Ecoregional site index models for Pinus pinaster in Galicia (northwestern Spain)

Abstract

Ten algebraic difference equations were used to develop site index models for even-aged stands of Pinus pinaster in two ecoregions of Galicia (northwestern Spain). Data from 204 stem analyses were obtained and a data structure involving all possible growth intervals was used to fit the equations. Generalized nonlinear least square methods were applied to take into account the error structure. Autocorrelation was corrected expanding the error term to allow a first-order autoregressive model adequate for the data structure. Different weighting factors were employed to satisfy the equal error variance assumption. Bias, root mean square error and Akaike's information criterion were calculated and cross-validation residuals were used to evaluate the performance of the equations. Ecoregional differences in the site index models were analysed using the non-linear extra sum of squares method and Lakkis-Jones test. The parameters of the models were significantly different between ecoregions. Relative error in site index predictions was used to select 20 years as the best reference age. Based on the analysis, an algebraic difference equation derived from the base model of Chapman-Richards with a different set of parameters for each ecoregion can be recommended. This model is polymorphic and with multiple asymptotes. It provides compatible site index and height growth estimates. © INRA, EDP Sciences, 2005.