title: Fitting and Calibrating a Multilevel Mixed-Effects Stem Taper Model for Maritime Pine in NW Spain.
abstract: Stem taper data are usually hierarchical (several measurements per tree, and several trees per plot), making application of a multilevel mixed-effects modelling approach essential. However, correlation between trees in the same plot/stand has often been ignored in previous studies. Fitting and calibration of a variable-exponent stem taper function were conducted using data from 420 trees felled in even-aged maritime pine (Pinus pinaster Ait.) stands in NW Spain. In the fitting step, the tree level explained much more variability than the plot level, and therefore calibration at plot level was omitted. Several stem heights were evaluated for measurement of the additional diameter needed for calibration at tree level. Calibration with an additional diameter measured at between 40 and 60% of total tree height showed the greatest improvement in volume and diameter predictions. If additional diameter measurement is not available, the fixed-effects model fitted by the ordinary least squares technique should be used. Finally, we also evaluated how the expansion of parameters with random effects affects the stem taper prediction, as we consider this a key question when applying the mixed-effects modelling approach to taper equations. The results showed that correlation between random effects should be taken into account when assessing the influence of random effects in stem taper prediction.
authors.affiliation: Unidad de Gestión Forestal Sostenible (UXFS), Departamento de Ingeniería Agroforestal, Universidade de Santiago de Compostela, Escuela Politécnica Superior, C/Benigno Ledo, Campus universitario, 27002 Lugo, Spain.
authors.forename: Manuel
authors.initials: M
authors.lastname: Arias-Rodil
authors.affiliation: Departamento de Ingeniería y Ciencias Agrarias, Universidad de León, Escuela Superior y Técnica de Ingeniería Agraria, Avda de Astorga, 24400 Ponferrada, Spain.
authors.forename: Fernando
authors.initials: F
authors.lastname: Castedo-Dorado
authors.affiliation: Grupo de Investigación en Sistemas Forestales Atlánticos (GIS-Forest), Departamento de Biología de Organismos y Sistemas, Universidad de Oviedo, Escuela Politécnia de Mieres, C/Gonzalo Gutiérrez Quirós s/n, 33600 Mieres, Spain.
authors.forename: Asunción
authors.initials: A
authors.lastname: Cámara-Obregón
authors.affiliation: Unidad de Gestión Forestal Sostenible (UXFS), Departamento de Ingeniería Agroforestal, Universidade de Santiago de Compostela, Escuela Politécnica Superior, C/Benigno Ledo, Campus universitario, 27002 Lugo, Spain.
authors.forename: Ulises
authors.initials: U
authors.lastname: Diéguez-Aranda
mesh_terms: D008962:Models, Theoretical; D028223:Pinus / Q000254:growth & development*; D018547:Plant Stems / Q000254:growth & development*; D013030:Spain; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water