Hydraulic regulation and bidirectional sap flow in a seasonally dry tropical forest

The soil-plant-atmosphere continuum (SPAC) describes water movement through plants from compartments of high to low water potential. In some cases, however, reverse sap flow may occur. Studies on reverse sap flow are scarce in seasonally dry forests, such as the Caatinga biome in northeastern Brazil. This study aimed to analyze sap flow dynamics in a representative Caatinga species, Caesalpinia pyramidalis Tul., using in situ measurements during both rainy and dry seasons, to investigate the occurrence of reverse flow. Sap flow was measured using the heat ratio method (HRM) with thermocouple sensors and a heating probe. Results showed that C. pyramidalis exhibits reverse sap flow at night and predawn throughout the year. Peak sap flow occurred during transitional seasons, associated with high atmospheric demand and moderate soil moisture. Sap flow was 113% higher during the rainy season compared to the dry season. Notably, nocturnal sap flow was more significant during drier months, even in the absence of leaves, suggesting an adaptive water use regulation. • Bidirectional sap flow observed in Caatinga tree species under natural conditions. • Reverse nocturnal flow occurs even during the leafless dry season. • Transitional periods show the highest sap flow rates and hydraulic activity. • C. pyramidalis adjusts water use to balance soil moisture and atmospheric demand. • Results suggest an adaptive hydraulic regulation enhancing drought resilience.