Contrasting Soils and Processes of Shrub Island – Interspace Landscapes In Southeastern California.

In arid landscapes, shrubs are often widely spaced due to competition for limited water.  The resulting pattern of “shrub islands” separated by “interspaces” results in two distinct, yet intimately associated, soil environments, each of which plays an important role in the hydrology and ecosystem behavior of arid landscapes.  The shrub islands support a rich and active biological community that mixes the soil, whereas the biologically impoverished interspace sites are physically stable.  Both sites may collect dust, though its fate in the soils is different.  If the interspaces have desert pavement, the dust accumulates under the pavement to form a vesicular horizon that severely restricts infiltration. Shrubs are very effective at trapping dust, but it is mixed by burrowing rodents to yield a highly permeable soil.  During rainstorms, water runs off from interspaces and infiltrates around shrubs.  Not only does this soil pattern preferentially deliver water to plant roots, it also leaches salt from their root environment and concentrates it in surrounding interspace soils. The salts include high levels of nitrate such that, even though the organic matter content is much higher in shrub island soils, nitrogen levels are orders of magnitude higher in interspace soils. In order to fully understand these arid landscapes, both soil environments must be considered.