Sustainable forestry: A model of the effects of nitrogen removals in wood harvesting and fire on the nitrogen balance of regrowth eucalypt stands

Abstract

Abstract Sustainable forest use is an integral part of Australia's recently adopted National Forest Policy; consequently, there is an urgent need for quantitative, ecologically based measures of sustainability. One process that may affect ecosystem sustainability is the removal of nutrients through forest harvesting and fire. This paper presents a model‐based analysis of the combined consequences of harvesting and fire management practices for the nitrogen (N) budgets of managed forest ecosystems. 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 criterion for the maintenance of site N reserves. That criterion can be used to estimate the accretion (or depletion) of site N over a single forest rotation, or to predict sustainable stem productivity for given N inputs and management practices. The analysis is applied to managed stands of karri (Eucalyptus diversicolor F. Muell.) in southwestern Australia to investigate whether natural N inputs are sufficient to maintain site N capital under current harvesting and fire practices. Model predictions for stands harvested at age 100 years with slash burns and regular 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 further research to improve estimates of several key model parameters and relationships.