A compatible system for estimation of individual tree volume was developed for Scots pine (Pinus sylvestris L.) in northwestern Spain. The system comprises a merchantable volume equation, a total volume equation, and a taper function. The use of the volume equation allows rapid estimation of tree volume, and stand volume by summing individual tree volumes, which is equal to the volume obtained by integrating the taper equation. The volume equation is very easy to use and is therefore preferred when classification of the products by merchantable sizes is not required. Data from 228 destructively sampled trees were used for model development. Fourteen compatible volume equations were evaluated, 13 of these equations were taken from the available literature, and the other was developed in the present study. A modified second-order continuous autoregressive error structure was used to correct the autocorrelation of the hierarchical data used. The model developed by Fang et al. (Z. Fang, B.E. Borders, and R.L. Bailey. 2000. For. Sci. 46: 1-12) best described the data. There model is therefore recommended for the estimation of diameter at a specific height, merchantable volume, and total volume of Scots pine stems in the area of study. Résumé :Un système d'équations compatibles pour estimer le volume des arbres individuellement a été développé pour le pin sylvestre (Pinus sylvestris L.) du nord-ouest de l'Espagne. Le système comprend une équation du volume marchand, une équation du volume total et une fonction du défilement. L'application de l'équation du volume permet une estimation rapide du volume des arbres (et du volume du peuplement par sommation) qui est égal au volume obtenu par intégration de l'équation du défilement. L'équation du volume est facile à utiliser et donc préconisée lorsque la classification des produits par taille marchande n'est pas requise. Les données de 228 arbres échantillons abattus ont été utilisées pour le développement du système d'équations. Quatorze équations de volume compatibles ont été éva-luées; 13 d'entre elles proviennent de la littérature et l'autre a été développée dans le cadre de la présente étude. Une structure modifiée d'erreur autorégressive continue de second ordre a été utilisée pour corriger l'autocorrélation des données hiérarchiques disponibles. Le modèle de Fang et al. (Z. Fang, B.E. Borders et R.L. Bailey. 2000. For. Sci. 46 : 1-12) décrit le mieux les données. Ce modèle est donc recommandé pour l'estimation du diamètre à une hauteur donnée, du volume marchand et du volume total des tiges de pin sylvestre dans l'aire d'étude. [Traduit par la Rédaction] Diéguez-Aranda et al. 1205

A total of 31 taper functions from 3 different groups of models (single, segmented and variable-form taper functions) were fitted to diameter-height data from 203 Pinus pinaster trees sampled across even-aged stands in Galicia (northwestern Spain). Most of the taper functions analyzed showed problems of multicollinearity as indicated by the condition number. A second-order autoregressive CAR(2) error process was incorporated into the models to minimize the effect of autocorrelation inherent in the longitudinal data used, and to provide valid tests of significance for model parameter estimates. In general, variable-form taper functions provided the most accurate predictions. The flexibility and predictive performance of the variable-form model developed by Kozak (For Chron 80(4):507-515, 2004) indicated its usefulness for estimating diameter at a specific height, merchantable volume, and total volume of Maritime pine in the study area.

