Simulation of competition for light in even-aged stands of Douglas Fir

Three versions of an explanatory model to simulate competition between trees within forest stands are presented, based on the distribution of photosynthetically active radiation (PAR) over the trees in the stand. From the amount of PAR absorbed, the rates of assimilation and volume increment are calculated for the trees, which are presented in size classes. Volume increments calculated this way for periods of 35–45 years are compared with measurements from permanent field plots to evaluate different versions of the model.In the first elementary version of the model, it is assumed that the foliage of all trees is distributed uniformly over the field area, neglecting any differences in individual tree height. In this case, the growth of each tree is proportional to its share of the total foliage area of the stand and approximately proportional to its size. This model version slightly underestimated the variation in diameters observed in the field plots.The second version of the model takes into account differences in tree height. As a result of the prior assumption that the foliage of all classes is distributed uniformly over the field area, this model version severely overestimated the shading of short trees by tall neighbours, and the variation in diameters was overestimated as compared to the field data.The third version of the model accounts for clustering of needles within individual crowns in addition to height differences between the classes. This approach gave the better fit to the field data for normally stocked and for dense stands.