Three alluvial piedmonts (bajadas or alluvial fans) studied in the Sonoran Desert near Tucson, Arizona are complex mosaics of distinct geological landforms. These landscape mosaics have been produced through the temporally episodic and spatially discontinuous aggradation of alluvial surfaces and the destruction of other parts of the landscape by erosion. These geomorphic processes produce abrupt juxtapositions of soils of different ages and degrees of profile development. Vegetation patterns correspond closely to this geomorphic mosaic. Larrea tridentata predominates on most Holocene-aged surfaces and all parts of highly dissected, early Pleistocene surfaces. This shrub is generally excluded from Pleistocene-aged surfaces containing soils with strongly developed argillic (clay-rich) horizons. The highest species diversity is encountered on some of the most unstable, erosional slopes of early Pleistocene surfaces. Comparisons among the three study areas indicated the importance of igneous lithology (highly weatherable intrusives vs. weatheringresistant extrusives) in controlling geomorphic processes, and ultimately, vegetation patterns. The areal extent of late Holocene alluvial aggradation and patterns of erosion and dissection of older Pleistocene deposits are strongly influenced by the weatherability of different lithologies and provide a strong control over the spatial scale of ecological patterns.Processes limiting the distributions and abundances of plants are directly linked to landscape characteristics in many ways. Landform age and stability affect the structure of populations of long-lived Larrea tridentata. Individuals of this shrub species can exhibit clone-like growth and increase considerably in size (diameter) over time spans of many centuries to millennia. The growth and persistence of these long-lived clones in some parts of the landscape apparently contribute to the exclusion of other species. However, development of large clones and dominance by L. tridentata are impossible or greatly inhibited in several landscape settings including: (I) extremely young alluvial deposits that have existed for too short a time for large clones to have developed, (2) hillslopes subject to considerable erosional disturbance, and (3) extremely thin soils underlain by impenetrable petrocalcic horizons (caliche), which magnify drought conditions and apparently contribute to episodic mortality in L. tridentata.Soil horizon development as determined by landform age controls the vertical movement and distribution of soil water, in turn affecting the distribution of various plant life forms. Clay-rich (argillic) horizons that have required tens to hundreds of thousands of years to form greatly limit the downward infiltration, vertical distribution, and the temporal availability of soil water. Despite surficial stability for extremely long periods of times, sites with strongly developed argillic horizons lack L. tridentata and are instead occupied by drought-deciduous or succulent plants that are capable of highly seasona...

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Interspecific competition for space limits the distribution and abundance of many benthic insects on stones in a western Montana stream. The sessile caddisfly larva Leucotrichia pictipes is territorial and aggressively eliminates conspecifics and other species from its foraging territories. Territoriality produces an intraspecific pattern of regular spacing and negative spatial associations between Leucotrichia and several other sessile insects: Parargyractis confusalis, Rheotanytarsus sp. and Eukiejferiella sp. Experimental removals of Leucotrichia resulted in higher densities of other sessile species and several mobile insects, including Baetis, Glossosoma, and Simulium. These insects show broad microhabitat overlap with Leucotrichia; competition with Leucotrichia limits their distributions and abundances within otherwise suitable microhabitats. Leucotrichia is the only species that monopolizes large areas of space.Physical disturbances interrupt the formation of competitive monopolies by Leucotrichia. During seasonal reduced flows, Leucotrichia is eliminated from stones and boulders in shallow water. Greater densities of a short-lived sessile species with multiple annual generations occupy this ephemeral spatial resource. On permanently submerged stones, Leucotrichia competitively affects a large part of the benthic fauna. On small stones that overturn with higher frequencies during flooding, Leucotrichia densities are reduced and species' abundances are highly equitable. Large, more stable substrates with greater densities of Leucotrichia are characterized by lower species evenness.

In the Sonoran Desert, the sahuaro cactus (Carnegiea gigantea) is commonly associated with canopies of trees and shrubs: so-called nurse plants. Although mechanisms by which nurse plants facilitate sahuaro establishment have been studied, possible competitive interactions between sahuaro cacti and nurse plants have not been conclusively demonstrated. In this paper I show that the close proximity of sahuaros leads to a relative increase in stem die-back as well as greater mortality in a common nurse tree, the foothill paloverde (Cercidium microphyllum). This interaction appears to accelerate the local loss of individual nurse trees, resulting in a predictable pattern of species replacement.

In the arid southwestern United States, subtle differences in soil horizon development affect seasonal soil hydrology and consequently influence plant performance and community structure. We measured canopy development, population structure, and seasonal ecophysiology (predawn water potential, pd , and midday net photosynthetic assimilation, A net ) of two co-dominant warm-desert shrubs, the evergreen Larrea tridentata and drought-deciduous Ambrosia dumosa, in five Mojave Desert soils varying in surface and sub-surface soil development, and we used process-based soil hydrology modeling output to determine longer-term soil water dynamics underlying soil/plant responses. We hypothesized that ecophysiological performance would covary with plant development, which would reflect soil hydrological characteristics.Among three sites on alluvial fan deposits of different geological ages (Young Alluvial, Ͻ4000 yr BP; Intermediate Alluvial, ϳ12 000 yr BP; Old Alluvial, ϳ40 000 yr BP), total canopy volume of Larrea (cubic meters per 100 m 2 ground area) was highest on the Young Alluvial site, in close agreement with soil modeling results showing that these coarsetextured, weakly developed soils permit deeper water infiltration. In older, stronger developed soils, infiltration and persistence of soil water was sharply reduced, which was reflected by lower individual Larrea plant volumes. However, during peak spring conditions, pd and A net were highest in Larrea at the Intermediate Alluvial site (Ϫ4.2 Ϯ 0.32 MPa and 3.2 Ϯ 0.91 mol·m Ϫ2 ·s Ϫ1 ), where soils had substantial surface and subsurface horizons, and at the Pavement site, where soils had strong surface layers but little subsurface development. Concurrent plant performance at the Young Alluvial site was unexpectedly low (Ϫ4.8 Ϯ 0.49 MPa and 1.7 Ϯ 0.56 mol·m Ϫ2 ·s Ϫ1 , respectively). During summer drought pd and A net remained high in Intermediate Alluvial plants, but were extremely low in Pavement site Larrea (Ϫ8.17 MPa and Ϫ0.04 mol·m Ϫ2 ·s Ϫ1 , respectively), due to curtailed infiltration of summer precipitation. These findings suggest that Larrea growing in older soils experience greater mortality and reduced growth but are not subject to strong intra-specific competition resulting from the persistence of large individuals apparent in younger, coarser textured soils.In contrast to Larrea, density of Ambrosia increased with soil horizon development, but smaller plant sizes resulted in similar canopy volume per area, and identical pd and A net across all soils where it occurred, suggesting greater plasticity to the transmittal of precipitation. These findings show that a strong geomorphology and soils context is essential for understanding the variation in plant responses and vegetation structure in desert environments.

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