Evidence for the natural occurrence of treeless grasslands in the Riverina region of south-eastern Australia

Grasslands in the Riverina region of south-eastern Australia have long been thought to be derived from woodland dominated by Acacia pendula A.Cunn. ex G.Don and Atriplex nummularia Lindl. following over-grazing and clearing in the 19th Century. Despite the broad acceptance of this view, there is little evidence for such a universal change having occurred. Phytosociological and historical evidence is presented here, which suggests that, although many of the existing grassland remnants are floristically similar to remnants of A. pendula woodland and are probably derived from woodland, natural treeless grassland also probably occurs in the Riverina. Such grassland is floristically distinct from remnant woodland and generally spatially separate, being prevalent in the southern Riverina. Although the delineation of natural and derived grassland boundaries may now be difficult, grassland vegetation in general is of immense biodiversity significance, containing a large number of highly localised rare or threatened species. A decrease in rainfall during the winter and spring growing season, as predicted by climate-change models, may be detrimental to natural grassland, which is restricted to the higher rainfall portion of the Riverina.