Stand basal area growth functions for Pinus sylvestris L. plantations in Galicia

Stand basal area growth functions for Pinus sylvestris L. plantations in Galicia A compatible basal area growth system was developed for single-species, even-aged Scots pine (Pinus sylvestris L.) stands in Galicia (northwestern Spain). The data used to develop the system were obtained from an inventory of 155 plots located in unthinned or lightly thinning stands and from a second inventory of 68 of these plots. The system consists of a function for predicting initial stand basal area and another for projecting stand basal area. The former is a growth function obtained by integration of a differential equation; the latter was obtained from the initialization function expressed as an algebraic difference equation. Compatibility between the two equations was ensured by estimating the projection function parameters separately, substituting their values into the initialization function, and then fitting this function to obtain the estimate of the remaining parameter. This method of proceeding gives priority to the projection function, and was selected because it was considered that the model projection would be used most frequently to estimate stand basal area from any given initial stand conditions obtained from a forest inventory. The model that provided the best results was based on the Korf’s function (with b1 as free parameter), which explained 93.5% of the variability of the stand basal area projected by the algebraic difference equation. This model estimates future stand basal area using initial basal area and initial and final ages as independent variables. The corresponding initialization equation explained 71.9% of the variability of the stand basal area at any point in time. In this case, the free parameter was expressed as a multiplicative inverse function of site index.