Application of three modelling approaches to simulating tree growth in central NSW, Australia

In Australia, tree planting is being considered for major national catchment planning and management initiatives, especially in low rainfall regions. Models that provide spatial estimates of tree growth and incorporate the effects of climate variability are required to support these initiatives. This study employed three different modelling approaches to assess tree growth for the Lachlan catchment in south-eastern Australia. Firstly, the Physiological Principles for Predicting Growth (3-PG) model was used in ‘grid mode’ with long-term average climate grids to produce surfaces of modelled growth (Model 1) for three scenarios: managed timber plantations, plantations for farm forestry and other purposes, and natural regeneration of native vegetation. Secondly, the 3-PG model was used in ‘point mode’ with 50 years of actual weather records to simulate the effects of climatic variability on tree growth at 30 sites across the region (Model 2). Simulated estimates of growth for these 30 sites (Model 2 outputs) were then regressed against a detailed mean annual rainfall surface and the resulting linear equations were used to develop surfaces for predicted growth across the catchment (Model 3). These three approaches were compared to assess their suitability for developing spatial estimates of tree growth and the effects of climatic variability for catchment assessment and planning. The study confirms that use of long-term average climate data to model tree growth across a region may be misleading and that regional assessments should incorporate the effects of climatic variability on tree growth.