Vapor pressure deficit dominates sap flow variability across forest biomes

• VPD is the key causal driver of sap flow in EBF, ENF, and MF ecosystems. • SWC impacts sap flow with delays; VPD shows both instantaneous and lagged effects. • Synergistic effects of SWC and VPD strongly influence sap flow dynamics. Sap flow (SAPFlow) is a critical process governing plant water use, influencing ecosystem hydrology, and regulating plant physiology through the exchange of water between the biosphere and atmosphere. Understanding the causal drivers of SAPFlow is essential for improving our ability to predict plant responses to environmental stressors, including drought and climate change. However, the causal relationships between environmental drivers and SAPFlow across diverse biomes remain poorly understood, particularly on a global scale. The present study uses 15 sites from the global SAPFLUXNET dataset to examine SAPFlow dynamics in three forest biomes: evergreen broadleaf forests (EBF), evergreen needleleaf forests (ENF), and mixed forests (MF). Using information theory-based process networks, we identify key causal drivers of SAPFlow. Our findings reveal that vapor pressure deficit (VPD) and soil water content (SWC) form a coupled system with SAPFlow, mediated by land–atmosphere feedback between soil and atmospheric aridity. VPD emerges as a dominant causal driver across all forest types, highlighting its pivotal role in the soil–plant-atmosphere continuum. Wavelet analysis further highlights distinct temporal interaction scales, with VPD exerting immediate influence on SAPFlow and SWC showing delayed effects. Our findings provide critical insights into plant-water interactions, with significant implications for refining land surface models and improving the accuracy of ecosystem response predictions under climate change scenarios.