Vj. Anderson et Kc. Hodgkinson, GRASS-MEDIATED CAPTURE OF RESOURCE FLOWS AND THE MAINTENANCE OF BANDED MULGA IN A SEMIARID WOODLAND, Australian Journal of Botany, 45(2), 1997, pp. 331-342
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.