15.1.4 Hydraulic lift

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Root are generally organs of water uptake but can sometimes recycle soil water from deep layers to surface layers. Root length density commonly declines exponentially with depth. Most roots are restricted to upper layers where nutrient resources are generally concentrated and soil structure is conducive to root proliferation.

However, in arid zones the upper soil profile is generally dry and soil water potential is low. Water may then prove to be more readily available at depth. In a northern hemisphere sagebrush plant (Artemisia tridentata), a number of roots pen-etrate to deeper moist soil, absorbing water and recycling it to upper layers at night-time. Soil adjacent to upper roots then acts as a short-term storage for water drawn from lower layers. During daytime, water released by roots the previous night is reabsorbed and transpired (Caldwell and Richards 1989). This process of nocturnal uptake by deep roots and release into the upper soil profile followed by re-uptake by shallow roots is called hydraulic lift and can be shown experimentally by comparing diurnal patterns of soil water potential where plants have been allowed to engage hydraulic lift with plants that have been deprived of that option.