Factors affecting the relative distribution of Atriplex vesicaria and Kochia sedifolia (Chenopodiaceae) in the arid zone of South Australia

Abstract

The factors controlling the relative distribution of Atriplex vesicaria and Kochia sedifolia were obscure, and explanations in the literature unsatisfactory. This paper describes the results of an experimental approach to an understanding of these factors. A survey of the soils on which A. vesicaria and K. sedifolia are found in the arid zone of South Australia revealed that the distribution of the two species is usually correlated with the depth to which the soil is wetted by the normal rainfall. K. sedifolia, a deep-rooted species, is frequently found on soils which can be wetted to a depth of 2 ft or more. A. vesicaria, a shallow-rooted species, is found on soils in which a heavy clay subsoil or hardpan impedes penetration of water beyond 12 in. From experimental evidence it appears that: (1) either species can grow successfully in pots of the surface soil associated with the other species; (2) each community utilizes water at the same rate; (3) there is no difference between the two species in the rate of defoliation in response to drought conditions, when held at night either in a dry atmosphere or in one with a relative humidity exceeding 85 %; (4) A. vesicaria can reduce the percentage moisture in the soil to a significantly lower level than can K. sedifolia when subjected to drought. These observations are discussed in relation to field conditions, and hypotheses are proposed to explain why these two species are usually found in distinct communities. More detailed work is necessary to test these hypotheses. I.