Soil Water Potential In Root Zone Research Articles

Examination and parameterization of the root water uptake model from stem water potential and sap flow measurements

AbstractA simple field‐based method for directly parameterizing root water uptake models is proposed. Stem psychrometers and sap flow meters are used to measure stem water potential and plant transpiration rate continuously and simultaneously. Predawn stem water potential is selected as a surrogate for root zone soil water potential to examine and parameterize the root water uptake–water stress response functions. The method is applied to two drooping sheoak (Allocasuarina verticillata) trees for a period of 80 days, covering both a dry season and a wet season. The results indicate that the S‐shape function is more appropriate than the Feddes piecewise linear function for drooping sheoak to represent the effect of soil moisture stress on its root water uptake performance. Besides, the water stress function is found to be not only a function of soil moisture but also dependent of the atmospheric demand. As a result, the water stress function is corrected for the effect of atmospheric conditions. Copyright © 2012 John Wiley & Sons, Ltd.

Stomatal characteristics and transpiration of three species of conifer seedlings planted on a high elevation south-facing clear-cut

Douglas-fir (Pseudotsugamenziessi (Mirb.) Franco), western hemlock (Tsugaheterophylla (Raf.) Sarg.), and Pacific silver fir (Abiesamabalis (Doug.) Forbes) seedlings were planted in the spring as 1-0 container-grown plugs on a south-facing high elevation clear-cut located on Mount Arrowsmith, Vancouver Island, British Columbia, and their stomatal responses to environmental and physiological variables were determined over two successive growing seasons. The stomatal responses of all three species to changes in environmental variables and time did not differ over the 2 years nor were there differences in response between seedlings planted a year apart. A simple multiplicative boundary-line model that related seedling stomatal conductance (gs) to measurements of hourly average solar irradiance, air temperature, vapour pressure deficit, and average root zone soil water potential accounted for over 70% of the variability in gs. When the number of hours from sunrise was included as an independent variable, over 85% of the variability in gs could be explained. Daily seedlings transpiration rates on a projected leaf area basis were successfully estimated by summing the product of the calculated average gs and D/(RvT′) where D is the vapour pressure deficit, Rv is the gas constant for water vapour, and T′ is the absolute air temperature.

Estimation of twig xylem water potential in young Douglas-fir trees

Methodology, based on water flow theory, is described which can be used to estimate twig xylem water potential for 10–15 m tall Douglas-fir (Pseudotsugamenziesii) (Mirb.) Franco) trees. Using pressure chamber measurements, values of needle xylem water pressure potential were found to be similar to twig xylem water pressure potential. For root zone soil water potential (ψs) > −0.4 MPa, measured predawn total twig xylem water potential (ψTtx) of these Douglas-fir trees was significantly less than ψs. A transpiration rate (E) dependent single soil to twig liquid resistance (Rst) accounted for the difference between predawn ψTtx and ψs. For ψs > −0.4 MPa, during the daytime when E was high, Rst could be described by a logarithmic function of ψs. The effect of capacitance on twig xylem water potential (ψtx) was small. A model incorporating the transpiration dependent Rst accurately estimated the course of ψtx on a clear day when ψs was −0.04 MPa.