GRASS-MEDIATED CAPTURE OF RESOURCE FLOWS AND THE MAINTENANCE OF BANDED MULGA IN A SEMIARID WOODLAND

It has been proposed for semi-arid and arid regions that the scarce re sources of water and nutrients normally limiting plant growth are dist ributed patchily in the landscape in order to maintain both stability and productivity (Tongway 1990). 'Fertile patches' are maintained by t he concentration of resources during flows and when resource control i s weakened their persistence is threatened. This proposition was teste d by comparing water relations of perennial life-forms in the nutrient and water rich 'island-bands' of mulga (Acacia aneura F.Muell. ex Ben th.) shrubs, with and without perennial grasses. Dawn xylem water pote ntial (dawn psi(x)) and stable hydrogen isotope composition data showe d that A. aneura and companion grasses competed for soil water, wherea s the poplar box (Eucalyptus populnea F.Muell.) trees accessed additio nal sub-soil water. During a drying cycle, dawn psi(x) of A. aneura on the upslope portion of 'island-bands' was -2 MPa where grass was abun dant, and -4.8 MPa where grasses were killed by heavy grazing. On the downslope portions, dawn psi(x) of A. aneura was similar at -4.2 MPa. Recruitment of A. aneura was occurring in the upslope portion, but was less where there were no grasses. There were more dead A. aneura shru bs on the downslope portion and in grass-free 'island-bands'. The resu lts suggest that removal of perennial grasses by grazing shortens the period of water supply to A. aneura shrubs in 'island-bands' and they prematurely die during low rainfall periods. Furthermore, continued gr azing pressure from domestic and non-domestic herbivores will prevent restoration of landscape processes necessary for the survival of these shrubs and associated flora.