## 4 Discussion
[PERSON] and [PERSON] [36] provide an approximate date of the beginning of March, and the leak duration is 10-14 days. The authors also add that this time depends on the weather and climatic conditions. [PERSON] et al. [9] specify the time of sap collection as 2-3 weeks. In this paper, we proposed to divide the birch sap leakage period into three stages: initial, main, and final. No scientific sources were found that would divide the sap leakage period into characteristic stages. However, it is important that other authors also note the variability of the amount of birch sap leaking during collection and the impact of climatic factors on this variability. According to [PERSON] [14], in the first period of sap collection, its amount increases with temperature, while in the second, with its increase, the amount of sap decreases. The efficiency of sap leakage is the highest in the initial period of its collection [16], and about 3 weeks after sap movement, the flow even decreases several times. The highest birch sap flow occurs 2-3 weeks after the start of sap flow in the tree trunk [17]. This is also confirmed by [PERSON] [15], who claims that during the first two-thirds of the harvesting season, the increase in temperature has a positive effect on the sap yield, while in the last third, it has a negative effect. [PERSON] and [PERSON] [20] state that the trees produce the most sap between 12 and 6 p.m. and the least between 0 and 6 a.m. [PERSON] [14] reported that the leakage during the day was about 25% higher than at night. He also added that in early March, the sap production increased, except for two days when the temperature at night reached \(-8\)\({}^{\circ}\)C and \(-0.9\)\({}^{\circ}\)C during the day, and the leakage was interrupted. The available literature data mostly come from 50-60 years ago, when the industry related to birch sap extraction was more widely developed in Central and Eastern Europe. Since then, not only have economic changes occurred but also climatic and environmental changes. Aspects such as the length of the winter period, average air temperatures in winter and spring, and groundwater levels are just examples of variables that can affect the physiology of trees and, consequently, the yield of birch sap. In the present work, a model for predicting daily birch sap collection was proposed based on a selected tree feature (total height), a stand feature (forest site), and a time-related feature (sap collection phase). The model indicates which features are related to the amount of sap collected, which allows for selecting the site, tree, and time when sap collection will be most efficient. The presented model can be used to analyze the profitability of birch sap collection. Studies aimed at determining the amount and modeling of factors influencing the amount of birch sap collected were conducted in Finland on an extensive sample, which was possible thanks to the involvement of a citizen scientist [37]. This model concerned the total amount of sap collected from one tree during the season, and the explanatory variables included the DBH and tree height. The seasonal sap yield of individual birch trees studied in Finland was, on average, 53.4 dm\({}^{3}\) in 2019 and 37.3 dm\({}^{3}\) in 2020. These values are several times lower than those of the birches examined in this study, where the average values ranged from 124.6 dm\({}^{3}\) in the broadleaved forest site to 176.8 dm\({}^{3}\) in the mixed coniferous forest site. The results obtained in this study allow us to state that in the case of large-scale birch sap harvesting, the selection of the forest site type has little practical significance, but it is important to use the high sap yield in the main leakage phase. However, a number of factors that may affect the quality of the obtained sap and, consequently, the safety of its consumption should certainly be taken into account [38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48].