Conservation assessment for triple-ribbed milkvetch (Astragalus tricarinatus, Fabaceae) in Joshua Tree National Park

Fraga, Naomi; La Doux, Tasha; Prine, Linda; Harding, Mitzi; Hoines, Joshua

Triple-ribbed milkvetch (Astragalus tricarinatus) is a perennial endemic to southern California, occurring in Riverside and San Bernardino counties in the transition zone between the Mojave and Sonoran deserts. A. tricarinatus was federally listed as endangered in 1998 due to narrow distribution, few individuals, and vulnerability to ground disturbance. Habitat and distribution of this species was thought to be limited to a few scattered individuals in canyons and sandy washes of southeastern San Bernardino Mountains and western Little San Bernardino Mountains. Fewer than 100 individuals had been documented by 1998. Since 2004, surveys have yielded more information on distribution, habitat, and abundance, and we now estimate 3100 individuals in 45 occurrences. We review A. tricarinatus biology, ecology, distribution, taxonomic history, conservation status, and provide management recommendations for populations occurring in Joshua Tree National Park (JOTR) based on current information and recent studies. We present original research on seed germination, population genetics, and a habitat model for A. tricarinatus, emphasizing plants occurring within JOTR. A monitoring protocol is provided for a JOTR pilot study to assess demographics and reproductive capacity. Germination trials indicate seeds require scarification to germinate. Clipping the seed coat at the hilum yielded 97–100% germination, while scarifying the seed coat using sandpaper yielded 68–96% germination. No treatment, a cold water soak, cold moist stratification, and boiling water followed by 24 hours of water soak had the lowest success rates at 0–4% germination. The population genetic study used inter simple sequence repeat markers and found individual populations were generally cohesive. However, the populations did not form reciprocally monophyletic population groups, suggesting either historic or contemporary gene flow among all three populations sampled. The GIS habitat model used elevation, slope, aspect, soil mapunit, and vegetation association to assign probability values for potential habitat within JOTR. These datasets helped characterize occupied habitat for A. tricarinatus. The model output was used to prioritize areas for ground-truthing, yielding two new occurrences of the species. Conservation efforts must focus on building on baseline data and on research, field surveys, and long-term monitoring. Proposed research includes expanding the current population genetic study across the range and among central and peripheral populations. Research should include a phylogenetic study to determine the closest relative of A. tricarinatus and its phylogenetic uniqueness, and to investigate drought tolerance across the species range. Management practices in JOTR will include a standardized monitoring protocol (presented in this report) and continued surveys of known occurrences. The habitat model will be used to search for new populations. A management plan identifying a Conservation Area with Core Habitat will be developed in accordance with the Coachella Valley Multiple Species Habitat Conservation Plan and the USFWS 5-year Review, to ensure protection of ecological processes and protection of corridors critical for the survival of A. tricarinatus.

Conservation assessment for triple-ribbed milkvetch (Astragalus tricarinatus, Fabaceae) in Joshua Tree National Park

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