Spatial ecology of a small desert shrub on adjacent geological substrates

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Abstract
Summary: 1 Spatial pattern analyses were used to generate hypotheses about the processes that shape the structure of a plant community in the Mojave Desert of North America, with a focus on the semi‐shrubAmbrosia dumosa. We analysed spatial distributions and sizes of this species relative to other semi‐shrubs, shrubs and annuals, and the relationships between spatial patterns and abiotic and biotic habitat characteristics.2 The analyses were based on maps of sample plots placed along a transect spanning two adjacent geological substrates: aeolian sand and gravelly, sandy to loamy alluvium. Of these two substrates, sand supported higher total biomasses ofAmbrosiaand of all woody perennials, while alluvium supported on average higher biomasses of winter annuals.3 Annuals and seedlings ofAmbrosiawere much more strongly aggregated withAmbrosiacanopies on sand than on alluvium, suggesting that these small plants were more strongly facilitated byAmbrosiaon sand than on alluvium.4 Ambrosiasemi‐shrubs were spatially segregated on sand but aggregated on alluvium, and the degree of segregation on sand increased with the total above‐ground biomass ofAmbrosiaper unit area, indicating that negative interactions betweenAmbrosiaplants were stronger in more productive habitats. Canopy sizes ofAmbrosiain all mapped plots increased with distance to the nearest conspecific neighbour, which suggests that neighbour interactions negatively affected plant sizes.5 Ambrosiaplants on sand were spatially aggregated withAcamptopappus sphaerocephalussemi‐shrubs, suggesting that at least one of these species may benefit from the association.Ambrosiaplants were spatially segregated fromLarrea tridentatashrubs on both substrates, possibly due to negative effects ofLarrearoots onAmbrosiaroots reported in previous studies.6 Subtle differences in substrate characteristics were correlated with strong differences in the spatial distribution ofAmbrosiaplants relative to their neighbours, which suggests that edaphic conditions may affect the spatial structure of the community by modifying complex positive and negative interactions between neighbouring plants.