Development of compatible taper and stem volume equations for caucasian fir stands in Şavşat region by using mixed-effect modeling techniques

In this study, segmented polynomial taper and stem volume equations have been developed by using non-linear mixed-effect modeling techniques in pure Caucasian Fir stands that spread to Artvin Şavşat region. Besides that single and double entry tree volume equations were developed for practical use in the same region. For this purpose, the measurement data of 240 felled Caucasian Fir sample trees were used. 7 different compatible taper equations tested after regulation of data. The Jiang et al.'s stem profile model produced the best prediction results. The adjusted coefficient of determination (R_(adj.)^2) 0.981, Standart Error of Estimate (SEE) 1.526, Bias (E ̅) 0.233 cm and Mean Absolute Error (|E̅|) 0,996 cm of the model were found for Caucasian fir trees. Considering the AIC, BIC and -2LnL error criteria, it was seen that the constant effect parameter b3 of the equation of Jiang et al. Calibration results have been investigated using different extra diameter values for single-parameter randomized 12 calibration options. The best estimation result among the options was obtained from the calibration option for three diameter measurements at the bottom. At the last stage, the coefficients of the single and double entry tree volume equations were found to be 0.977 and 0.993, and the standard error values of the estimation were 0.171 and 0.07. As a result of the statistical tests, it has been determined that the models produced within the scope of the study are suitable for the Caucasian fir stands in Şavşat and can be used with confidence.