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Root water uptake in a Douglas fir forest Nnyamah, Joseph U.


The forest water balance and root water relations were studied in a thinned (840 stems/ha) and an unthinned (1840 stems/ha) stand of a Douglas fir forest during two consecutive summers. Soil water content and potential data were used to compute water extraction rates and patterns for the root zone in each stand over a four-week drying period. The results showed a gradual downward shift of the zone of maximum root water uptake as the soil dried. There was good correlation between water uptake rate and root density. Water flux into the bottom of the root zone, estimated by the use of Darcy's Law, increased from 8 to 15% of the evapotranspiration at the thinned site and from 2 to 8% at the unthinned site. Soil profile water depletion, corrected for flux out of or into the bottom of the root zone, agreed well with evapotranspiration computed from micrometeorological energy balance data. Water withdrawal from trunk storage accounted for only 2% of the total evapotranspiration over the four-week drying period. In the first two weeks, evapotranspiration from the thinned stand was 11% less than that from the unthinned stand, but was 18% more in the last two weeks. At a particular soil water potential, individual trees at the thinned site transpired an average of 25% more than those at the unthinned site on fine sunny days. When water uptake was compared over the four-week period, it was found that the individual trees at the thinned site were transpiring 35% more than those at the Unthinned site. Measurements of soil and root xylem water potentials were made using a Wescor HR-33T dew point microvoltmeter and PT 51-10 hygrometers. Tensio-meters were used to measure soil water potentials at values greater than -1 bar. Twig water potential was measured by the pressure chamber technique. Root water potential measurement required tangential insertion of the hygrometer into the root xylem and sensor protection from plant resins using gypsum powder. Soil water potentials measured with hygrometers were compared with potentials computed using gravimetric soil water content measurements and laboratory soil water retention data, while root water potential measurements were compared with those made on roots with the pressure chamber. The comparisons showed good agreement to within 0.3 bar over an 8-bar range. Soil water matric potential on both dew point and psychrometric modes showed good agreement. Soil and root resistances to water uptake were studied in both stands. Resistances were obtained from water potential differences and evapotranspiration fluxes. Root xylem water potential, like twig water potential, showed a definite diurnal trend. Soil water potential approached the root water potential as the soil dried. Soil resistance remained very small in comparison to root resistance even at a soil water potential of -11 bars; however, it was found that "contact resistance" could account for as much as half of the total soil to root xylem resistance. Root resistance varied during the daytime becoming increasingly.important toward nightfall. The plot of rate of water uptake versus soil to root xylem potential difference showed a linear relationship extending through zero. Root resistance remained relatively constant as the soil dried.

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