Ecological Responses of Two Mojave Desert Shrubs to Soil Horizon Development and Soil Water Dynamics

Abstract

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, ψpd, and midday net photosynthetic assimilation, Anet) 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, <4000 yr BP; Intermediate Alluvial, ∼12 000 yr BP; Old Alluvial, ∼40 000 yr BP), total canopy volume of Larrea (cubic meters per 100 m2 ground area) was highest on the Young Alluvial site, in close agreement with soil modeling results showing that these coarse-textured, 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, ψpd and Anet were highest in Larrea at the Intermediate Alluvial site (−4.2 ± 0.32 MPa and 3.2 ± 0.91 μmol·m−2·s−1), 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 (−4.8 ± 0.49 MPa and 1.7 ± 0.56 μmol·m−2·s−1, respectively). During summer drought ψpd and Anet remained high in Intermediate Alluvial plants, but were extremely low in Pavement site Larrea (−8.17 MPa and −0.04 μmol·m−2·s−1, 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 ψpd and Anet 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.