dynamics, namely the combination of growth and tree mortality as constrained by the self-thinning line (STL) and sapwood allometry of the dominant overstory tree species under non-limiting rainfall regimes. A sensitivity analysis shows that observed variations in the relative streamflow anomaly trend can be explained by parameters controlling: (i) the shape of the STL; (ii) regeneration success; (iii) radial tree growth rate; and (iv) fire severity. We conclude that the observed variation in long-term post-disturbance streamflow behaviour might have resulted from different trajectories of forest dynamics and suggest that to minimize uncertainty in future water-balance predictions, eco-hydrological models for even aged forests include a mechanistic representation of stand demography processes that are constrained by forest inventory data.