Tree/pasture interactions at a range of tree densities in an agroforestry experiment. II. Water uptake in relation to rooting patterns.

Patterns of water uptake throughout a drying period of approximately one year were investigated under trees and pasture at three tree densities in an agroforestry experiment, and related to tree and pasture rooting patterns and water use. A greater proportion of soil water was extracted from deep in the soil profile under the densely planted trees, owing to lower soil water contents in upper horizons and deeper and more dense rooting systems than at lower tree densities. As the drought period progressed, the ratios of tree transpiration rate and pasture evaporation rate to equilibrium evaporation rate tended to decrease at each tree density as soil water contents in upper horizons decreased, and an increasing proportion of water was extracted from deeper soil horizons. At each tree density, the rate of water uptake per unit root length was lowest in surface soil horizons and tended to increase with increasing soil depth. The rate of water uptake per unit root length tended to increase with time in deeper, wetter soil horizons and decrease with time in surface soil horizons as soil water content decreased.