Generalized height-diameter relationship for Pinus sylvestris plantations in North-western Spain

Abstract

A height-diameter relationship for Pinus sylvestris L. plantations in Galicia (northwestern Spain) was developed using the breast height diameter and some stand characteristics as regressors. The data used were obtained from 155 sample plots located in unthinned stands, or stands thinned lightly from below, throughout the study area; 68 of the sample plots were measured twice. Nine different compatible height-diameter equations, which estimate the dominant height when the diameter at breast height is equal to dominant diameter, were evaluated for inclusion in a dynamic stand growth model for the species. Selection of the best equation was based on both numerical and graphical analyses of the residuals. The data were best described by a modified version of the equation of Gaffrey (1988), which includes the diameter at breast height, the dominant diameter and the dominant height as regressors. This model explained 94% of the total variability of the data used in both the fitting and the cross-validation phases. As two trees within the same stand and that have the same diameter are not necessarily of the same height, a stochastic component was added to the deterministic height-diameter relationship. By using this approach, more realistic height predictions were obtained from diameter classes.