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ŠUMARSKI LIST 9-10/2023 str. 45 <-- 45 --> PDF |

height as a third variable does not increase the accuracy of the tree volume estimation. For the usability analysis and comparison of regression models, in addition to the standard criteria shown in Table 5, the deviations estimated from the real volumes of trees by diameter classes were calculated (Table 7) and the significance of the differences was tested using a t-test of pairs between the real and estimated volumes of merchantable trees, also by diameter classes (Table 8). Based on the obtained results, with the applied regression models, larger deviations and statistically significant differences of real and estimated values are observed for thinner trees, which can be explained by the determined coefficients of volume variation by diameter and height classes shown in Figure 2. As is the case with the previously analyzed criteria, the Swiss NFI model with three variables stands out. According to Kaufmann (2001), the volumes of individual trees can be more precisely estimated with the aforementioned three variables (d _{1.3}, h and d_{7}), because the differences in the shape of the tree can be easier distinguished with the measurement of the upper diameter. Statistically significant deviations occur only in the trees thinner than 25 cm. Concerning the specified model, the maximum deviation of the estimated values from the real ones is in the diameter degree of 17.5 cm, which makes 3.20%. In most diameter classes, the deviation is less than 1%. It is to be expected that the influence of the diameter at 7 m height on the accuracy of determining the volume of the tree is less pronounced |