Constraints on transpiration of Eucalyptus globulus in southern Tasmania, Australia

The constraints on transpiration were studied in plantation grown Eucalyptus globulus trees over the summer of 2004/2005 at a research site in southern Tasmania. Diurnal patterns of leaf water potential and tree water use,measured using heat pulse techniques, were examined monthly in rain-fed and irrigated trees growing under similar atmospheric conditions. Soilmatric potential declined during the summer in rain-fed plots but remained high in irrigated plots. Pre-dawn leaf water potentials decreased in rain-fed trees and this was associated with increasing soil water deficit. The difference between pre-dawn and midday leaf water potential declined with decreasing pre-dawn water potential, suggesting isohydric regulation of plant water potential. Transpiration and canopy conductance were lower in rain-fed trees than irrigated trees and the decline in transpiration and canopy conductance was related to pre-dawn leaf water potential. There was marked hysteresis in the relationship between transpiration and D in both rain-fed and irrigated trees. Hysteresis was also observed in the relationship between transpiration and leaf water potential. However, in this case hysteresis was only evident in rain-fed trees. For the relationship between transpiration and D, hysteresis was larger at high D’s than at low D’s in both rain-fed and irrigated trees and was not related to diurnal changes in soil to leaf hydraulic conductance. Diurnal changes in leaf conductance, however, may play an important role in controlling stomatal sensitivity to D and may help to explain the hysteresis in the relationship between transpiration and D. Soilto- leaf hydraulic conductance of rain-fed trees declined in response to decreasing predawn leaf water potential. We propose that loss of hydraulic conductance is an important mechanismfor