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Resistance to water uptake : a comparison of ponderosa pine and Douglas-fir seedlings and an investigation into the effects of planting

University of British Columbia

Resistance to water uptake in ponderosa pine (Pinus ponderosa Laws.) and coastal Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) seedlings was studied in a controlled environment using an Ohm's Law analogy to liquid phase water flow for calculating resistance from measurements of soil and needle water potential and, uptake rate. This study compares resistance in ponderosa pine to similarly measured resistance in Douglas-fir from another study. Resistance to water uptake in planted Douglas-fir seedlings was also investigated. Ponderosa pine shows virtually constant needle water potential through the range of soil water potential where uptake occurs. A modified Weibull function was found to give good overall results for predicting uptake as a function of soil water potential using nonlinear regression techniques. Generally, uptake rates are high compared to Douglas-fir at soil water potentials greater than about -0.5 MPa but decrease more rapidly as the soil dries. Resistance to water uptake appears to be more or less constant and much less than in Douglas-fir at soil water potentials greater than -1.5 MPa. At lower soil water potentials resistance increases rapidly; at -2.0 MPa, resistances for the two species are about equal. The increase in resistance below -1.5 MPa of soil water potential in ponderosa pine may be largely a result of increases in plant and soil-root contact resistance. Resistance in planted Douglas-fir seedlings was studied using three experimental treatments. The control treatment consisted of seedlings which had grown in pots for about 15 months. Planting was simulated by carefully lifting and re-potting a randomly selected group of seedlings. Half of the planted seedlings were randomly allocated to the third treatment which consisted of subjecting seedlings to vibration. Planted seedlings showed considerably greater (by about six times) resistance than controls. Planted and vibrated seedlings, however, were statistically identical to the control seedlings. Because of similarity in soil physical properties among treatments, it is doubtful that decreased soil resistance in planted and vibrated seedlings could totally account for their lower resistance compared to planted seedlings. Therefore it is inferred that reduced soil-root contact resistance is responsible for the lower total resistance.

Text

2010-04-22

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http://hdl.handle.net/2429/24027

Soil Science

University of British Columbia

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