Germination Studies on some Annual Species from an Arid Region of Western Australia

Abstract

Annual floras of the arid regions of Western Australia have been described by Gardner (1942) and Wilcox (1963), who noted that the typical flora which occurs after winter rains is predominantly of dicotyledons, whereas monocotyledons, especially grasses, make up a large proportion of the summer-germinating annuals. Analogous differences between summer and winter floras occur in arid regions of North America, and Went (1948, 1949) attributed the differences to germination requirements. As there is no equivalent study of the germination requirements of annuals in the arid zones of Australia, the following analysis was carried out to evaluate the relation between germination requirements and the composition of the summer and winter floras. The area studied in detail is a station property, Mileura, in the Murchison Region of Western Australia (latitude 26022' S; longitude 117?20' E) (Fig. 1). The area forms part of a broad watershed drained by the Murchison River into the Indian Ocean. There is little relief in the land surface, and the soils are mainly leached red-earths overlying a siliceous hard-pan. The land types of the Murchison Region have been described by Mabbutt et al. (1963). On their land system classification, the majority of the study area is of the sandy red-earths of the Belele and Ero Land Systems, while the remainder is composed of the rocky Sherwood Land System and the saline river flats of the Beringarra Land System. The area falls within a zone receiving an average of less than 200 mm of rainfall annually. Arnold (1963) gives a general account of the weather of the whole Murchison Region. She points out that the rainfall results both from winter and summer falls, with tropical cyclones and local thunderstorms accounting for the precipitation in the summer months from November to March, and extra-tropical depressions bringing rain in the cooler months from May to August (Fig. 2). Rainfall is extremely variable, great differences between the falls occurring each year. Arnold notes that droughts are common if the condition of introduced stock is taken as a criterion. Moreover, Davies (1968) points out that sufficient rain falls to recharge the drainage channels in at least 80% of the summers and 90% of the winters. There is a wide difference between the mean monthly maximum and minimum temperatures in the two seasons. This, allied with the two periods of rainfall, gives sharply delineated growing seasons (Fig. 2). Temperatures in the top 0-5 cm of soil can rise to very high levels in the summer months, with mean maxima of over 65? C for four months of the year. The major annual vegetation over most of the station is 'annual short grass-mixed forb' and 'short grass-chenopod' pastures described by Wilcox & Speck (1963), with the