Variation in Vesicular Horizon Expression by Desert Ecoregion in the Western United States.

Surface horizons with vesicular porosity are a widely observed feature of arid and semi-arid shrublands.  The low infiltration rates associated with this horizon lead to low water supply, high salinity, and poor conditions for plant growth, which reinforce the barren “shrub interspace” that occurs between “shrub islands” in desert and desertified landscapes.  This study was conducted to determine trends in vesicular horizon expression across the different desert ecoregions of the western United States and to develop hypotheses to explain the observed trends.  Vesicular horizon expression was quantified using a field index that can be calculated from standard soil morphological description terminology and applied to 1087 Official Series Descriptions distributed across the Chihuahuan, Sonoran, Mojave, Central Basin and Range, and Northern Basin and Range deserts.  The possible role of thermal expansion of vesicular pores in controlling the observed trends was evaluated by applying a theoretical model based on the ideal gas law to data from Remote Access Weather Stations.  Results of this study suggest that vesicular horizon expression is strongest in the cold deserts (i.e., Central and Northern Basin and Range).  The large temperature fluctuations during rainy periods in these deserts may contribute to the strong tendency of these soils to form vesicular horizons.  Climate variability may help explain trends in vesicular horizon expression across the western United States.