Suggestions for choosing an appropriate level for modelling forest stands.

Models are abstractions of reality. In order to be useful, models must include essential elements of the real world system that are to be mimicked to meet some specified modelling objective. The pattern in a data set can often be described with a relatively simple model. Models of forests have been constructed for numerous management and research objectives. To determine an appropriate modelling unit (e.g. cell, organ, tree, stand, landscape), one must define the modelling objective and the forecasting time frame. Often the level of modelling detail possible is dictated by the data available. However, there are guiding principles that can aid in selecting an appropriate level for modelling. These principles include: (i) developing as parsimonious a model as possible; and (ii) adjusting the number of state variables for the forecasting period involved. The application of these principles is discussed within the framework of forest growth and yield models. As an illustration of the relationship between model accuracy and complexity, data from a loblolly pine [Pinus taeda] spacing trial in Virginia, USA, were used to predict stand volume.