Verification of 3-PG growth and water-use predictions in twelve Eucalyptus plantation stands in Zululand, South Africa

Abstract

The South African forestry industry perceives Process-based forestry models as important tools for improving predictions of growth and water use in forest plantations. The 3-PG process-based forest model was evaluated in South Africa using limited Eucalyptus and Pinus trial data. Some uncertainties exist, as to whether 3-PG will prove to be a practical forest management tool on forestry estates. The question is, will the 3-PG process be capable of modelling growth over a wide range of site growth? And if there are practical methods for estimating or measuring all of the required parameter values for the model.12 stands of Eucalyptus grandis by camaldulensis hybrid clones, representing early, mid and late rotation age, and covering a wide range of site growth potential in KwaZulu-Natal- South Africa, were intensively studied over a period of 12 months, so as to answer the above questions. Measurements of initial and final biomass, leaf area index, biomass allometric ratios, litterfall, sap-flow rates, pre-dawn xylem pressure potential and weather conditions were made throughout the year. Two site parameter values required indirect estimation, since relevant information was lacking and not available. A practical approach is described. Annual tree growth predicted by the model, and daily ranges of sap flow, were compared to field measurements. Predictions of annual growth increment were acceptable for 11 of the 12 stands. The twelfth stand was under predicted because it had a high annual growth increment, high leaf area and year long access by the trees to shallow groundwater, likely credited by physiological adaptations to the long-term absence of water stress at the site. The authors conclude that 3-PG can simulate growth and water use. Preliminary sets of model parameter values for a range of diverse stands may be estimated over a relatively short period using simple field equipment and techniques. Further testing of these parameter values at physiologically significant times over the remainder of the rotation is recommended to test model output and permit fine-tuning of parameters