ABSTRACT

The spatial structure of a plant community contains a record of the conditions that enable individual plants to coexist.  This record reflects past- and present-, biotic and abiotic conditions, as well as their interactions.  To reduce the number of confounding factors, past studies of spatial patterns have been based on the assumption that substrate characteristics did not affect the distribution of individual plants relative to other plants.  This assumption, however, is contradicted by the observation that substrate characteristics can affect spatial patterns in plant communities by influencing germination, performance, and interactions of plants.
To determine whether substrate differences affect spatial distributions of plants relative to other plants, I chose two adjacent creosote bush scrub communities on different geomorphic surfaces, aeolian sand and alluvium, in the Mojave Desert as my study system.  The focus of the study was on the dwarf-shrub Ambrosia dumosa, its intraspecific patterns and the spatial distributions of other shrubs and annual plants relative to Ambrosia.  Patterns were analyzed using a new technique based on replicated and completely mapped sample plots along a 4 km transect transversing the two substrates.
Ambrosia shrubs were spatially segregated from conspecific neighbors on sand, but strongly aggregated on alluvium.  In contrast, substrate differences did not affect the spatial segregation of Ambrosia from Larrea tridentata shrubs.  Annuals and Ambrosia seedlings were much more strongly associated with shrubs on sand than on alluvium, and were most abundant on the northern side of shrub canopies.  A directional association of shrub neighbors was detected on sand, where Acamptopappus sphaerocephalus shrubs occurred predominantly on the northwestern side of Ambrosia and/or Acamptopappus neighbors.
Patterns of seedling emergence were strongly affected by shrub canopies and the microtopography of shrub mounds, which caused large directional differences in soil surface temperatures.  The resulting, positive, directional association of seedlings with shrubs may account for the observed directional orientations among shrub neighbors.  Acamptopappus shrubs at times benefited from the presence of close Ambrosia neighbors, possibly by utilizing water leaking from Ambrosia roots into shallow soil layers, but competed with Ambrosia neighbors at other times.  No evidence for current interactions among Ambrosia shrubs could be detected.