Modeling the Interdependence of Hydropedology and Root Distributions of Larrea Tridentata in the Mojave Desert

The root distribution of perennial desert plants is ultimately tied to the search for water. Soil development and resulting morphology are functions of time. Morphological features are known to affect the soil water storage and infiltration in arid soils, but they have not been linked with the root distributions of perennial plant. For this study, the depth and lateral spread of Creosote ( Larrea tridentata) roots, a common evergreen shrub of the Mojave Desert, were hypothesized to be coupled with the depth of water penetration and associated hydropedology (i.e. soil age and structure). Soil morphology, soil hydrologic properties, L. tridentata root distributions, and canopy size were measured from the plant canopy to the interspace at three Mojave Desert sites. Each site consisted of a paired geomorphic chronosequence containing a young (Holocene) weakly developed soil and an older (Pleistocene) strongly developed soil on the same geomorphic unit. Particle-size distribution, coarse fragments, bulk density and horizon thickness were used to parameterize the HYDRUS and the Simultaneous Heat and Water (SHAW) models. A 50-year climatic records were developed for each site using the long-term meteorological records and the CLIGEN stochastic weather simulator. Numerical simulations were conducted to evaluate soil water distributions in relation to measure rooting patterns for each chronosequence. Numerical simulations indicated greater depth of water penetration at young sites, coinciding with the measured root distributions. Water penetration was limited to approximately 1.5 m on the young surface and 0.75 on the old interspace surfaces throughout the 50-year simulations. Deeper penetration occurred under all plant mounds, but no drainage below 2 m occurred at any site. Results from infiltrometers showed that younger coarser textured surfaces had saturated conductivities (Ksat) nearly twice that of older surfaces. Plant mound microsites were more sand-rich than interspace locations although K_ {sat} was not significantly different. Larrea tridentata height, volume, root numbers and depth were all greater in weakly developed Holocene soils than older strongly developed Pleistocene soils. Thus, the degree of soil development, and hence the soil age, exerts significant controls on Larrea root patterns by dictating the available soil water.