Modeling compatible stem profile of scots pine stands in Ardahan, Turkey using nonlinear mixed-effects models

The objective of this study is to develop segmented polynomial taper equations which enable detailed volume estimations using the nonlinear mixed-effects modeling approach for pure Scots pine stands in Ardahan region of Turkey. For this purpose, the data obtained from 137 felled sample trees were used. The Jiang et al.'s stem profile model produced the best prediction results for the species. The adjusted coefficient of determination, Standart Error of Estimate, Bias, and Mean Absolute Error values of the model were found as 98.3 %, 1.955 cm, 0.043 cm, 1.300 cm, respectively. Based on AIC, BIC and -2LnL criteria, the model including random-effects in two parameters (b1 ve b3) were the best for the tree species. This mixed-effects model with two random parameters showed homogeneous residual variance and autocorrelation was almost removed. In addition, among 20 different calibration choices, the best results were obtained with the choice involving the measurement of two diameters such as the diameters at breast high (d1.30) and d5.30. It is also proposed to construct both single and double entry tree volume equations. These selected models accounted for 96.9 and 99.2 % of the total variance in single and double entry tree volume equations, respectively. Tree volume equations were tested with independent data set within the border of Ardahan Forest District Directorates, and concluded that these equations can be used for these stands at the 0.05 significant level.