RATIONAL VOLUMETRIC MODELS: TESTED IN FOREST STANDS OF Acacia mearnsii DE WILD IN SOUTHERN BRAZIL

ABSTRACT Tree volumes can alternatively be obtained by ratio estimates, instead of applying linear regression modeling. An interesting question was considered by the authors when comparing the precision of tree volumes obtained by these two statistical approaches, that allowed us to formulate a new hypothesis to conduct this study: “Is it possible to obtain better volume estimates if the form factor is associated only to the exponent of the independent variable that presents higher correlation with the stem volume?”. The research was conducted in commercial stands of black wattle (Acacia mearnsii De Wild) in the municipalities of Cristal, Encruzilhada do Sul and Piratini to answer this question. Forest stands were sampled in a sequence of ages after planting, varying from 1 to 14 years. In each forest stand, trees were felled, and the following characteristics were evaluated: diameter at breast height, total height, and stem scaling. The three age groups were specified according to the behavior of the form factor values. It was proposed three models to be solved by ratio estimates. A comparison of the proposed models with the Schumacher-Hall model, fitted by regression analysis, was done. The model in which the form factor was associated to the exponent of the diameter variable produced the best results, owing to a powerful correlation resulted of this confounding with volume as a dependent variable. Ratio estimates resulted in more accurate tree volumes predominantly, when compared with those obtained by the traditional Schumacher-Hall model fitted through regression analysis. Therefore, we recommend the application of model 2 fitted by ratio estimate.