Abstract
Taper models are one of several necessary tools in modern forest inventory, giving information on diameter at any point along the tree stem and this information can also be used to estimate stem volume. In this study, we used nonlinear mixed-effects (NLME) modeling approach to minimize existing statistical problems in constructing taper equations. A segmented taper model of Max and Burkhart (1976) was fitted using this approach to consider for within- and between-tree variation in brutian pine (Pinus brutia Ten.) stem taper. Based on evaluation statistics, the model including random-effects parameters β 1, β 3 and β 4 were found to be the best. Inclusion of random parameters were not completely eliminated heterogenous variance and autocorrelation in residuals. Incorporating variance function and a continuous autoregressive error structure (CAR(1)) to NLME Max and Burkhart model removed the heteroscedasticity and autocorrelation in residuals. Upper stem diameters were used to localized stem taper model to individual tree. For this, two different measurement scenarios were evaluated as one and two upper stem diameter measurements. Inclusion of random parameters were improved the predictive capability of taper model in particularly the middle and lower sections of stem based on upper stem diameter measurements. The calibration using upper stem diameter measurements can improve the tree-level accuracy of stem taper model is therefore recommended.
Highlights
Brutian pine (Pinus brutia Ten.) is a one of the commercially tree species in the southern Turkey
The data were collected from natural brutian pine from Bucak Forest Enterprise in Mediterranean Region of Turkey which represent the northern portion of brutian pine natural range. 280 destructively sampled brutian pine trees used in this study
The mean response presented the lowest root mean square error (RMSE) in the upper section of the bole at relative heights between 80-100%. These results showed that two stem diameter measurements are required to accurately calibrate the taper equation for brutian pine
Summary
Brutian pine (Pinus brutia Ten.) is a one of the commercially tree species in the southern Turkey. This species has a significant share in the growing stock of this region, occupying an area of about 5.8 million ha. We need reliable information regarding growth and yield of the species to sustainable management of brutian pine forest, but this information is currently lacking in Turkey. Tree volume estimation is one of important tools of forest growth and yield modeling. Total volume estimates are important for evaluating growth and yield, net primary production, stand dynamics, above ground biomass, and sustainable management of timber resources. Ground biomass estimations are used in carbon sequestration and climate change models and fuel production in place of traditional fossil fuel sources (Pancoast 2018)
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