RESISTANCE TO WATER UPTAKE IN A DOUGLAS FIR FOREST

Abstract

Soil and root resistances were studied in two stands of a Douglas fir forest, Soil water potential was measured with a tensiometer-pressure transducer system in the 0 to −1− bar range, and with Wescor HR-33T dew point microvolmeter and hygrometers at values less than −1 bar. Root xylem water potential was measured with hygrometers, and twig xylem pressure potential was measured by the pressure-chamber technique. Transpiration rates were calculated from energy balance and stomatal diffusion resistance measurements. Total bulk-soil-to-root-xylem resistances were calculated from the water potential differences and transpiration fluxes. Root density was determined from intensive sampling. Root xylem water potential, like twig xylem pressure potential, showed a definite diurnal trend. Soil water potential approached toe root water potential as the soil dried. Assuming the absence of contact resistance between roots and soil, the soil resistance remained very small in comparison with root resistance, even at a soil water potential of −11 bars. An analysis using a contact resistance model suggested that contact resistance could account for one-half the total bulk-soil-to-root-xylem resistance when the soil was moist (-0.3 bar) and almost three-quarters when dry (-11 bars. Analysis of midday data during the drying period showed that the rate of water uptake was linearly related to the difference between soil and root xylem water potentials, and that total soil-to-root-xylem resistance remained relatively constant with decreasing soil water potential.