Modelling nitrogen fertilization responses in temperate Eucalyptus plantations using APSIM

Nitrogen (N) fertilization is an important and flexible management option for increasing the growth, wood yield and profitability of many types of forest plantations, including temperate eucalypts in Australia. Process-based models are increasingly being used to simulate plantation growth. However, few have been tested on fertilizer experiments with a range of responses across a broad range of environmental conditions. The APSIM Eucalyptus model was tested for its ability to simulate observed growth responses to N fertilization by plantations of Eucalyptus globulus and E. nitens. Twenty–four experiments in temperate Australia with treatments of 0 or 400 kg ha⁻¹ N were established in 1- to 3-year-old plantations (early treatments), with treatments reapplied after three years (late treatments), and monitoring continued for another 2 years. Twelve experiments that were well replicated and randomized provided an adequate prediction of percentage responses to fertilizer (-8–51 % observed; −13–39 % predicted; R² = 0.71). Most of the response (89–100 %) was attributed to the first application of fertilizer. For plantations receiving early and late fertilization, average predicted water stress accounted for 71 % of variability in mean annual increment of stem volume. Modelling scenarios suggested that early fertilization was essential for maximizing growth at most sites. Field experimentation should continue to improve knowledge of N-responsiveness in changing climates, and to evaluate the model’s suitability for simulating those responses.