Soil water, plant functional traits and their interaction jointly shaped sap flow density

Plant water use strategies varied profoundly among different species in response to soil moisture gradients. Understanding the patterns of plant water use is critical for future prediction of ecosystem hydrology under climate change. In this study, we compiled a comprehensive database of sap flow density (SFD) measurements across diverse climates, encompassing 101 tree species from 99 sites, and investigated how plant functional traits and environmental variables regulated SFD. The results showed that daytime mean SFD varied substantially among species, ranging from 13.46 (Cunninghamia lanceolata) to 1997.35 g cm⁻² day⁻¹ (Dipterocarpus alatus), with an average of 267.72 g cm⁻² day⁻¹. SFD was higher in angiosperms (mean = 318.47 ± 21.16 g cm⁻² day⁻¹, n = 168) than in gymnosperms (mean = 184.95 ± 10.15 g cm⁻² day⁻¹, n = 103). This difference was significant under high soil moisture, but none was observed under low soil moisture, reflecting a fundamental trade-off between hydraulic efficiency and drought tolerance. In addition, SFD increased with soil moisture in angiosperms, but not in gymnosperms. Plant functional traits explained 41.9 % and 29.3 % of variation in SFD in angiosperms and gymnosperms, respectively. However, the trait-SFD relationships varied remarkably between angiosperms and gymnosperms. SFD of the angiosperms was regulated by the leaf area, while that of the gymnosperms was jointly regulated by stem wood density, stomatal conductance, and leaf dry mass content. Notably, soil moisture amplified the trait-SFD relationships in angiosperms, emphasizing the role of soil moisture in shaping trait expression and tree water use. We concluded that angiosperms tended to maximize the utilization of water resources, while gymnosperms may be more focused on adapting to relatively drier environments.