The predicted increase in hydrometeorological stress in many of the world’s ecosystems necessitates the assessment of plant water exchange indicators in a wide environmental range to reveal the adaptation potential of terrestrial ecosystems under possible climate change scenarios. The aim of this study was to assess the effect of abiotic factors on the water exchange parameters of Scots pine trees (Pinus sylvestris L.) in mid-boreal old-growth forests of Karelia during two growing seasons with contrasting hydrothermal conditions (2023–2024). For this purpose, we analyzed the variability of predawn and midday water potentials of foliated shoots of 170-year-old pine trees in 14 permanent sample plots established in 5 groups of bilberry- and lingonberry-type pine forests formed in different edaphic and hydrological conditions. The effects of deficient and excessive precipitation on water exchange in pine were shown for a gradient of soil hydrothermal conditions. Where precipitation was in deficit, the lowest values of the water potential in predawn (–1.06...–1.22 MPa) and midday hours (–1.46...–1.54 MPa) were observed in groups of pine forests on podzols. At the same time, the daily gradient of the water potential (0.35–0.44 MPa) was found to be similar in the groups of pine forests on podzols and on peat soils. Under abundant precipitation, midday water deficit (–1.21...–1.38 MPa) was the highest in the group of pine forests formed in contrasting environments of a sandy podzol and on peat soils. The seasonal dynamics (May July) exhibited an increase in water deficit in pine trees of all groups of biogeocenoses with a simultaneous increase in the daily water potential gradient in May (0.41–0.64 MPa), June (0.64–0.79 MPa) and July (0.75–1.07 MPa). The increase in the latter was mostly associated with soil temperature (R2 = 0.59, p < 0.001) relative to soil water availability (R2 = 0.12, p < 0.001). The article discusses the adaptation mechanisms for maintaining hydraulic homeostasis in woody plant.