Lepidium papilliferum is a rare mustard endemic to Idaho. Variation is widespread across Lepidium papilliferum and the closely-related Lepidium montanum var. montanum, and distinctions between species and subspecies are particularly difficult to quantify (Moseley 1994; Al-Shebaz and Gaskin 2010). There are two main population systems of Lepidium papilliferum in Idaho, both composed of several smaller populations—one set, “Jarbidge,” in Owyhee County; and another, “Snake River Plain (SRP),” in Ada, Canyon, Gem, Payette and Elmore counties. Differences in certain morphological characteristics between these two separate sets of populations have been noted, raising the possibility that the Jarbidge populations actually constitute a distinct variation of Lepidium papilliferum. I measured 22 morphological characteristics on 60 specimens of mature plants and conducted a Principal Components Analysis (PCA) to assess the range of variability for these characters across the different Lepidium papilliferum and Lepidium montanum var. montanum populations. Significant differences were evident between Jarbidge and SRP Lepidium papilliferum populations, suggesting that these indeed constitute two varieties of Lepidium papilliferum. Additionally, preliminary results from a set of SRP collections not seen since 1950 suggest that they constitute a new (likely extinct) describable variety of Lepidium montanum.

Morphological analysis suggests that the rare endemic Lepidium papilliferum (Brassicaceae) constitutes two distinct varieties