Drying of root-soil interface; hydraulic conductance decreases
Fig. 2.2.7. Schematic diagram of the change in xylem water potential with the increased water flow, when: (A) the hydraulic conductance is constant, (B) the hydraulic conductance falls when the interface between root and soil dries out, and (C) additional roots contribute to the transport as the soil dries regions include particularly the meristematic regions of root tips and the axial meristems of side roots. The transmission cells in the endo-dermis and exodermis are also able to transport water in both directions, i.e. roots are not only able to take up water from the soil, but may lose water to the soil. This is in fact an important ecological process and occurs particularly at times of low transpiration. It is termed hydraulic lift, where water is taken up from wet soil in deep horizons, lifted by a water potential gradient to the upper soil horizon, where it is released into the dry soil. Water release of plants to the soil was first observed in dry climates (Richards and Caldwell 1987), but it is also important in temperate climates. For Acer saccharum it was observed (Dawson 1993) that the isotopic composition of soil water in the region of the canopy does not correspond to the rain water, but to the much deeper ground water (Fig. 2.2.8). The further the distance from the root regions of the tree, the more similar is the isotopic composition of the xylem water of the herbaceous vegetation to that of the precipitated water. Obviously, during the night, larger amounts of water are transported by tree roots from the moist soil in the proximity of the water table into the dryer upper layer of soil. The isotopic label of the water of the top soil changes correspondingly. The water is utilised during the day not only by the tree, but also by the vegetation
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