Diameter growth models induced by competition for four Salix clone monocultures

In this study, growth models are presented that predict the growth of a mean shoot of the willow stool. The growth models were constructed using an empirical approach and were developed both with and without inclusion of competition indices. The competition indices were based on inter-tree distances, on differences in shoot size of the subject mean shoot and size of the other shoots on the stool in question, and also on differences between the subject stool and neighbouring stools. The best growth models could explain 19 to 51% of the variation in growth, therefore the remaining 49% of the variation was unexplained. No trend was observed in the residual deviations, which indicates that the functions fitted the data well. According to the computations, the growth reactions for different kinds of competition were not the same for all clones studied. Thus it may be possible to find combinations of clones that could utilize the site factors more effectively as a mixed stand than any clone could as a pure stand.