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

Abstract

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.