Opening Rains Research Articles

Effect of Temperature and Depth of Burial on the Persistence of Seed of Avena fatua L. In Western Australia

Two wild species of Avena predominate in the temperate south-west region of Western Australia; A. fatua L. is a major weed of cereal crops, while A. barbata Pott ex Link is found mainly on vacant, undisturbed land and in pastures (Paterson 1976). These habitat preferences may be due largely to the differing seed germination characteristics of the two species (Paterson 1974). A. fatua has considerable seed dormancy; less than 4000 of each seed crop germinates with the following opening rains and a further 10-30% germinates later in the season. In contrast, almost complete germination of A. barbata occurs with the opening rains and pre-sowing cultivation virtually eliminates this species in a cereal crop. The dormancy of A. fatua seed is known to continue through the following growing season regardless of soil temperatures (Paterson 1974). Banting (1962) and Forbes (1963), in Canada and England respectively, stated that a minimum of five years of continuous pasture is required before the number of viable seeds in the soil falls to a level where resulting plants do not constitute a weed problem in a subsequent cereal crop. Thurston (1966) in England and Quail & Carter (1969) in Eastern Australia believe that a much shorter period, possibly only one to two years, is equally effective. This paper reports on a study of the long-term persistence of A. fatua in Western Australia and the effect of temperature and depth of seed burial on persistence.

Growth, Water Relations and Nutrition of Three Grassland Annuals as Affected by Drought

Although the Mediterranean type of climate is typified by cool-wet winters and hot-dry summers, drought may be frequent in the cool season, influencing the botanical composition of annual-type vegetation (Bentley & Talbot 1951; Heady 1958; Williams & Elliott 1960; McKell, Whalley & Williams 1971). In California, U.S.A., variations in the amount and pattern of precipitation, especially the date of germinating rains, have been associated with the phenomena referred to as 'good clover', 'good filaree', and 'good grass' years (Talbot, Biswell & Hormay 1939). In Australia, early opening rains result in 'clover years', whereas a 'false break' (early rains followed by a long period of dry weather) favours drought-tolerant species and leads to the death of clover seedlings (Rossiter 1966). Of great significance is earliness of flowering and seed production, which are partly a function of the soil moisture reserve. Seed production can have a profound effect on subsequent fall germination (Wagnon & Biswell 1943; Rossiter 1959; Donald 1960). Rossiter (1966), in an excellent review of annual-type ecology, also discussed a probable temperature x moisture x species interaction and seed dormancy as factors influencing botanical composition. McKell, Major & Perrier (1959) have demonstrated that fertilization of annual-type range increases the depletion of soil moisture. Among the other factors that can also influence botanical composition, Biswell (1956) implicated grazing, fire, and mulch removal. Since variability in precipitation is associated with extreme fluctuations in species abundance, it is suggested that the common annual species respond differently to moisture stress. One advantage that Erodium has over Trifolium and Bromus under conditions of drought and poor fertility is its superior rooting habit (Cameron & McGowan 1964; Ozanne, Asher & Kirton 1965; Rossiter 1966; Cayley 1968; McCown & Williams 1968). Information is lacking on the effect of moisture stress per se on the growth of winter annual species. Winter annuals are admittedly mesophytic, but the importance of drought tolerance as a factor in determining the relative success of annual species should not be minimized. The ability to tolerate dry spells is of great economic importance, as Meadly (1945) indicated in investigations of the virtual elimination of clover and subsequent dominance by Erodium in some pastures in western Australia. He attributed the change to an early 'break' followed by a dry spell. Costly range improvement programs can be made more costly by the additional reseeding necessitated when such a pattern reduces reserves of seed. This investigation was conducted to develop information on the effects of moisture stress and population density on the early growth of three winter annuals that are * Present address: Department of Agronomy, University of Thessaloniki, Greece.

Studies in the ecology of the Riverine Plain. IV. Basal area and density changes of Danthonia caespitosa Gaudich, in a natural pasture grazed by sheep

The changes in basal area and density of Danthonia caespitosa were measured over the 9-year period 1949-1957 by charting permanent quadrats in plots which were ungrazed, and lightly, moderately, and heavily grazed. Compared with the control, the grazing treatments had no effect on basal area, and a significant but not substantial effect on the contribution made by the various age groups of the dominant grass. There were no real differences between the age groups of D. caespitosa on continuously and intermittently grazed treatments, and it is suggested that the design of grazing experiments might be simplified, cognizance being given to the stage of degeneration reached by the pasture, and to the possibility that seasonal deferment procedures might cause substantial changes in botanical composition.. In the autumn or winter of 1951 the mature population of D. caespitosa was almost wiped out and the build-up of basal area and density over the 1951-1957 period was very slow. Dry summers and the late arrival of the opening rains in autumn appeared to be responsible for substantial losses in D. caespitosa. Abnormally high rainfall during the preceding growth season appeared to accentuate these losses. In the gilgai microrelief death rates were greater on the depression than on the shelf. It is suggested that the D. caespitosa grassland was degraded, and reached a "steady state" condition under the influence of cattle, sheep, rabbits, and drought. If grazing treatments are to induce changes in such a plant population, such treatments must be a wide departure from the previous procedures which have given rise to the existing plant community. The results are discussed in relation to grazing experiments in Queensland, New South Wales, South Australia, and the north-west of Western Australia.