ABSTRACT
1 Spatial pattern analyses were used to generate hypotheses
about the processes that shape the structure of a community in the
Mojave Desert of North America, with a focus on the semi-shrub Ambrosia
dumosa. We analyzed its spatial distributions and sizes
relative to other plants of
different growth forms, as well as relationships of spatial patterns
with abiotic and biotic habitat characteristics.
2 The analyses were based on replicated, completely-mapped,
spatial samples placed along a transect spanning two adjacent
geological substrates: aeolian sand and gravely, sandy to loamy
alluvium. Plots on sand had higher total biomass of Ambrosia
and of all woody perennials than those
on alluvium, while plots on alluvium on average had higher biomass of
winter
annuals.
3 Annuals and seedlings of Ambrosia were much more
strongly aggregated with Ambrosia canopies on sand than on
alluvium, suggesting that these small plants were more strongly
facilitated by Ambrosia on sand than on alluvium.
4 Ambrosia semi-shrubs were spatially segregated on sand
but aggregated on alluvium, and the degree of segregation increased
with the total
aboveground biomass of Ambrosia per area, indicating that
net-negative interactions among Ambrosia neighbours were
stronger in more productive habitats. Canopy sizes of Ambrosia
in all sample plots increased with distance to the nearest conspecific
neighbour, which suggests that neighbour interactions negatively
affected plant sizes.
5 Ambrosia semi-shrubs on sand were spatially aggregated
with Acamptopappus sphaerocephalus semi-shrubs, suggesting that
at least one of these two species may benefit from the spatial
association.
Ambrosia plants were spatially segregated from Larrea tridentata
shrubs on both substrates, possibly due to negative effects of Larrea
roots on Ambrosia roots reported in previous studies.
6 Subtle differences in substrate characteristics affected the
spatial distribution of Ambrosia plants relative to their
neighbours, which suggests that edaphic conditions affect community
structure by modifying interactions
among neighbouring plants. Spatial pattern analyses as well as
separate
experimental studies provided evidence for complex positive and
negative
plant interactions, the balance of which appears to shift in response
to
edaphic factors.