Night-time transpiration can decrease hydraulic redistribution


A. R. Howard. Fax: +1 503 838 8072; e-mail:


C3 plants dominate many landscapes and are critically important for ecosystem water cycling. At night, plant water losses can include transpiration (Enight) from the canopy and hydraulic redistribution (HR) from roots. We tested whether Enight limits the magnitude of HR in a greenhouse study using Artemisia tridentata, Helianthus anomalus and Quercus laevis. Plants were grown with their roots split between two compartments. HR was initiated by briefly withholding all water, followed by watering only one rooting compartment. Under study conditions, all species showed substantial Enight and HR (highest minus lowest soil water potential [Ψs] during a specified diel period). Suppressing Enight by canopy bagging increased HR during the nightly bagging period (HRN) for A. tridentata and H. anomalus by 73 and 33% respectively, but did not affect HRN by Q. laevis. Total daily HR (HRT) was positively correlated with the Ψs gradient between the rooting compartments, which was correlated with light and/or atmospheric vapour pressure deficit (VPDa) the prior day. For A. tridentata, HRT was negatively correlated with night-time VPDa. Ecological implications of the impact of Enight on HR may include decreased plant productivity during dry seasons, altered ecosystem water flux patterns and reduced nutrient cycling in drying soils.