## 1 Introduction  
Using non-wood forest products plays a significant role in sustainable development, especially in the context of regional development [1, 2, 3, 4, 5, 6]. Searching for additional (non-timber) sources of income for forest management is one of the priorities listed by the European Union in the New Forest Strategy and an action indicated by social certification systems for sustainable forest management [7, 8]. It is also important to establish cooperation with potential business partners from the food and pharmaceutical industries; this is also a postulate in numerous publications related to forest management [9]. In the above context,birch sap deserves special attention. In European conditions, it is obtained mainly from silver birch (_Betula pendula_ Roth). The possibilities of its actual and potential use in practice, including in the food and cosme industries, are vast [3, 10, 11, 12], and the profit achieved from its harvest may exceed the profit from the sale of birch timber in the same area [13]. Research on the utility value of birch sap and the influence of different factors on its leakage intensity and quality has been carried out in many research centers [3]. [PERSON] [14] stated that the forest site influences the intensity of sap leakage and the quality and health condition of the tree stand; he observed the highest sap production in healthy, 30-40-year-old birch stands. [PERSON] [15] observed that in moist habitats, the leakage of birch sap is more abundant, while in drier sites, the amount of sap is smaller--but with higher sugar content. According to [PERSON] [14], sap production depends on the average thickness of the trees. However, [PERSON] [16], in a 65-year-old tree stand, did not confirm the influence of tree thickness on the intensity of sap flow, pointing to its relationship with the height of the trees. The exposure of the tree stand was also analyzed: according to [PERSON] [14], the highest production of birch sap was recorded on gentle northern slopes and the lowest on southern ones. However, [PERSON] [17] and [PERSON] et al. [18] state that more sap can be obtained from trees growing on the edge of a forest or open space than from those growing inside a tree stand. [PERSON] and [PERSON] [19] also note that not only does the microclimate have an impact but also the sunlight reaching the tree trunks. The period during which birch sap can be obtained varies. [PERSON] [14] indicates that harvesting should begin in early spring, during the rapid melting of snow, cold nights, and warm days until the leaves develop. The leakage time is approximately 16-24 days, sometimes longer, and is influenced by climatic conditions. [PERSON] [15] states that the sap leakage season of birch begins in mid-April and, in unfavorable climatic conditions, at the beginning of May. Regardless of the location, it lasts 17-24 days--sometimes up to 29 days. The efficiency of sap flow is the highest in the initial period of its harvest. About 3 weeks after sap leakage starts, the yield decreases radically [16]. The most significant leak of birch sap occurs 2-3 weeks after the sap begins to flow in the tree trunk [17]. In the first period of sap obtaining, its amount increases with the temperature, while in the second period, the amount of sap decreases with its increase [14]. 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 sap yield, while in the last third, it has a negative effect. [PERSON] and [PERSON] [20] and [PERSON] [14] report that the intensity of birch sap leakage varies throughout the day. [PERSON] et al. [9] found that the volume of sap obtained was not significantly influenced by the temperature on the day of the leak or the night immediately preceding the leak. However, they observed a relationship between the intensity of the leak and the temperature measured several dozen hours earlier. Moreover, they found an almost complete inhibition of birch sap leakage after a night frost.