The diameter distributions of 125 permanent plots installed in birch dominated (Betula alba L.) stands in Galicia were modelled with the two-parameter Weibull distribution. Four different fitting methods were used: that based on percentiles of the distribution, non linear regression, maximum likelihood and the method of moments. The most accurate fit was obtained with the non linear regression (NLR) approach, considering the following statistics in the comparison: bias, mean absolute error (MAE), mean square error (MSE) and number of plots rejected by the Kolmogoroff-Smirnoff (KS) test.The scale parameter (b) and the shape parameter (c) obtained with the most accurate method (non linear regression), were first modelled with simple linear models and then related to commonly measured prediction variables (quadratic mean diameter, dominant height and stand density) with the parameter prediction model (PPM). The parameters fitted with the method of moments were recovered with the parameter recovery model (PRM) from the first and the second moments of the distribution (mean diameter and variance, respectively). Results indicated that both methods were successful in predicting the diameter frequency distributions. The PRM was more accurate than the PPM method.Key words: diameter class model, two-parameter Weibull distribution, fitting methods, parameter modelling. ResumenModelización de las distribuciones diamétricas en masas de Betula alba L. en el noroeste de España con la función Weibull biparamétrica Las distribuciones diamétricas de 125 parcelas permanentes instaladas en masas puras de abedul (Betula alba L.) en Galicia fueron modelizadas con la distribución Weibull de dos parámetros. Se emplearon cuatro métodos de ajuste: basados en percentiles de la distribución, regresión no lineal, máxima verosimilitud y el método de los momentos. Los ajustes más precisos fueron obtenidos con regresión no lineal, considerando los siguientes estadísticos en la comparación de los resultados: sesgo, error medio absoluto, error medio cuadrático y número de parcelas rechazadas por el test de Kolmogoroff-Smirnoff.El parámetro de escala (b) y el parámetro de forma (c) obtenidos con el método más preciso (regresión no lineal), fueron relacionados con variables de masa de frecuente medición (diámetro medio cuadrático, altura dominante y densidad) mediante modelos lineales sencillos aplicando la metodología de predicción de parámetros. Los parámetros ajustados con el método de los momentos fueron recuperados con modelos de recuperación de parámetros a partir del primer y del segundo momento de la distribución (diámetro medio y varianza, respectivamente). Los resultados indicaron que ambos métodos fueron satisfactorios para predecir las distribuciones de frecuencias de diámetros. El mé-todo de recuperación de parámetros fue más preciso que el método de predicción de parámetros.Palabras clave: modelo de clases diamétricas, distribución Weibull biparamétrica, métodos de ajuste, modelización de parámetros.

-In the present study, the distribution of C and nutrients in biomass (above-and below-ground) and soil were estimated in native Quercus robur stands in NW Spain. This information can be used to design future strategies to preserve and improve these forests, which are subjected to severe degradation as a consequence of poor management and fire damage. Biomass equations for tree components were fitted simultaneously using the data corresponding to 31 trees felled in 4 representative mature stands. The C accumulated in aerial biomass, roots, litter layer and mineral soil made up 40, 10, 8 and 42% respectively, of the C in the whole forest system. Tree biomass contained higher amounts of macronutrients than the available reserves in the soil. Stem wood was the major tree compartment for most nutrients, with the exception of Ca, which was mainly accumulated in stem bark. In comparison with commercial forest plantations in the region, particularly high amounts of nutrients were accumulated in the litter layer because of the high amounts of litterfall and higher foliar concentrations of nutrients. Quercus robur L. / biomasse / nutrition forestière / carbone / régression apparemment sans rapport

The purpose of this study was to compare the accuracy of the Weibull, Johnson's S B and beta distributions, fitted with some of the most usual methods and with different fixed values for the location parameters, for describing diameter distributions in even-aged stands of Pinus pinaster, Pinus radiata and Pinus sylvestris in northwest Spain. A total of 155 permanent plots in Pinus sylvestris stands throughout Galicia, 183 plots in Pinus pinaster stands throughout Galicia and Asturias and 325 plots in Pinus radiata stands in both regions were measured to describe the diameter distributions. Parameters of the Weibull function were estimated by Moments and Maximum Likelihood approaches, those of Johnson's S B function by Conditional Maximum Likelihood and by Knoebel and Burkhart's method, and those of the beta function with the method based on the moments of the distribution.The beta and the Johnson's S B functions were slightly superior to Weibull function for Pinus pinaster stands; the Johnson's S B and beta functions were more accurate in the best fits for Pinus radiata stands, and the best results of the Weibull and the Johnson's S B functions were slightly superior to beta function for Pinus sylvestris stands. However, the three functions are suitable for this stands with an appropriate value of the location parameter and estimation of parameters method.