Seasonal and diurnal patterns of soil water potential in the rhizosphere of blue oaks: evidence for hydraulic lift

In a 3-year study, seasonal and daily soil water fluctuations in a California blue oak woodland were investigated by measuring soil water potential (Psi (s)) at hourly intervals. Soil water potential remained relatively high well into the annual summer drought, with values above -0.5 MPa until June even in a dry year. As drought progressed, (s) (at 25, 50, 75, and 100 cm depth) decreased to less than -3 MPa, providing evidence for continued blue oak root activity throughout the summer. We observed diurnal Psi (s), fluctuations (gradual increase at night and rapid decrease during daytime) characteristic of hydraulic lift, a process by which plant roots redistribute water from wet to dry soil layers. These diurnal fluctuations were observed at all four soil depths and began to appear when Psi (s) reached approximately -0.3 MPa. When Psi (s) reached approximately -3 MPa, fluctuations became "offset" from those typical of hydraulic lift. These offset fluctuations (apparent at low water potentials when temperature fluctuations were large) closely followed diurnal fluctuations in soil temperature. We propose that these offset patterns resulted from a combination of hydraulic lift cessation and an over-correction for temperature in the model used to calculate Psi (s) from raw sensor data. The appearance and disappearance of hydraulic lift fluctuations seemed to depend on Psi (s). While soil temperatures and dates at which hydraulic lift appeared (and disappeared) were significantly different between wet and dry years, Psi (s) values associated with hydraulic lift appearance were not significantly different. Hydraulic lift occurred too late in summer to benefit annual forage grasses. However, water released by blue oak trees at night could slow the rate of soil water depletion and extend blue oaks' growing season.