Sap Flow Density of the Prevailing Tree Species in a Hemiboreal Forest under Contrasting Meteorological and Growing Conditions

This study aimed to assess the adaptive capacity of the dominant tree species in Lithuania, namely Scots pine, Norway spruce, and silver birch, to current climate conditions based on their changes in transpiration expressed through the tree sap flow intensity. The species-specific responses were investigated at two typical edaphically different forest sites with water-limited and water-saturated soils. Contrasting events like overflow in 2017 and drought in 2019 provided an opportunity to detect the adaptative capacity of the monitored tree species to these meteorological extremes. Norway spruce trees, due to having both the most intense sap flow at the beginning of the growing season and the longest period of active transpiration, demonstrated the highest annual transpiration rate, regardless of the hydrological regime of the site. Their decreased resilience to subsequent biogenic damage caused by pests due to a significant decrease in sap flow density during intense and prolonged droughts may reduce their importance in Lithuanian forestry. Silver birch trees, which demonstrated a reduction in sap flow after a drought following the untimely drop of their leaves and the end of active vegetation, even at the end of a prolonged warm period, can be seen to not have appropriate adaptations to current climate conditions. Scots pine trees are the best adapted to mitigating the recent threats of climate change.