A model to evaluate factors controlling growth in Eucalyptus plantations of southeastern Australia

A process model of Eucalyptus plantation growth was developed, including limitation of growth by nitrogen and phosphorus, understorey competition for water and nutrients and the occurrence of juvenile foliage in young eucalypts. The model considers plant, litter and soil nutrient cycles and can thus make long-term projections of the impacts of nutrient removal associated with different management practices. The model was parameterized for Eucalyptus sieberi forests in East Gippsland, Victoria, Australia, growing on very phosphorus-deficient soils. Model results suggest that: (1) The presence of juvenile foliage, with high specific leaf area, greatly increases growth rates during the early exponential growth phase. However, wood accumulation increases little in mature forests because the productivity of the adult canopy is not affected by the juvenile phase. (2) Understorey competition for water and nutrients causes a decrease in overstorey growth, but has a smaller effect on long-term wood production, due to feedbacks between production and the removal of nutrients in harvested wood. (3) The use of short rotation cycles would cause soil P stores and productivity to decline, although this decline may not be noticeable over the first century. The predicted rotation length required to maintain long-term productivity is inversely related to the rate of atmospheric deposition of P. (4) Application of phosphorus is projected to increase production, but only if nitrogen inputs are also increased to prevent the latter from becoming limiting in later rotations. As there are uncertainties in some of the model parameters, these projections should be viewed as relative effects, rather than site-specific predictions.