A Survey for Early-flowering Rare Plants of Palouse and Canyon Grasslands in Northern Idaho

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We conducted a survey for understudied plant species of intermountain grassland and adjoining dry forest in northern Idaho. The target species were early-flowering plants of conservation concern, primarily Piper’s milkvetch (Astragalus riparius), Palouse milkvetch (Astragalus arrectus), and Asotin milkvetch (Astragalus asotinensis). These were our first surveys for Palouse milkvetch. Surveys for Asotin milkvetch were conducted by Björk (2010) and Pekas et al. (2012). We have surveyed for Piper’s milkvetch in the past, and consider it extirpated from the state. The objective of this survey was to provide distribution and habitat data for these species that could be used in assigning conservation ranks and making recommendations for their protection. The targeted habitats were native bunchgrass grassland and adjoining dry forest. The targeted time frame was early May to late June, when the three Astragalus species were known to flower. Survey sites were determined by habitat and accessibility. Fifteen sites were surveyed by two botanists on foot. Seven additional sites were found by driving roads in the vicinity of herbarium collections. For each rare plant population we completed a standard rare plant observation report form used by the Idaho Natural Heritage Program. The form contained directions to the site, ownership information, population size and extent, associated species, and threats. Approximately 300 ac (120 ha) were surveyed across 22 sites. No Piper’s milkvetch was found. Palouse milkvetch was found at 10 sites. Habitats included bunchgrass grassland and Ponderosa pine (Pinus ponderosa) forest with bunchgrass understory. Two large populations were found in bunchgrass habitats degraded by introduced grasses – either wheatgrass (Elymus sp.) or tall oatgrass (Arrhenatherum elatius). All ten sites were on private land or road rights-of-way. Asotin milkvetch only occurred at one site, but was abundant there over about 250 ac (100 ha). It appears to be closely tied to calcareous substrates, which are of limited extent in the region. Other rare plants found incidentally included Spalding’s catchfly (Silene spaldingii)–a Threatened species, Palouse goldenweed (Pyrrocoma liatriformis), and Palouse thistle (Cirsium brevifolium). These were also documented using Heritage Program methodology. Piper’s milkvetch remains ranked “extirpated” in Idaho. Protection of Palouse milkvetch will be dependent on the cooperation of interested landowners. Protection of Asotin milkvetch will require protection of its Idaho fescue bunchgrass habitat.

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Reference Code: U14LIC01IDUS <br>

Full Citation: Lichthardt, J., K. Gray, and K. Pekas. 2014. A survey for early-flowering rare plants of Palouse and Canyon Grasslands in Idaho. Idaho Natural Heritage Program, Idaho Department of Fish and Game, Boise, ID. 13 pp. plus appendices. <br>

Location: ELECTRONIC FILE - BOTANY: MULTIPLE SPECIES <br>

Keywords: Palouse Prairie, Palouse Grassland, Canyon Grassland, rare plants, Astragalus arrectus, Palouse milkvetch, Astragalus asotinensis, Asotin milkvetch, Astragalus riparius, Piper’s milkvetch, Cirsium brevifolium, Palouse thistle, Pyrrocoma liatriformis, Palouse goldenweed, Silene spaldingii, Spalding’s catchfly <br>