The 3-PG Process-Based Model

This chapter presents and discusses in detail the model known as 3-PG. The acronym is an abbreviation for Physiological Processes Predicting Growth. Besides its simple, general structure, a significant factor in the widespread adoption of 3-PG has been that implementations of the model have been made freely available to all who wanted to use it. This chapter provides an overview of why 3-PG has the structure it does, describes that structure and summarizes the various data and species-specific parameters required to run the model. It discusses the assumptions that underlie the sub-models and functional relationships used in it, and it discusses the manner in which species-specific parameter sets can be established. Any model must consist of a set of statements that constitute hypotheses about the way the system being modeled works. Wherever possible these should be in a form that is testable, either by direct measurements designed to test particular sub-models, or indirectly by measurements that evaluate the model as a whole at the level of its outputs. Accordingly, this chapter provides a description of how a species-specific parameter set and 3-PG as a whole can be tested. Applications of 3-PG across a wide range of environments and species are summarized in this chapter. This allows assessment of the extent to which it fulfills the criteria for evaluation, whether it provides a framework within which one can set and evaluate current knowledge and information about tree physiology and the factors that affect and determine stand growth, and whether it is a useful practical tool. Finally, this chapter considers changes that could be made to various parts of the model and assess the implications of these changes in terms of the number and availability of the parameter values that would be required in relation to possible gains in the accuracy and precision of predictions.