Rates and Patterns of Water Uptake in a Douglas‐Fir Forest

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Abstract
The forest water balance was studied in a thinned (840 stems/ha) and an unthinned (1,840 stems/ha) Douglas‐fir forest [Pseudotsuga menziesii (Mirb.) Franco] during two consecutive summers. Soil water content was measured with the neutron meter. Soil water potential was measured with tensiometers and dew‐point hygrometers over a range of 0 to −15 bars. These data were used to compute water extraction rates and patterns for the root zone 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. The fully developed root system of Douglas fir showed less hydrotropic response than the developing root systems of annuals reported in the literature. There was good correlation between water uptake rate and rooting density. During the drying period, 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 with‐drawal from trunk storage accounted for only 2% of the total evapotranspiration over the four‐week drying period.