ABSTRACT
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 of the
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 four-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 at which distances and in which
directions to their neighbors plants can grow and survive.