Effects of Fuel-Reduction Burning on a Eucalyptus obliqua Forest Ecosystem in Victoria

Abstract

An autumn fuel-reduction burn of low intensity (200-250 kW m^-1) was performed in a Eucalyptus obliqua forest near Gembrook, Victoria. The aim of the study was to evaluate the effect of a single burn on floristics, biomass, N content and N₂ fixation. The fire burnt 50% of the area in a mosaic pattern, significantly reducing understorey vegetation cover (by 90%) and plant density (by 70%) in burnt areas immediately after the fire. Understorey cover was restored to 40% of the original value 1 year later, but 33% of the understorey species were still absent from burnt areas. In the whole site mosaic, biomass declined by 30 t ha^-1 (3 kg m^-2) (10%) and N content by 100 kg ha^-1 (10 g m^-2) (18% excluding soil N, 2% including soil N). These losses were due to significant losses of biomass and N from the understorey only (88%, 85%), standing dead trees (57%, 62%), fallen wood (73%, 60%) and litter (69%, 70%). One year later, there was no significant increase in either biomass or N content. Burnt areas had five times the total nitrogenase activity of unburnt areas, owing to significantly greater specific nitrogenase (C₂H₂ reduction) activity, three times the nodule weight and 20 times the plant density of unburnt areas for the dominant legume (Pultenaea scabra). Using a calibration ratio for C₂H₂:N₂ of 2.68 :1 derived from glasshouse growth studies, N₂ fixation for P. scabra was estimated as 15 g ha^-1 year^-1 in burnt areas and 3 g ha^-1 year^-1 in unburnt areas, with a mean of 9 g ha^-1 year-' for the whole site mosaic. Adding superphosphate to burnt areas increased estimated N₂ fixation significantly by 14%, mainly by increasing nodulation. Losses of N due to the burn (100 kg ha^-1) were considerably greater than gains from increases in N2 fixation (6 g ha^-1 year^-1) one year after the burn. Even allowing for N₂ fixation by other, infrequent legumes and greater N₂ fixation in subsequent years, these data suggest that the N lost in the burn is more likely to be replaced by inputs from soil reserves and rainwater than from N₂ fixation by legumes.