Trends in the recruitment of Astrebla spp. in relation to seasonal rainfall.

Abstract

Differences in both the occurrence and size of the recruitment of Astrebla spp. (mainly A. lappacea) in Astrebla grassland have been reported from three geographically different locations. The studies reported in this paper were undertaken to develop an understanding of the factors affecting the recruitment of Astrebla spp. at Cunnamulla and Julia Creek at the southern and northern extremities of Astrebla grasslands in Queensland. The germinable seed bank of Astrebla spp. at Cunnamulla in October 1981 was 250 seeds/m2 compared with only 60 seeds/m2 at Julia Creek while the germinable seed bank of Iseilema spp. was 3020 seeds/m2 at Julia Creek compared with < 1 seed/m2 at Cunnamulla. Grazing had no effect on the germinable seed banks of Astrebla spp. but grazing at Julia Creek increased the germinable seed bank of Iseilema spp. Recruitment of Astrebla spp. was recorded at both locations. At Cunnamulla, an unusually large recruitment of 15.0 plants/m2 of Astrebla spp. was recorded in July 1983 following a severe drought in the previous summer. At Julia Creek, a density of 1.4 seedlings/m2 of Astrebla spp. was measured in 1984 and this may have been the first recorded recruitment of Astrebla spp. in the ten year history of the permanent quadrats. Recruitment at Julia Creek was associated with a low density (< 3 plants/m2) of Iseilema spp. I conclude that the seasonal pattern of rainfall, as it influences the size and composition of the soil seed bank, is the major factor affecting recruitment of Astrebla spp. An integrated understanding of Astrebla spp. persistence across its range is presented.