Combined effects of tree canopy and root systems on soil hydraulic properties: A field monitoring of a Populus alba

Canopy and roots play crucial roles in regulating soil hydraulic processes, yet their combined effects on soil hydraulic properties remain poorly understood. In this study, the combined effects of canopy interception, transpiration, and root activity on soil hydraulic properties were investigated through field monitoring of a Populus alba. The results showed that the contact angle on the adaxial and abaxial leaf surfaces decreased from 104.3° to 75.1° and from 125.8° to 83.7° during the growth period, respectively, resulting in a significant increase in the canopy interception ratio. During rainfall, canopy interception and root water uptake limited increases in soil water content and decreases in matric suction beneath the canopy, but this effect diminished with increasing rainfall. Following rainfall, transpiration-induced root water uptake accelerated the dissipation of soil water content and the recovery of soil matric suction, exhibiting exponential and logarithmic relationships with leaf area index (LAI), respectively. The influence of canopy and roots on soil water and suction is predominantly observed during the high LAI period from June to October and was primarily within 150 cm soil depth. Although root activity increased soil saturated hydraulic conductivity by improving porosity, canopy interception reduced unsaturated hydraulic conductivity under high soil suction conditions (>60 kPa) during early rainfall. This study highlights the critical role of the combined effects of canopy and roots in regulating soil water content, suction, and permeability, which have important implications for understanding ecosystem hydrological processes and guiding ecological restoration practices.