Desert piedmonts are a mosaic of interspersed vegetation and interspaces. The distribution of perennial plants in arid regions is ultimately tied to available soil water. Alluvial fan complexes, commonly found in the arid southwest, evolve as a series of geomorphic surfaces of varying age and pedologic development. However, the development of soil structure depends on the proximity to plant canopies where biotic processes such as bioturbation and the accumulation of organic matter under canopies result in a mound-like formation. Interspace soils evolve from the same coarse-textured deposits into smooth, desert pavements underlain by a fine-textured vesicular Av horizon resulting from the accumulation of eolian dust. Infiltration capacity in interspace soils is reduced as a function of geologic age resulting from abiotic process. Differences in soil structure and texture between canopy and interspace microsites can be significant, thus affecting the water balance. In this study, we sought to answer the question: does the spatial structure in soil hydraulic properties from canopy to interspace depend on the soil surface age? To answer this question, transects of unsaturated hydraulic properties [K(h)] were measured radiating away from creosotebush (L. tridentata) shrubs into interspaces along a chronosequence in the Providence Mountains located in the Mojave Desert, USA. Three soils along this intensively studied chronosequence were studied, including a young (Qf6, ~4 ka), intermediate (Qf5, ~10 ka) and old (Qf3, ~75 ka) geomorphic surface. Results/Conclusions
Significant gradients of soil properties were observed from canopy to interspace microsites to 1.2 times the mound diameter, beyond which random processes dominated. Significant (P<0.01) decreases in bulk density and) increases (P<0.01) in organic matter were observed with increasing distance from shrub centers. A consistent trend of decreasing Gardner’s alpha from shrubs was also measured, regardless of soil age. However, the saturated hydraulic conductivity [K(0)] under plant canopies remained relatively constant at ~10 cm hr-1, while K(0) in interspace microsites was dependent on geologic surface age, ranging from >100 cm hr-1 on the younger Qf6, to <1 cm hr-1 on the older Qf3. Therefore, in unsaturated conditions, K(h) underneath the canopy decreases more quickly than in interspace sites. The results show that a predictable spatial structure in K(h) exists in intercanopy/interspace microsites, that K(h) is soil age dependent, and the results can be used in part to illustrate the different hydrologic characteristics between these two microsite locations, even though they differ in spatial locations by just a few 10s of cm.
