Effect of groundwater freshening on riparian vegetation water balance

In the floodplains of the lower River Murray (Australia), riparian open forests and woodlands dominated by River Red Gum (Eucalyptus camaldulensis Dehn.), Black Box (E. largiflorens F. Muell.) and River Cooba (Acacia stenophylla A. Cunn. Ex Benth.) have been severely impacted by salinization and drought. The 'Bookpurnong Experiment' was developed to test the hypothesis that watertable lowering combined with groundwater freshening would reduce tree water stress and improve floodplain vegetation health. Tree water use and water stress following groundwater freshening were examined using measurements of sapflow, pre-dawn leaf water potentials and stable isotope composition of xylem water. Water use and water stress varied markedly across the floodplain. The open mixed forest lining the bank of the River Murray, with access to fresh groundwater, used five times more groundwater over the measurement period and maintained greater canopy vigour than the open mixed woodland at inland sites. The River Cooba trees approximately 80 in from the river responded to watertable lowering of 0.65 in and to groundwater freshening by increasing transpiration. Black Box trees at the same site experienced reduced plant water stress, with no increase in transpiration. However, at a third site, approximately 170 in from the river, where the saline watertable (total dissolved solids = 36300 mg I(-1)) was lowered by I in, no change in plant water stress or pattern of plant water use was observed. These results indicate that groundwater lowering combined with groundwater freshening can provide an accessible water source for water-stressed floodplain vegetation. Copyright (C) 2009 John Wiley & Sons, Ltd.