The Increase of Leaf Water Potential and Whole-Tree Hydraulic Conductance Promotes Canopy Conductance and Transpiration of Pinus Tabulaeformis During Soil Droughts

Transpiration of urban Pinus tabulaeformis was strongly controlled by leaf stomata and its positive response to leaf water potential and whole-tree hydraulic conductance during soil droughts. Many studies have elucidated the response of urban tree transpiration to environmental factors, but little is known about how soil water alters canopy conductance (Gc), leaf water potential, the whole-tree hydraulic conductance (K), and their influences on canopy transpiration (Ec). In this study, sap flow, leaf water potential, and environmental factors were measured in a 60-year-old Pinus tabulaeformis plantation in a semi-arid urban environment of northern China. We found Ec, Gc, and K (0.11 ± 0.01 mm d–1, 0.13 ± 0.01 mm s–1, and 0.007 ± 0.001 kgm–2 h–1 MPa–1, respectively) under soil water-stressed conditions were significantly lower than that (0.32 ± 0.01 mm d–1, 0.34 ± 0.01 mm s–1, and 0.042 ± 0.004 kg m–2 h–1 MPa–1, respectively) under non-water-stressed conditions (p < 0.05). Leaf water potential at predawn and midday (Ψm) and the hydrodynamic water potential gradient from roots to shoots was relatively constant, with averages of –1.25 ± 0.26 MPa, –1.87 ± 0.19 MPa, and 0.62 ± 0.20 MPa, respectively. Gc was negatively related to vapor pressure deficit (VPD) but was positively correlated to soil volume water content (VWC) and wind speed when soil water was relatively sufficient. As soil drought progressed, Gc was more impacted by VWC and was negatively associated with air temperature to reduce water loss, but it was positively related to Ψm and K. These findings indicated that urban P. tabulaeformis could control transpiration by strict stomatal regulation and maintain a constant water potential gradient to avoid a hydraulic breakdown during soil droughts.