## 4 Discussion
However, the authors of other publications indicate the influence of certain factors on the amount of sap obtained. It is worth recalling the cause-effect sequence described by [PERSON] [14], mentioned earlier: the more fertile the soil and the better the tree quality, the larger the crown, root system, and average thickness of the tree, which results in greater sap production. He also describes how during the day, birches with a large crown yield, on average, more sap (1.932 L) than trees with a medium crown (1.023 L) and trees with a small crown (0.641 L). He also adds that even with a smaller total amount of sap obtained, caused by an unsuitable site and lower rainfall, the size of the crown affects the production of sap in the tree. [PERSON] [14] showed, in his studies, from birches with a breast height of up to 15 cm, the daily leakage was 2.34 dm\({}^{3}\); from 16 to 20 cm, it was 2.52 dm\({}^{3}\); from 21 to 25 cm, it was 3.14 dm\({}^{3}\); and from 26 to 30 cm, it was3.55 dm\({}^{3}\). In a study on the efficiency of birch sap flow, depending on the breast height of the tree [16], no statistically significant differences in the efficiency of sap flow were found between individual DBH classes (8-26 cm, 26-30 cm, and 32-42 cm; stand age, 65 years). The author indicates that efficiency is a very variable feature, and height is the best feature based on which we can determine correlations with efficiency. Another aspect worth noting is the difference in the efficiency of sap flow between trees with very similar DBH values and those growing several or several dozen meters apart (in the same habitat with the same microclimate). The experiences of various authors indicate that these differences can even be several times greater. [PERSON] [16] reported that 5.8 L of sap was obtained from birch with a DBH of 26.9 cm, and only 1.9 dm\({}^{3}\) was obtained from a neighboring tree with a DBH of 28.2 cm per day. The authors noticed similar relationships in their studies. In the mixed coniferous forest site, tree THN3 was characterized by an average daily leakage of 10.88 dm\({}^{3}\)/24 h, while the trees with the largest DBH values(TCK1, TCK2, and TCK3) did not exceed a value of 4 dm\({}^{3}\)/24 h. Similar relationships can be observed in the other two forest sites (mixed broadleaved forest and broadleaved forest). In this study, the age of the trees was similar, averaging about 80 years. According to [PERSON] [14], healthy, 30-40-year-old birch stands have the highest sap production. Studies conducted in 2017 on the efficiency of birch sap flow depending on the age class of the stand suggest that there are no statistically significant differences in the efficiency of sap flow between stands that significantly differ in age (34 and 84 years), while the daily sap yield is a feature of high variability [35]. The start date of birch sap leakage may be variable but mainly depends on the season and air temperature. [PERSON] [14] conducted his research from 3 March to 20 April. He states that the season affects sap production, and the most favorable time is early spring (a period of rapid snow melting with cool nights and warm days). Sap leakage ends after the leaves unfold, and sap production in the tree starts again in autumn after the leaves fall, but the intensity is much lower. He defines the early spring leakage period as 16-24 days (with the possibility of extending this period depending on the impact of climatic conditions). According to [PERSON] [14], the optimal period for sap collection is 18-20 days. [PERSON] [15] states that the birch sap season begins in mid-April and in unfavorable climatic conditions in the first days of May. The collection period (regardless of the location) lasts 17-24 days, sometimes even up to 29 days. [PERSON] and [PERSON] [19] state that the commencement of sap collection depends on climatic and atmospheric conditions, and in Poland, sap can only be collected in early spring, before the leaves develop, i.e., in the period from mid-March to the end of April.