EVALUATING A GROWTH-MODEL FOR FOREST MANAGEMENT USING CONTINUOUS FOREST INVENTORY DATA

Model evaluation should include qualititative as well as quantitative examinations of the model. The qualitative parts should comprise a cri tical appraisal of model logic as well as theoretical and biological r ealism of the model. The quantitative parts should comprise statistica l tests and comparisons of predictions with observations independent o f those used to fit the model. Comprehensive model evaluation requires several alternative approaches and criteria. Model evaluation is not one simple procedure, but consists of a number of interrelated steps t hat should not be separated from each other or from model construction . It is stressed that models can only be evaluated in relative terms, and their predictive value is always open to question. Thus, model eva luation is an ongoing process. A case study with the PBRAVO growth mod el for maritime pine (Pinus pinaster Ait.) in the Leiria forest, Portu gal, illustrates the utility of selected criteria and graphical techni ques. Based on theoretical examinations and tests with data from conti nuous forest inventories, we conclude that the Leiria version of the P BRAVO model does not adequately represent reality and that forecasts l ack sufficient accuracy for forest management purposes.