Application of the 3-PG model to a Eucalyptus grandis stand subjected to varying levels of water and nutrient constraints in KwaZulu-Natal, South Africa

Abstract

The 3-PG model has shown considerable potential as a tool for predicting forest productivity. As part of a model validation exercise, 3-PG was applied to an irrigated and fertilised Eucalyptus grandis stand in the KwaZulu-Natal Midlands. Simulations were run over a two-year period using appropriate initialisation data and a parameter set developed for E. grandis for the summer rainfall region of South Africa. The model was validated using observed leaf area index, and foliage and above-ground woody biomass, and then used to predict stand volume. Evaluation of 3-PG model outputs against field measurements indicated a generally good correlation for growth data across the control, irrigated, fertilised, and irrigated and fertilised trees. At 3.9 years, the model-predicted leaf area index over all treatments ranged from 3.8 to 5.1, similar to the observed range of 4.3 to 4.9. Stand volume at 3.9 years of age was under-predicted in the non-irrigated treatments. Actual stand volume at this age ranged from 100 to 118 m³ ha^-1 across all treatments, compared to model estimates of 79 to 121 m3 ha-1. Use of a dynamic fertility rating may improve model predictions and there is possible scope for improvement in the water-balance component of the model. Results from this study have shown 3-PG to be a robust model by producing accurate predictions of growth under varying levels of resource availability.

Key Words: Process-based modelling, irrigation, fertilisation, model validation, E. grandis

Southern African Forestry Journal No.203 2005: 3-13