Stand and tree-level variability on stem form and tree volume in Pinus pinea L.: A multilevel random components approach

Abstract

Stem analysis data from 536 sampled trees located in 36 permanent sample plots were used to develop a taper function which allows flexible end products volume estimation for stone pine (Pinus pinea L.) in Spain. To alleviate inference problems derived from high correlation among observations a multilevel linear mixed approach, including random coefficients varying at both plot and tree levels, was used. The proposed taper function expresses the section diameter as a function of breast height diameter and relative height of the section, showing a logical behaviour at both breast and total height. Between and within plot stem form variability was explained by including explanatory covariates as basal area or distance independent competition indices. Mixed models allow calibration of the model for new locations, by predicting random coefficients if additional stem form measurements are available. Several alternatives of calibration, considering section diameters measured at different heights in a variable number of trees per plot, were compared between them, and with the basic marginal model and the marginal model including covariates. The best calibration alternative was to use additional section diameter measurements taken at 0.5 m height above ground. This model results in substantial improvement in stem form and single tree volume predictive ability over previously existing volume functions for the species, allowing size end-use classification for timber products.