Variable self-thinning explains hydrological responses to stand replacement in even-aged forests

That streamflow from even-aged forests is a function of forest age (the Kuczera curve) is an accepted paradigm in Australian forest hydrology and less commonly in the Northern Hemisphere. However, none of a dozen stand replacement experiments in Australian obligate seeder forests have faithfully reproduced the Kuczera curve. Here we test an alternative model based on the self-thinning line, showing that both the Kuczera curve and results of subsequent stand replacement experiments can be explained by observed changes in self-thinning after disturbance. Self-thinning in even-aged forests is dictated by the rate of reduction in stocking density as mean tree size increases. Comparison of stand basal area and stocking densities in stands of Eucalyptus regnans F. Muell, regenerated after wildfires in 1851 and 1939, suggested significant differences in self-thinning lines between age cohorts. In three large catchments, a simple forest growth and water use model only reproduced observed substantial long-term reductions in streamflow after stand replacement (mean reduction from 1940 to 1982 of 233 mm year−1) when observed cohort-specific changes in the intercept of the self-thinning line (STL) were incorporated (modelled mean reduction 235 mm year−1). Use of a single STL intercept in both pre- and post-disturbance simulations did not produce the observed streamflow responses (modelled mean reduction 37 mm year−1). Averaged over six E. regnans experimental catchments in which the dominant forest was either partly or completely removed and replaced with the same species, the observed decadal maximum streamflow reduction was only 7 mm year−1, compared to 22 mm year−1 using the model with observed STL intercepts but 200 mm year−1 using the Kuczera curve. Consequently, hydrological responses to disturbance are less predictable than previously assumed, as the reasons for changes in self-thinning behaviour are not yet understood. We speculate that in this case it is related to variability in seed supply and competition with shorter-lived understorey species in the early stages of regeneration.