Positive and negative plant interactions contribute to a north-south-patterned association between two desert shrub species.

Abiotic factors are often thought to be the predominant forces shaping desert plant communities. But both positive and negative interactions between plants are frequently observed in deserts, and it is an open question whether they can strongly affect the spatial structure of a desert community. The goal of this study was to answer this question for a plant community in the North American Mojave Desert. Two semi-shrub species, Ambrosia dumosa and Acamptopappus sphaerocephalus, were the focus of this study. At the study site, seedlings emerged predominantly on the northern side of shrubs, indicating positive effects of canopy shading on emergence, but survival of Ambrosia seedlings was much higher in open areas than at the edge of conspecific shrubs. Negative intraspecific interactions also affected Ambrosia shrubs, which did not increase in size over a 4-year period unless the nearest conspecific neighbor had been removed. These negative intraspecific interactions among different life stages of Ambrosia appear to contribute to spatial segregation observed among shrubs of this species. In contrast, Acamptopappus shrubs and their seedlings were aggregated with Ambrosia shrubs, and occurred more often on the northern side of Ambrosia than expected by chance. Removal of Ambrosia neighbors positively affected growth of Acamptopappus, but only when the neighbor was removed on the northern side. For Acamptopappus, an Ambrosia neighbor on the southern side may have some positive effects, which appear to neutralize the negative effects found for northern neighbors. These positive effects were likely at least partly due to shading. Removal of Ambrosia neighbors negatively affected predawn xylem pressure potentials of Acamptopappus, but this effect was only found during one growing season and was briefly reversed during the next. In summary, negative intraspecific interactions appear to cause spatial segregation of Ambrosia shrubs, while a combination of positive and negative interactions apparently contribute to the directional association between Ambrosia and Acamptopappus. Thus plant interactions in this desert appear to shape community structure in at least two dimensions by influencing the distances and in which directions to their neighbors plants can grow and survive.