n Southern African Forestry Journal - Modelling growth and water use in four stands with the 3-PG model

Volume 2001, Issue 191
  • ISSN : 0038-2167



Accurate prediction of growth and yield of forest plantations remains important to the forestry industry for such purposes as assessing the benefits of silvicultural practices, matching species to site, understanding economic risks, predicting profitability and scheduling harvests. Conventional methodology is based on statistically-derived stand growth curves used to define a site index reflecting growth potential. Limitations of the method are widely acknowledged: factors constraining productivity are not identified; changes in environmental conditions affecting growth may not be taken into account; and growth predictions at sites not previously afforested may be poorly predicted. The hydrological impact of forest plantations remains a controversial issue. Existing prediction models do not take sufficient stand and site detail into account to usefully predict water use patterns on a scale that is practical to forest managers. Several relatively simple simulation models based on the major physiological processes behind growth and water use of forest stands have emerged recently, and are claimed to be useful tools for forest managers and water resource planners. One of these, the 3-PG model (Physiological Principles in Predicting Growth) is assessed here for its ability to predict growth and water use of <i>Pinus patula&lt;/i&gt; at four widely separated test sites. Model parameter values are fixed according to available field data, reported data for other species of pines, or as a result of a fitting process to match simulated to observed patterns of growth and water use in four diverse stands of this species. Simulation results are very encouraging, and a further phase of model testing on a wider range of test sites is recommended to improve parameter estimates and demonstrate the usefulness of the model.

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