Search

The majority of pattern analyses of total density data collected from geographically separated populations of Atriplex vesicaria, both in the western Riverine Plain of New South Wales and in areas of north-western New South Wales, provided evidence of bi- or trimodal contagion. More detailed analyses of pattern in age-related phases of these populations exhibited most intense patterning in 'seedling' phases, with a progressive reduction of pattern intensity in subsequent ('pioneer', 'building' and 'mature') phases of the entire population. It has been found that the pattern of young individuals may be correlated with patterned variation in microtopography, as well as with water and nutrient availability, and that the indirect effects of grazing animals may be responsible for the intermediate scales of the mosaic pattern in populations of sexually mature individuals. The performance of A. vesicaria plants was assessed experimentally in relation to a range of natural soils, with associated watering and defoliation treatments, and in controlled nutrient solution culture. The results suggest that performance (as estimated by dry weight yield) is affected significantly by soil type, water stress and defoliation. In an experimental context the prime effect of soil variation on yield appeared to be mediated by variation in soil cation balance, a patterned balance that may have been historically induced in the field situation.

The analysis of density data from a variety of Australian dry-land vegetation types provides evidence of widespread randomness when the component distributions are compared with a Poissonian model. While such randomness may be reflecting traditionally held beliefs concerning structure in colonizing and "relict" populations, randomness may develop as a result of a differential elimination of individuals between high and low density phases in an originally patterned population. It is suggested that structure in vegetation cannot be estimated reliably by simple observation alone, and this statement is evidenced by the ubiquity of random (and to a lesser extent contagious) distributions which are found in Australian dry-land communities, in contradistinction to the widely held view that such distributions are necessarily essentially regular in their character.