The Disappearing Grassy Balds of the Bunya Mountains, South-Eastern Queensland

Abstract

The physical setting of 61 grassy balds on the Bunya Mountains in south-eastern Queensland was surveyed during 1995, and a further 73 balds were assessed from aerial photographs taken in 1951 and 1991. Balds were analysed to search for consistent environmental patterns that may account for their existence. There was little association between measured environmental variables (including altitude, surface rock cover, slope, landscape position, surrounding vegetation, bald area) and the occurrence of balds, although large balds have a weak association with steep slopes and relatively low rock cover. Grassland was on shallower soil than forest. Some, but not all balds were surrounded by rainforest. No consistent relationship was evident for all measured balds surrounded by eucalypt forest. About 26% of the area of balds existing in 1951 had been invaded by forest by 1991. The extent of the invasion was generally higher for balds surrounded by eucalypt forest than balds surrounded by eucalypt and rainforest or rainforest only, but showed little relationship with other environmental variables. The invading tree species groups could not be associated with environmental variables. The lack of environmental correlation of bald location and size, and the rapid change in bald area is not supportive of hypotheses relating bald maintenance to intrinsic physical conditions. There is relatively poor species diversity of Eucalyptus on the Bunya Mountains and this narrow ecological range of species may be a limitation on the colonisation of the grasslands. Indirect evidence for the role of grazing and fire as agents of bald maintenance are discussed, and it is concluded, that fire, and not grazing, is likely to be a major influence.