Some factors affecting water yield from mountain ash ( Eucalyptus regnans) dominated forests in south-east Australia

Abstract

An experiment in mountain ash forests in Melbourne's water supply catchments in south-east Australia investigated the impact on long-term water yield of reducing forest density. Fifty-four per cent of basal area was removed from a 17 ha catchment (Black Spur 1) by patch cutting, and the patches were regenerated with mountain ash. A 50% reduction was implemented in Black Spur 3, an 8 ha catchment, through uniform thinning. Uniform thinning was shown to be more effective in enhancing streamflow than patch cutting. A streamflow increase of 25–30% (130–150 mm year −1) was observed after treatment in both catchments. Eleven years later, a treatment effect of 15% was still evident in the selectively thinned catchment (Black Spur 3), but the effect had completely decayed in the patch cut catchment (Black Spur 1). Research by Melbourne Water has established that streamflow is significantly influenced by forest age. It is hypothesised that this relationship, and the observed decay after patch cutting in Black Spur 1, is largely the result of variation in transpiration. To study the relationship between forest age and transpiration in detail, sap velocity was measured over two summers in four mountain ash plots using the heat-pulse method. The trees on these plots were 50, 90, 150 and 230 years old. On days when measurements were made, the mean sap velocity was not significantly different in the 50-, 90- 230-year-old stands, but was significantly smaller by 14% in the 150-year-old stand. Overstorey sapwood area gradually decreased with plot age, and was 57% lower in the 230-year-old plot than in the 50-year-old plot. When combined with the sap velocity measurements, these data indicated that over the six warmest months of the year, transpiration in the 50-year-old plot was 190 mm more than in the 230-year-old plot. These results support a hypothesis that differences in streamflow between 50-year-old and 230-year-old mountain ash forest can largely be accounted for by differences in transpiration. Further heat-pulse studies in young regrowth and in thinned and unthinned 1939 regrowth will be needed if the streamflow changes in Black Spur 1 and Black Spur 3 are to be fully explained.