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

Abstract

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 (ψ_s) > −0.4 MPa, measured predawn total twig xylem water potential (ψ_Ttx) of these Douglas-fir trees was significantly less than ψ_s. A transpiration rate (E) dependent single soil to twig liquid resistance (R_st) accounted for the difference between predawn ψ_Ttx and ψ_s. For ψ_s > −0.4 MPa, during the daytime when E was high, R_st could be described by a logarithmic function of ψ_s. The effect of capacitance on twig xylem water potential (ψ_tx) was small. A model incorporating the transpiration dependent R_st accurately estimated the course of ψ_tx on a clear day when ψ_s was −0.04 MPa.