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in the case of thinner and, subsequently, shorter trees. Regarding the models with two variables, the Spurr II model is the most favorable. In four diameter classes, the deviation is greater than 3% (Max=-7.70%), that is, the deviations are not random (p < 0.05) in five diameter classes. The results are similar for the Schumacher-Hall model, in three diameter classes the deviation is greater than 3% (Max=-8.28%), that is, the deviations are not random (p < 0.05) in six diameter classes.
A study on the volume functions of spruce trees in Switzerland showed that, by creating regional volume functions with two variables (d1.3 and h), the gain in precision is negligible, so the regional functions were not derived, but the aforementioned general functions with three inputs (Kaufmann 2001). It is to be expected that even in the conditions of Bosnia and Herzegovina, given the large variations in habitat and stand conditions even in small areas, the creation of regional volume functions with two variables (d1.3 and h) would not result in great accuracy.
The development of regression equations (volume tables) for the estimation of the volume of sessile oak trees in the territory of Bosnia and Herzegovina, which is characterized by the specific habitat conditions and the method of management of the sessile oak forests, is necessary because in practice, for the estimation of the volume, tariff tables are used, which are made on the basis of two-entry volume tables for oak from Germany by Schwappach from 1905. A particularly significant fact for the estimation of the volume of trees in the observed area is the atypical way of managing the sessile oak forests. Selection cutting is applied and, as a consequence, uneven-aged and dominantly mixed stands are formed. The tested models with two variables that have a wide application for equalizing the volume of merchantable wood of a tree depending on the diameter at breast height and height of the tree, such as Spurr II (MAE = 0.1647, RMSE = 0.2796 and R2 = 0.9732) and Schumacher-Hall (MAE = 0.1628, RMSE = 0.2776 and R2 = 0.9736) functions, give satisfactory accuracy, considering the fact that these are general models intended for estimating the volume of trees in the entire area of Bosnia and Herzegovina. By introducing a third variable into the model, diameter at seven meters height, the accuracy of the tree volume estimation increases significantly (MAE = 0.1081, RMSE = 0.1932 and R2 = 0.9872). Due to the measurement of the diameter at a height of seven meters, the volume of work in the field to collect the necessary data increases significantly, so this model is not suitable for use in forest inventories.
Acknowledgements – Zahvalnice
We would like to express our gratitude to numerous collaborators of Public Forest Enterprise “Forests of the Republic of Srpska” who participated in the data collection.
Funding – Financiranje
This study was financed from special purpose funds for forests of the Ministry of Agriculture, Forestry, and Water Management in the Government of the Republic of Srpska. The project is entitled „Creation of volume and assortment tables for sessile oak in the Republic of Srpska“.
Conflict of interest – Sukob interesa
The authors declare that they have no competing interests
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