## 4 Discussion  
The average daily birch sap leakage efficiency in individual sites varied. It amounted to 3.05-11.50 dm\({}^{3}\)/24 h for mixed coniferous forest, 0.78-12.43 dm\({}^{3}\)/24 h for mixed broadleaved forest, and 1.24-8.99 dm\({}^{3}\)/24 h for broadleaved forest. According to [PERSON] [14], the intensity of sap leakage is influenced by the site and the quality and health of the stand (expressed by the crown size and the trees' average thickness). He also defines a kind of cause-and-effect chain: the more fertile the soil and the better the quality  
Figure 3: Prediction of daily sap volume collected per tree in relation to total tree height (**A**), forest site (**B**), and the sap collection phase (**C**). \begin{table}
\begin{tabular}{l c c c} \hline \hline  & & \multicolumn{2}{c}{**Sap Volume**} \\
**Predictors** & **Estimates** & **CI** & \(\mathbf{p}\) \\ \hline (Intercept) & \(-\)3.36 & \(-\)19.17-12.45 & 0.677 \\ Height & 0.17 & \(-\)0.35-0.69 & 0.521 \\ Site [mixed broadleaved forest] & 1.22 & \(-\)1.96–4.40 & 0.453 \\ Site [mixed coniferous forest] & 2.50 & \(-\)0.90–5.90 & 0.149 \\ Sap collection phase [phase 2] & 3.86 & 3.38–4.34 & **<0.001** \\ Sap collection phase [phase 3] & 1.67 & 1.09–2.25 & **<0.001** \\  & **Random Effects** & & \\ \(\sigma^{2}\) & & 7.54 & \\ \(\tau_{00\,\mathrm{Tree.Id}}\) & & 9.48 & \\ ICC & & 0.56 & \\ N\({}_{\mathrm{Tree.Id}}\) & & 27 & \\ Observations & & 783 & \\ Marginal R\({}^{2}\)/conditional R\({}^{2}\) & & 0.162/0.629 & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Parameters of the mixed-effects model in the prediction of the daily volume of birch sap obtained from one tree. of the trees, the larger the crown, root system, and average thickness of the tree, which results in more intense sap production. The authors' research results do not confirm this thesis. Based on the data (Table 1), we determined the average daily birch sap leakage efficiency from all 27 examined trees (all three examined sites) to be 5.16 dm\({}^{3}\)/24 h. Broken down by site, the values are as follows: mixed coniferous forest--6.09 dm\({}^{3}\)/24 h; mixed broadleaved forest--5.08 dm\({}^{3}\)/24 h; broadleaved forest--4.30 dm\({}^{3}\)/24 h. [PERSON] [14] also states that the amount of leaking sap is also influenced by water conditions in the soil, the age of trees, and their exposure. Moreover, on drier soils, the amount of sap is smaller, while the sugar content increases. This is also confirmed by [PERSON] [15]. The amount of sap that can be obtained during the entire season is also difficult to determine unequivocally. The average daily sap leakage efficiency is presented in Table 2. During the 29 days of collection in the mixed coniferous forest site, an average of 176.76 dm\({}^{3}\) of sap was obtained from one tree; in the mixed broadleaved forest, an average of 147.29 dm\({}^{3}\) of sap was obtained, and in the broadleaved forest, an average of 124.64 dm\({}^{3}\) of sap was obtained. Taking into account the above data and the high variability of birch sap leakage from individual trees (Table 1), it can be assumed that during a 29-day collection of sap from one tree, approximately 150 dm\({}^{3}\) of birch sap can be obtained. This is a larger amount than that provided by many authors in the literature. [PERSON] [33], in his work, obtained a result of 15-20 dm\({}^{3}\) from one tree during the season, [PERSON] [14] obtained 78 dm\({}^{3}\), [PERSON] and [PERSON] [20] obtained 80 dm\({}^{3}\), and [PERSON] [34] obtained 120 dm\({}^{3}\). Many factors can influence this diversity, including the biometric features of trees, forest sites, the exposure of trees, water conditions, the length of the sap collection season, and the number of holes made and their location in relation to the cardinal directions. This study did not confirm the statistical significance of the influence of factors such as diameter at breast height, total height, crown base, crown length, relative crown length, and slenderness on the efficiency of birch sap leakage.