A tree crown ratio prediction equation for eucalypt plantations

Based on a data set from spacing trials and permanent plots of Eucalyptus g lobulus Labill., several nonlinear equations for crown ratio prediction (ba sed on exponential, logistic, Richards and Weibull functions) were tested. The total data set was used to fit and select the equations. The equations were evaluated in terms of measures of fit and prediction ability: adjusted -R-2, residual mean square, sum of PRESS residuals and sum of absolute PRES S residuals. The normality of the studentized residuals was analyzed using normal QQ plots. The presence of heteroscedasticity associated with the err or term was checked by plotting the studentized residuals against the predi cted values. The significance of the estimated parameters was verified. Mod el error was characterized in terms of bias and precision. The Richards fun ction was selected. This equation is age and density dependent, reflecting the importance of intertree competition; an initial tree dimension and a me asure of stand productivity were also required as explanatory variables.