This study presents a yield model for aboveground biomass production from three species the Eucalyptus in northern and western regions of Uruguay, based on sampling records from intensive crop plantations. High-density eucalyptus plantations represent a forestry alternative for the production of forest biomass. This work assessed the survival and growth of three eucalyptus species (Eucalyptus benthamii Maiden & Cambage, E. dunnii Maiden and E. grandis Hill ex Maiden) planted at densities of 2220, 3330, 4440 and 6660 trees ha−1, for a period of 57 months in northern (Tacuarembó) and western (Paysandú) regions of Uruguay. Linear and logarithmic equations of individual volume were fitted by site and species. The survival of E. grandis, E. benthamii and E. dunnii was not related to planting density, and the highest mortality values occurred in Tacuarembó. The effects of competition among trees were more evident at the highest planting density for E. grandis. In all species, the reduction in diameter was more marked than that of height, as planting density increased. Tree volume showed the same trend, and this was higher with higher planting densities. At Tacuarembó, the volume was the highest with E. benthamii at 6660 trees ha−1 (416.4 m3 ha−1), and, at Paysandú, the highest production was obtained with E. grandis (370.7 m3 ha−1) and with the densities of 4440 and 6660 trees ha−1 (305.9 and 315.3 m3 ha−1, respectively). With all species and planting densities, there was an increase in the accumulated volume during the 57-month study period; however, growth curves indicate that the maximum production per unit time and, therefore, the optimum harvest time occurred at 48 months. In this work, it has been shown that the use of intensive short-rotation plantations of eucalyptus for the production of biomass in Uruguay is suitable in soils prioritized for forestry.

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Biomass additivity is a desirable characteristic of a system of equations for predicting components and total biomass, since equations independently adjusted generate biologically inconsistent results. The aim of this study was to fit and compare three methods for modelling biomass: (i) total biomass individual regression, (ii) total biomass regression function calculated as the sum of separate biomass components, and (iii) simultaneous equations of biomass components based on Nonlinear Seemingly Unrelated Regression. A total of 208 trees of Eucalyptus dunnii and Eucalyptus grandis were harvested and destructively sampled to record above-ground biomass. Results indicate that a system of equations adjusted by simultaneous equations provides accurate biomass estimations, guaranteeing additivity. This model system showed good fit and good prediction performance, given that the correlation coefficient was higher than 97 % for total above-ground biomass, for both species; whereas root mean square error was 23.9 kg and 30.2 kg for E. grandis and E. dunnii, respectively. A system of biomass equations was developed for each eucalyptus species, such that the sum of the estimations of the biomass components equaled the estimate of above-ground biomass. Results showed that the systems of equations have high potential for improving the accuracy of individual tree above-ground biomass estimates for both species.

SUMMARYThe aim of this study was to compare the accuracy of Weibull, reverse Weibull, S B Johnson, Gamma and Beta distributions, fitted with the method of moments and maximum likelihood, for describing diameter distributions in stands of Pinus taeda, Eucalyptus globulus, and Eucalyptus dunnii in Uruguay. Data from 429 permanent plots of P. taeda, 185 plots of E. globulus and 271 plots of E. dunnii were used. To compare the quality of fit for the probability distribution function (pdf), the Kolmogorov-Smirnov and Reynolds error indices were used. The function S B Johnson presented a better fit on the plots of P. taeda, while the inverted Weibull function was more accurate for Eucalyptus species. The method of moments was more accurate than maximum likelihood.Key words: pdf, distribution, Weibull, S B de Johnson. RESUMENEl objetivo de este estudio fue comparar la precisión de las distribuciones de Weibull, Weibull reversa, S B de Johnson, Gamma y Beta, ajustadas por método de los momentos y máxima verosimilitud, para describir distribuciones diamétricas de Pinus taeda, Eucalyptus globulus y Eucalyptus dunnii en Uruguay. Se utilizaron datos de un total de 429 parcelas permanentes de P. taeda, 185 parcelas de E. globulus y 271 parcelas de E. dunnii. Para compararla calidad del ajuste de las funciones de densidad de probabilidad (fdp) se utilizaron la prueba de Kolmogorov-Smirnov y el índice de error de Reynolds. La función S B de Johnson presentó un mejor ajuste en las parcelas de P. taeda, mientras la función Weibull reversa fue más precisa para las especies de Eucalyptus. El método de los momentos resultó con mayor precisión que el método de máxima verosimilitud.Palabras clave: fdp, distribución, Weibull, S B de Johnson. INTRODUCCIÓNConocer el número de árboles correspondiente a cada clase diamétrica de un rodal es de gran utilidad para el planeamiento y manejo de plantaciones, ya que permite estimar la variabilidad entre individuos, los posibles productos a obtener y, por lo tanto, el valor de la plantación. Las funciones de densidad de probabilidad (fdp) son utilizadas en el ámbito forestal para estimar la probabilidad de ocurrencia de un número de individuos y la probable distribución que presentará según sus diámetros, describiendo en términos matemáticos la estructura del rodal (Vallejos et al. 2004).Dentro de las fdp más utilizadas se destacan las funciones de S B de Johnson, Weibull, Weibull reversa, Beta y Gamma, siendo las distribuciones de Weibull y S B de Johnson las más empleadas debido a una mejor calidad de ajuste (Vanclay 1994). Sanquetta et al. (2014) trabajaron con las funciones Gamma, Log Normal, Normal y Weibull (dos y tres parámetros), para describir las distribuciones diamétricas de Acacia mearnsii De Wild.. La función de Weibull es ampliamente utilizada en el ámbito forestal, su versión reversa (Kuru et al. 1992, Xu et al. 1992, en la cual el origen de la curva de distribución diamétrica se cambia del diámetro a la altura del pecho (DAP) mínimo al máxi-mo, presenta como ventajas: a) mayor fac...