One of the most striking aspects of desert vegetation is the apparently regular spacing of shrubs. The species which has been commented on most in this respect is Larrea divaricata Cav., the creosote bush (Leopold 1963; Went 1952, 1955; Baker 1966). Its widereaching root system is referred to by Cannon (1911), who excavated several systems and showed that in some situations the roots could extend 2 or 3 m in all directions. Such regular spacing, if it exists (and casual observation certainly suggests that it does) must be the result of some sort of mutually disadvantageous interaction between the individuals. Went suggested that allelopathy was involved in this pattern in a widely quoted statement made in 1955: 'Another Death Valley plant endowed with a remarkable root system is the evergreen creosote bush. It has wide reaching roots which can extract water from a large volume of soil. The creosote bush is spread with amazingly even spacing over the desert; this is especially obvious from an aeroplane. The spacing apparently is due to the fact that the roots of the bush excrete toxic substances which kill any seedlings which start near it. The distance of spacing is correlated with rainfall; the less rainfall the wider the spacing. This probably means that rain leaches the poisons from the soil so that they do not contaminate so wide an area. We commonly find young creosote bushes along roads in the desert, where the road builders have torn up the old bushes.' Rainfall measurements have been carried out at many stations in the California desert for varying numbers of years. We chose our experimental sites as close as possible to rainfall stations, preferably stations where records had been kept for some years (see map, Fig. 1). Eight sites were sampled in the Mojave Desert and four in the Sonoran desert. Each site was selected to be as near to the weather recording station as possible, to be on as level an area as possible, and to have minimum dissection by drainage channels.
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