The spatial structure of a plant community observed at any point in time is the product of many processes in its past. These include biotic processes, such as plant dispersal, growth, mortality and herbivory, as well as other factors and processes, such as substrate, topography, climate, fire, disturbance, or land-use history. More often than not, spatial patterns in the structure of a plant community will be the product of complex and interacting processes. The processes that are most important for shaping community structure are likely to leave observable imprints in spatial community structure. It should be possible therefore to use spatial pattern analyses in order to generate testable hypotheses about the processes that shape community structure.
The subject of this study was Ambrosia dumosa (A. Gray) Payne (Asteraceae), a drought-deciduous semi-shrub that is a very widespread and often dominant species in the Mojave and Sonoran Deserts of North America (Shreve 1925; Turner et al. 1995). We examined its spatial distributions and its sizes relative to conspecific neighbours as well as neighbours of different plant growth forms on two adjacent geological substrates in the western Mojave Desert. Effects of abiotic and biotic factors on spatial patterns were examined by analysing replicated, completely mapped, spatial samples, adopting an approach developed by Diggle et al. (1991). This study differs from previous spatial pattern analyses in plant ecology in that it was designed to test whether patterns differed between habitats or were correlated with other biotic and abiotic variables, instead of testing whether patterns deviated from null models of complete spatial randomness (e.g. Pielou 1977; Diggle 1983; Dale 1999). The replicated-maps technique was adopted because (i) deviations from randomness are the rule rather than the exception in spatial community structure (Greig-Smith 1979; Hutchings 1997), (ii) it allows examination of specific hypotheses about community structure based on mechanisms or ecological theory, and (iii) replication in general allows one to make inferences about a whole plant community or landscape, instead of just describing patterns in the plots that were actually sampled.
The underlying assumption of our study was that habitat quality affects plant performance and therefore the nature and intensity of interactions between neighbouring plants. Recent studies suggest that the nature of plant interactions may differ predictably between otherwise comparable habitats of lower and higher plant productivity. Interactions can have positive and negative effects on plant neighbours, often simultaneously, resulting either in net-positive or net-negative effects (Holzapfel & Mahall 1999). The relative importance of positive and negative interactions between plants appears to depend in part on habitat quality (Callaway & Walker 1997; Brooker & Callaghan 1998; Goldberg et al. 1999). Net positive plant interactions are most often found in habitats with high degrees of abiotic stresses and low productivity, and it has been postulated that this may be because interactions with plant neighbours can buffer effects of abiotic stress (Bertness & Callaway 1994; Callaway & Walker 1997; Brooker & Callaghan 1998). In contrast, net negative plant interactions occur in all kinds of habitats (Goldberg & Novoplansky 1997; Newman 1973; Tilman 1988; Goldberg et al. 1999), although Grime (1973) proposed that they may be more intense in more productive habitats that have a lower degree of abiotic stress.
If these generalizations apply to the study system, spatial patterns that are indicative of net negative interactions should be more pronounced on the substrate on which plants are more productive. Two kinds of spatial patterns are thought to be indicators of such net negative interactions: (i) spatial segregation (i.e. ‘regular’ distribution) of individual plants (Fowler 1986; Phillips & MacMahon 1981; Rejmánek & Lepš 1996; Pacala 1997), and (ii) positive correlations between the distances separating nearest neighbours and the combined sizes of these neighbours (Pielou 1960). We tested the hypothesis that these two kinds of spatial patterns are more pronounced in habitats where plants are more productive. Interactions between Ambrosia dumosa and its neighbours at the same site were also studied experimentally (Holzapfel & Mahall 1999; Schenk & Mahall 2002), which allowed us to relate patterns to processes.