Astragalus holmgreniorum is a rare species of milkvetch known by the common names Holmgren milk-vetch and paradox milk-vetch. It is native to a tiny section of desert shrub woodland on the border between Utah and Arizona, in the far northern Mojave Desert. There are six populations remaining. It is a federally listed endangered species.
This is a perennial herb with no stem, just leaves emerging directly from the root crown. The leaves are up to 13 centimeters long and are made up of several leaflets each measuring up to 1.6 centimeters in length. The inflorescence is a raceme of 6 to 16 flowers. Each pea-like flower has a purple corolla about 2 centimeters long. The peduncle bends down and lies on the ground as the fruits develop. Each fruit is a legume pod up to 5 centimeters long.
Threats to the existence of this species include damage to the habitat from off-road vehicles, highway maintenance, power line installation, and grazing, and invasion by introduced species.
Description
The Astragalus holmgreniorum is a herbaceous non-woody perennial that produces small purple flowers during the spring. This stem-less perennial grows close to the ground and after flowering, it dies back to the root crown. Astragalus holmgreniorum grows close to the ground with its leaves pinnately compound, on opposite sides of the stem in a row. The leaves are 4 to 13 cm long and have 9 to 15 leaflets that are 0.6 to 1.6 cm long and are obovately shaped.
The purple flowers are pedicels that consist of short floral stalks that grow on a branched supporting stalk. They have a distinct pea-like shape with 5 petals ranging from 1.8 to 2.4 cm in length and 0.6 to 0.9 cm in width. The flowers grow in groups of 6 to 16. Astragalus holmgreniorum produces fruits that are two-chambered legume pods that keep up to 38 seeds. The pods are 3 to 5 cm long and 0.6 to 0.9 cm wide.
Life cycle
Astragalus holmgreniorum is a short-lived plant. They experience low survivorship from germination to one-year-old juvenile or reproductive adult. It is rare for these plants to live past their third growing season Fewer than 2 percent of seedlings live into their fourth growing season. Many studies have shown that seed production and seed bank persistence are two key life-history transitions that are affecting population survival.
As for the life cycle, seedlings usually emerge in late winter, but for plants entering their second year of growth or older, they appear several weeks before the new seedlings. This occurs in late February or early March. Astragalus holmgreniorum flowers between March and April. They set fruit by the end of April and their seed pods remain until the end of May. The plants then die back to their roots between late May and mid-June Not all plants produce flowers. Usually, plants that have survived at least one dormant season produce them. The seeds are dormant at maturity and they lose their hard seededness over a period of 10 or more years in the soil.
Population size
In 1993, three populations of A. holmgreniorum were discovered and listed. Land managers and botanists have since divided these three populations of A. holmgreniorum into six smaller populations because of the distance between each population. In 2001, when the plant was listed as endangered, the United States Fish and Wildlife Service (FWS) estimated a population size between 10,030 and 11,030 adult plants. The most recent report on A. holmgreniorum, in 2021, estimated a total adult population of 7,100 range-wide.
Geographic distribution and range
The six populations of A. holmgreniorum are found in Washington County, Utah and Mohave County, Arizona in an area just under 25 km2 (10 sq. mi). Three major concentrations of the plant are broken into six populations. The largest concentration is along the Utah-Arizona state border. There are three populations within this grouping: State Line, Gardner Wall, and Central Valley.There are two populations in the city of Santa Clara, UT: South Hills and Stucki Spring. The last population is isolated east of St. George, UT called Purgatory Flat. There is no known data for the historic distribution of A. holmgreniorum. However, there is a past loss of habitats with similar conditions to the suitable habitat of A. holmgreniorum, indicating it may have had a larger range.
Ecology
Diet
Astragalus holmgrenorium is a photoautotrophic plant. This means that it relies on photosynthesis to produce its energy from sunlight.