Application of volume growth and survival graphs in the evaluation of fourprocess-based forest growth models

Volume growth and survival (VGS) graphs, which show volume growth rate and risk of mortality for individual trees (or tree size classes), have been pr oposed as a tool for assessing the validity of models that describe the dev elopment over time of tree size distributions within forest stands. We exam ined the utility of the VGS method in evaluating four process-based models. The performance of the models FORSKA, 4C, MORG, and PipeQual is analyzed a gainst long-term data from a Scots pine stand in Eve, Finland, and the mode ls FORSKA and 4C are also assessed with respect to data from a beech stand in Fabrikschleichach, Germany. Comparison of the measurement-based VGS grap hs with those produced from the model-based data shows that although the mo dels yield similar stand-level predictions, they can differ widely in their projections of individual tree growth and size distributions. Examination of the discrepancies between models and data in the context of the VGS grap hs reveals several areas in which the models could be improved. We conclude that the method is useful in model evaluation, especially if used in combi nation with indicators of stand structure, such as the height/diameter rati o.