Relating Water Potential Gradients in Mountain Ash (Eucalyptus regnans F. Muell.) To Transpiration Rate

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Abstract
Osmotic tensiometers recording in situ stem water potential, Ψst, were implanted at two heights in the trunks of mountain ash (Eucalyptus regnans) in mature and 1939 regrowth forests. Measurements over one summer showed that Ψst gradients, obtained from the mean Ψst values at each height, reached over 50 kPa/m, with the diurnal maxima generally occurring between 1500 and 1600 h. When Ψst gradients were averaged over 24 h it was found that, unless temperatures were extremely high or the foliage was wet, the average diurnal gradient was linearly related to the average vapour pressure deficit. From this relationship the average diurnal gradient during two earlier summers was estimated to be 23 kPa/m. The forest transpiration rate during this time was estimated to be 4.1 mm/day and when these data, together with sapwood area data, were put into a simple tree transpiration model, the relative conductivity of the sapwood was found to be approximately 7.9 × 10-12 m2.