R. ROE1 and H.I. DAVIES2
1CSIRO Division of Plant Industry, Black Mountain, Canberra, A.C.T. 2601.
2CSIRO Division of Mathematics and Statistics, 306 Carmody Road, St Lucia, Queensland, 4067, Australia.
The density of Astrebla spp. (Mitchell grasses) declined to a low level on "Gilruth Plains" Research Station in the central part of the Warrego region of Queensland during the period 1941–1974.
A survey of other areas in 1974 confirmed a similar low level on all of 7 proper ties in the central district and 7 of 8 properties in the southern district, suggesting that a decline had also occurred in these districts. In contrast, 5 of 5 properties in the northern Warrego district recorded substantially higher Astrebla densities. These probably reflected a more favourable rainfall regime.
The decline in Astrebla density was due to the failure of recruitment. Low winter rainfall may have contributed to this failure. Successive wet summers may also have had an adverse effect through competition from the high population of miscellaneous perennial species which resulted from these conditions. In a limited study, moderate rates of stocking showed no adverse effect on recruitment.
The practical implications of a decline in density are discussed. It would make the pastoral industry more liable to the effects of drought.