Forest growth in relation to weather and soils is studied using a physiological simulation model. Growth potential depends on physiological characteristics of the plant species in combination with ambient weather conditions (mainly temperature and incoming radiation). For a given site, growth may be lower because of incomplete canopy closure, shortage of water and nutrients, and the occurrence of growth-disturbing factors such as pests, diseases, and damage to the plants, e.g. by windthrow or frost. Attention is focused on the main growth-limiting factors, i.e. canopy closure, and the availability of water, nitrogen and phosphorus, so that differences in growth between different sites can be explained as a function of the properties of plant and soil, and of the ambient weather at a particular site. The model is applied to even-aged Douglas fir stands in the Netherlands because of the availability of field data for testing and evaluating it.The life cycle of trees and forests encompasses many years, and in order to be able to study overall stand dynamics, the model aims at simulating growth over periods of several decades. This allows the results of the model to be evaluated against data from permanent field plots, that are also being used in conventional, descriptive research on growth and yield. Furthermore, simulating forest growth over such long periods makes the results from the model comparable with the results of practical forest management. Variations in growth during the year are caused by changes in incoming radiation, temperature and water availability in the soil. To simulate this, time intervals of one day are used for the main part of the simulation model.The particular value of simulation models in forestry lies in the possibility they offer of combining different aspects of growth in an overall approach, and of studying stand dynamics over a long period of time without having to rely entirely on expensive and time-consuming field trials. Moreover, in a situation where the environment for forest ...