THE RAPID AND LONG-LASTING GROWTH OF GRASSES FOLLOWING SMALL FALLS OFRAIN ON STONY DOWNS IN THE ARID INTERIOR OF AUSTRALIA

Stony downs consist of grassy areas that alternate with areas that hav e a substantial stone cover. The stone-covered areas are impermeable, and most rain falling on them runs off, substantially increasing the e ffective rainfall in adjacent grassy areas. As a result, 20-25 mm of r ain on stony downs wetted the soil around the grass to a depth of 140- 170 mm and allowed sustained grass response. This is much less than th e 35-40 mm of rain required for the same response on red clay or grey clay plains. Grasses respond very rapidly after rain. Some have green shoots the day after rain, and all have responded by the second day. E phemerals dry off in 4-6 weeks, but most tussock grasses still have so me green foliage 8-10 weeks after rain. Deeper rooted tussock grasses remain green for so long because most of the moisture that reaches dee per roots after rain remains there. Most moisture loss is through the soil surface and is recognizable as a drying front that descends throu gh the soil profile. Soil above the drying front is nearly air dry (< 5% moisture) while soil below the front has substantial moisture (14-1 6%). By about a month after rain in summer, the drying front is at a d epth of about 80-120 mm. This is near the tips of the roots of ephemer al grasses and the ephemerals then dry off rapidly. Only the tips of t he leaves of deep rooted grasses like Mitchell grass (Astrebla spp.) d ry off. Their leaves continue to remain mostly green during most of th e second month after rain and they do not dry off completely until the third month when the drying front reaches the bottom of the main root system.