Stratification criteria to fit taper functions on pine boles

Abstract

This paper aimed to evaluate the accuracy of taper functions fitted with and without stratification on Pinus sp. trees. Three strata were allocated using diameter at 1.3 m above the ground, artificial form factor and Schiffel´s form quotient as stratification criteria. Schöepfer, Kozak et al. Hradetzky, Garcia et al., Ormerod and Demaerchalk taper functions were tested and the one which best fitted to the total population was selected by statistics standard error of estimate in percentage (s_yx%), adjusted coefficient of determination (R^²_aj.) and residual scatterplots. After this, the selected function was fitted with data stratified and the gain of accuracy was evaluated by two statistical methods. As results, Hradetzky´s function adjusted better when compared to the others. Equations obtained for the total population and for strata are statistically different. Stratification by form factor and by form quotient provide considerable reduction of errors, reaching up to 50%, however it was not observed expressive reduction for stratification by diameter at 1.3 above the ground. So, the stratification by form factor is recommended for fitting taper functions to boles of Pinus sp.