Contribution of a second diameter in estimating individual volume in young stands of Tectona grandis L.f.=Contribuição de um segundo diâmetro na estimação de volume individual de plantios jovens de Tectona grandis L.f

Estimating the individual volume of trees with precision and without bias is a prerequisite of a quality forest inventory. The most common methods are based on application of the average form factor, or on single or double entry equations. This study proposes models with two diameters as independent variables to estimate the commercial volume of Tectona grandis, with a view to improving precision and reducing bias. Thirty-five trees from commercial plantation of 4-5 years old, in the town of Redenção, in the State of Pará, were rigorously cubed using the Smalian method of scaling. Diameters at different heights were correlated with actual volume. Three new volumetric models based on two diameters were proposed, without the addition of tree height as a variable. These models were compared with others, which are traditionally used in forest inventories. The model with the best fit uses the diameter at breast height (dap) and at a height of 4.0 m (d4.0) as independent variables. The operational difficulties of using d4.0 are discussed. Models that use the dap and the diameter at 2.0 m (d2.0) were considered the most satisfactory, as they are more accurate and practical than classic models, and operationally more feasible than the model using d4,0. The conclusion was that there are other diameters that are more correlated with volume than dap, and that the introduction of a second diameter improves volume estimates, even without the inclusion of height as a variable.