Spacing patterns in Mojave Desert plant communities: near-neighbor analyses

Abstract

The identities, sizes and densities of woody shrubs > 10 cm high were recorded at four sites in the Mojave Desert of comparable elevation and topography, all located on flat, sandy granitic alluvium. For each commoner species, the identity, size and distance of c. 50 first and 50 second near-neighbors were recorded. Using the known densities of species in the quadrats, the observed neighbor frequencies were compared to those expected from random disposition of species. The comparison showed that some species are the ‘preferred’ neighbors of others, while yet other species are ‘avoided’ as neighbors. Some species ‘prefer’ conspecifics as neighbors, but the larger dominant shrubs almost invariably ‘avoid’ conspecifics and tend to a uniform spacing. Positive interactions (preferred neighbors) are 2–3 times commoner than negative interactions (involving avoided neighbors). It is hypothesized that the likely mechanism of these spacing patterns is differential compatibility of root systems, although specificity of germination sites may also play a role; this mechanism is hypothesized to be of importance in the maintenance of diversity in desert shrubs.