SUSTAINABLE FORESTRY - A MODEL OF THE EFFECTS OF NITROGEN REMOVALS INWOOD HARVESTING AND FIRE ON THE NITROGEN-BALANCE OF REGROWTH EUCALYPTSTANDS

Sustainable forest use is an integral part of Australia's recently ado pted National Forest Policy; consequently, there is an urgent need for quantitative, ecologically based measures of sustainability. One proc ess that may affect ecosystem sustainability is the removal of nutrien ts through forest harvesting and fire. This paper presents a model-bas ed analysis of the combined consequences of harvesting and fire manage ment practices for the nitrogen (N) budgets of managed forest ecosyste ms. The model, called N-BAL, evaluates the balance between N removals due to harvesting and fire (prescribed and regeneration burns), and N inputs (both natural and as added fertilizer), and leads to a criterio n for the maintenance of site N reserves. That criterion can be used t o estimate the accretion (or depletion) of site N over a single forest rotation, or to predict sustainable stem productivity for given N inp uts and management practices. The analysis is applied to managed stand s of karri (Eucalyptus diversicolor F. Muell.) in southwestern Austral ia to investigate whether natural N inputs are sufficient to maintain site N capital under current harvesting and fire practices. Model pred ictions for stands harvested at age 100 years with slash burns and reg ular prescribed burns range from a rotation-averaged depletion rate of 22 kg ha(-1) year(-1) to an annual accretion of 14 kg ha(-1) year(-1) , depending on assumed N inputs and fire frequency and intensity. The mean annual N balance is highly sensitive to rates of natural N inputs , fire intensity and inter-fire period, and less sensitive to rotation length. These results are tentative and highlight the need for furthe r research to improve estimates of several key model parameters and re lationships.