The silvicultural practices that are usually included in the growth models are thinning and fertilization. Individual-tree models for predicting fertilizer response were developed by Shafii et al. Model for reference growth predicts the level of tree growth if trees were not fertilized at all, including the relevant factors affecting growth except fertilization. The effect of fertilization is described by a growth modifier function. It predicts the relative growth response after fertilization.
Response function predicts the magnitude and the temporal distribution of growth response.
Duration and temporal distribution of the fertilization effect are predicted using Weibull probability density function. To include the magnitude of the response, Weibull function is multiplied by an equation referring to the effects of fertilizer dose, type of fertilizer and site quality. The possibility to predict the individual-tree growth response to thinning in Scots pine stands by using a similar model structure is currently under research. First, the data should be a representative sample of the forests in the region to which models will be applied. Forest inventory data fulfil this requirement.