IS THE FIELD WATER-USE OF EUCALYPTUS LARGIFLORENS MUELL,F. AFFECTED BY SHORT-TERM FLOODING

Transpiration of and water sources for Eucalyptus largiflorens F. Muel l trees at three sites on a semi-arid floodplain of the lower River Mu rray in southern Australia were investigated during the course of a fl ood. Two of the sites were flooded for 32 days while the third was not flooded. At both flooded sites transpiration was not suppressed durin g the period when the sites were flooded, indicating that anoxia did n ot occur. Available literature suggests that E. largiflorens has very low O-2 consumption, and that lateral roots can absorb O-2 from soil w ater at very low partial pressures. The oxygen stored in the unsaturat ed region of the soil appeared sufficient, in this case, to meet the r equirements of the trees. Transpiration at the flooded sites did not i ncrease in comparison with that at the non-flooded site in the period following flooding. We suggest that this was possibly due to the above -average rainfall experienced in the months preceding the flood and/or physiological adaptations to the highly saline conditions that preven ted the trees taking full advantage of the additional water. The sourc es of water used by the trees were investigated using the naturally oc curring stable isotopes of water and measurements of soil water suctio n. These showed that prior to the flood shallow cain-derived soil wate r was being used at all three sites. Ten days after flooding ceased tr ees at both flooded sites were using shallow flood-derived soil water while at the non-flooded sites they were using groundwater. Results fr om the flooded sites 7 and 22 weeks after flooding ceased showed that as the flood- and rain-derived water became limited the trees suppleme nted their supplies with groundwater from the capillary fringe. The no n-flooded trees used various combinations of rain-derived soil water a nd groundwater during this period.