Lepidium papilliferum, known as slick spot peppergrass, grows on visually distinct microsites within remnant communities of relatively undisturbed and moderately disturbed Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis). This species is a Federal Category 2 candidate for listing as Threatened or Endangered. Its known range is southwestern Idaho where rapid habitat alteration is occurring. Typical Lepidium microsites are variously described as "slick spots", "playettes", and "natric sites", implying a soil-chemistry explanation for the typical sparsity of vegetation and the low water permeability sometimes observed. This study examines soil factors that affect the distribution of Lep/oVu/n-microsites. It describes the soil morphology and chemistry of Lepidium-sWck spots and nearby shrub interspaces of three Lepidium populations that occur on distinct landscape units of the Lower Snake River Plain. Slick spots are best distinguished by a near-surface distribution of soluble sodium salts, thin vesicular surface crusts and shallow well-developed argillic horizons. High salinity and high sodium concentrations relative to soluble calcium and magnesium occur within 40 cm of the soil surface and the argillic horizons probably qualify as natric soil horizons. Searches for new Lepidium populations and habitat might best be focussed on remnant Pleistocene land surfaces where current soil surveys document occurrences of saline and natric soil series among the general matrix of non-saline Aridisols. This study, conducted in 1991, compares soil morphology and non-visual soil properties of Lep/d/um-bearing slick spots with those of shrub interspaces without Lepidium. The purpose is to identify what soil properties, if any, define the landscape-level distribution of Lepidium and to identify whether these properties are correlated with visual site-cues. Study Area This study examines three geographically distinct Lepidium populations, identified as Simco Road, Tenmile Ridge and Kuna Butte (Figure 1). They are documented by the Idaho Conservation Data Center as occurrences #015, #024, and #032, respectively, and are A-ranked populations, meaning that plant numbers are relatively high and that the surrounding shrubsteppe community is of high quality (Moseley, 1994). These populations occur at elevations from 2,900 to 3,300 ft (884 to 1 ,000 m) on distinct geologic units of the western Snake River Plain. The Kuna Butte population is on low, gently sloping (2 to 5%) basalt ridges. The soils formed on late-Pleistocene surfaces of loess or silty alluvium over basalt (Collette, 1980; Othberg, 1994). Lava boulders, smaller lava rocks and gravels are scattered across the terrain. The Tenmile Ridge population occurs on steeper nose and sideslopes of the oldest, early Pleistocene alluvial terrace of the Boise River. The Tenmile gravel formation is typically capped by 1 to 2 m of loess and generally has a well developed silica-cemented hardpan called a duripan (Othberg, 1994), although the loess cap appears to be significantly...
