Hydraulic Characteristics of Mountain Ash (Eucalyptus regnans F. Muell.) Derived From in situ Measurements of Stem Water Potential

Abstract

An osmotic tensiometer has been successfully used for in situ measurements of stem water potential, Ψ_st, in mountain ash. During spring and early summer when soils were saturated, fluctuations in Ψ_st, at a given height in different trees were very similar. When Ψ_st, values were measured at different positions around the circumference of individual trees during late summer, considerable variation between the different positions was recorded. The data at this time also suggest that night temperatures above 17°C can lead to stomatal opening, but this was not confirmed by direct measurement. Possible reasons for apparent discrepancies between leaf and stem water potential measurements are discussed. The time lag between vapour pressure deficit and Ψ_st was around 2 h whenΨ_st values were high, but fell to 10 min or less for Ψ_st values below about - 1.2 MPa. Analysing this time lag in terms of a resistance-capacitance model of water flow made it possible to estimate the storage-to-vessel transfer resistance as 1.7 × 10⁷ Pa s m³ kg^-1.