Adaptive Capacity of Scots Pine Trees to Meteorological Extremes in Highly Oligotrophic Soil in Hemi-Boreal Forest

Hemi-boreal forests are subjected to increasing pressures from hotter and longer droughts followed by heatwaves over vegetation periods and warmer winters related to climate warming. Via physiological adjustments, they show different degrees of adaptation, resistance, and tolerance to their extremes, which can push forests beyond their resilience thresholds, especially at highly arid growing sites like Lithuania’s Curonian Spit which characterized by wind-made sandy dune. This study addresses the threshold of the adaptive capacity of Scots pine (Pinus sylvestris L.) as the main species in hemi-boreal forest, to these meteorological extremes, what is critical for pine forest management at different growing conditions. Using stem sap flow and dendrometer measurements (2018–2024) at hourly scale, we assessed sap flow density, stem volume increment, and water use efficiency (WUE) over different period to detect key period most significant affecting their annual values. Multiple linear regression revealed that, during the vegetation period, elevated temperatures reduced WUE, whereas higher mean temperatures during the dormant period, combined with increased precipitation in the growing season, significantly enhanced stem volume increment and WUE in general, explaining up to 75% of their variation. When winter meteorological effects were excluded, the explained variation in stem volume increment and WUE decreased by nearly twofold, highlighting the critical role of dormant-season conditions in Scots pine resilience. The hypothesis about the heightened susceptibility of Scots pine to summer drought conditions after warm winters was rejected. These findings provide tools for risk assessment and environmental compliance control to evaluate Scots pine capacity to adapt to and mitigate new threats of climate change.