Water uptake by overstory and understory trees on shallow soils

Many forest soils are shallow, with soil development limited by a dense layer close to the soil surface. Do these layers limit water uptake to the shallow surface soil or can tree roots take up water in these layers? Does soil water uptake in shallow soil differ with forest vegetation and can this be attributed to root distribution and timing of tree physiological activity? Do trees in these soils respond similarly to drought? Are understory trees subject to more intense competition for water? The goal of this study was to address these questions to better predict water dynamics under different forest vegetation on shallow soils.;A soil water budget approach was used to determine water uptake by a sugar maple plantation, and red pine plantations with understory and with understory removed. Soil water content to a depth of 0.45 m was measured with TDR continuously from June 1992 to November 1995 in a shallow soil in central New York. Simulations were used to estimate water uptake from deeper in the soil profile. Sap-flow of understory trees was measured using a heat-balance method.;Predicted water uptake below 0.45 m averaged about 1 mm per day in all stands even during prolonged drought, though roots were present below this depth. The root density distribution impacted water uptake at depth. There were small differences in soil water uptake patterns that could be attributed to differences in rooting patterns and canopy duration. Osmotic adjustment may occur in both overstory trees in response to prolonged drought. Post-stress recovery to full transpiration occurred rapidly in both over and understory trees. Other responses to severe drought differed among species: reduced leaf area in sugar maple; reduced transpiration in red pine; earlier and more gradual reduction in leaf area, higher sap-flow and tolerance for lower leaf water potentials in black cherry saplings; later and more abrupt leaf senescence and maintenance of higher leaf water potentials at the expense of sap-flow in red maple saplings. The overstory pine responded to removal of understory through increasing root density and water uptake in the surface soil.