HARVESTING EFFECTS ON PHOSPHORUS AVAILABILITY IN A MIXED EUCALYPT ECOSYSTEM IN SOUTHEASTERN AUSTRALIA

Native eucalypt forests in the foothills of eastern Victoria, Australia, are being managed for sustained production of hardwood timbers on at least 80-year rotations. The likely effects of intensive harvesting on soil phosphorus reserves were investigated. Distribution of P in a mixed eucalypt ecosystem was determined and removals in sawlogs and residual roundwood were estimated. Availability of soil P was studied in detail by determining fluoride, acid-soluble and organic forms, plus P adsorption isotherms and P levels following sequential extractions under optimal soil: water ratios and extraction times. The above-ground biomass had a P content of 21.2 kg ha-1, and removals of P in harvesting were estimated to range from 1.5-3.0 kg ha-1, depending on harvesting intesting intensity. It was estimated that a further 8 kg P ha-1 would have been lost by burning logging residue to prepare a seedbed for natural regeneration. Soil phosphorus available for long-term forest growth was estimated to be 57 kg ha-1 based on results of sequential soil extractions under optimal conditions. This is a new approach to estimating long-term available P in forests, and we have described this estimate by the term 'forest-accessible phosphorus'. The combined losses from harvesting and burning represented 18% of the forest-accessible phosphorus. On this basis it was concluded that these eucalypt ecosystems could be intensively harvested for at least five rotations before a measurable change in site productivity would be detected as a result of P depletion. © 1990.