CPC Plant Profile: Humboldt Milkvetch
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Plant Profile

Humboldt Milkvetch (Astragalus agnicidus)

Astragalus agnicidus has clusters of small white flowers and pinnately compound leaves. Photo Credit: Andrea Pickart
Description
  • Global Rank: G2 - Imperiled
  • Legal Status: N/A
  • Family: Fabaceae
  • State: CA
  • Nature Serve ID: 132940
  • Date Inducted in National Collection: 04/04/1991

Throughout history, humans have systematically eradicated plants and animals that seem to pose a threat to their livestock or crops. Bears, wolves, coyotes, and even condors have all been victims of ranchers' guns and traps. Astragalus agnicidus has fallen victim to the rancher's shovel and herbicide use. Fortunately, recent changes in scientific thought and public opinion have helped people to see these species not as "threats" but as integral parts of native ecosystems, and therefore, worthy of our protection. For over seventy years, the only known site containing Astragalus agnicidus was a privately owned 8-acre ranch in Humboldt County, CA. In the 1930s and 40s, the owners logged and cleared portions of their land in order to raise sheep. When several sheep were found dead, the concerned owners enlisted the help of a local County Agriculture Extension employee. He identified the "culprit" and encouraged them to eradicate the plant (although many members of the genus Astragalus are toxic to livestock, some now believe that in this instance, the real killer may have been a lupine). But the damage was done, both literally and figuratively. When the species was formally named in 1957 it was given the specific epithet of agnicidus, which means lamb killer (the lamb killing locoweed is a much less appealing common name than Humboldt milkvetch). The landowners appeared to have been successful in their quest to eradicate this plant, as in 1954, only one plant was found. No more were found and it was presumed extinct until it was re-discovered. In 1985, bulldozers were used to clear a fallen tree that was considered a fire hazard. Two years later, 25 Astragalus agnicidus plants were found in the area and another 75 were found in a nearby clearing. The canopy opening created by the fallen tree coupled with the soil disturbance caused by the heavy equipment that removed the tree created suitable conditions for the seeds, which had lain dormant in the soil for at least 30 years, to germinate (see the research and ecology sections for more). Like many others who now see the value in conserving all aspects of native ecosystems, the same family that once worked to eradicate the plant is now enthusiastic about conserving it. The population is currently protected by a voluntary agreement between the landowner and The Nature Conservancy and the land is being managed for the species' continued survival. Astragalus agnicidus was removed from the Federal Endangered and Threatened Candidate list in 1996 (U.S. Fish and Wildlife Service, 1996) largely because of this protection agreement. In 1999 and 2000, two new sites containing A. agnicidus were discovered on timberland that had recently been logged. It appears that apparently extinct species can arise, phoenix-like, out of the ashes of oblivion from naturally occurring as well as artificial seed banks.

Participating Institutions
Updates
  • 09/01/2020
  • Orthodox Seed Banking

Based on an September 2020 extract of the California Plant Rescue Database, California Botanic Garden holds 2 accessions of Astragalus agnicidus in orthodox seed collection. There are as many as 654 seeds of this species in their collection - although some may have been used for curation testing or sent to back up.

  • 09/01/2020
  • Orthodox Seed Banking

Based on an September 2020 extract of the California Plant Rescue Database, Rae Selling Berry Seed Bank & Plant Conservation Program holds 17 accessions of Astragalus agnicidus in orthodox seed collection. There are as many as 6900 seeds of this species in their collection - although some may have been used for curation testing or sent to back up.

  • 08/19/2020
  • Genetic Research

Genetic analysis of one population of A. agnicidus utilizing isozymes (enzymes) and the role of mating system in maintaining genetic variability were examined by Bencie (1997). Astragalus agnicidus plants analyzed showed low genetic variability. Flowers were experimentally open-pollinated or self-pollinated, and five fitness variables were measured: seed set, seed weight, germination, survival, and seedling weight. Fitness was reduced to some extent in the selfed progeny for all variables, but the difference was only significant in regards to seedling survival.

  • 08/19/2020
  • Reproductive Research

Germination requirement study. 30 minutes scarification in sulfuric acid followed by 20 days of stratification at 4C lead to an 88.5% germination rate. Less than 5% of seeds germinated if scarification was not performed (Hiss 1990). Light requirement study. High light treatments showed greater seedling emergence, earlier leaf emergence, and produced more robust plants compared to low or moderate light treatments (Enberg 1990). Estimation of reproductive output (number of pods and number of seeds per pod) for one year (Pickart and Stauffer 1994). Genetic analysis of one population of A. agnicidus utilizing isozymes (enzymes) and the role of mating system in maintaining genetic variability were examined by Bencie (1997). Astragalus agnicidus plants analyzed showed low genetic variability. Flowers were experimentally open-pollinated or self-pollinated, and five fitness variables were measured: seed set, seed weight, germination, survival, and seedling weight. Fitness was reduced to some extent in the selfed progeny for all variables, but the difference was only significant in regards to seedling survival.

  • 08/19/2020
  • Propagation Research

Germination requirement study. 30 minutes scarification in sulfuric acid followed by 20 days of stratification at 4C lead to an 88.5% germination rate. Less than 5% of seeds germinated if scarification was not performed (Hiss 1990). Light requirement study. High light treatments showed greater seedling emergence, earlier leaf emergence, and produced more robust plants compared to low or moderate light treatments (Enberg 1990). Estimation of reproductive output (number of pods and number of seeds per pod) for one year (Pickart and Stauffer 1994). Genetic analysis of one population of A. agnicidus utilizing isozymes (enzymes) and the role of mating system in maintaining genetic variability were examined by Bencie (1997). Astragalus agnicidus plants analyzed showed low genetic variability. Flowers were experimentally open-pollinated or self-pollinated, and five fitness variables were measured: seed set, seed weight, germination, survival, and seedling weight. Fitness was reduced to some extent in the selfed progeny for all variables, but the difference was only significant in regards to seedling survival.

  • 08/05/2020
  • Seed Collection

Based on an August 2020 extract of the California Plant Rescue Database, Rae Selling Berry Seed Bank & Plant Conservation Program has collected 17 seed accessions of Astragalus agnicidus from 5 plant occurrences listed in the California Natural Diversity Database. These collections together emcompass 158 maternal plants

Nature Serve Biotics
  • 05/02/2017

Previously not seen since 1954, Astragalus agnicidus was rediscovered in 1987. As of 2005, it is known only from two counties in California and about 50 occurrences. Many new populations have been found, but few are ranked ""good"" or better. Many plants are reported, though numbers are somewhat in question and their long-term viability is unknown. Astragalus agnicidus occurs in forested areas where the canopy has recently been opened. It is threatened by logging treatments such as herbicide application.

Edward Guerrant, Ph.D.
  • 01/01/2010

Ecological succession (Pickart et al. 1991). Plant destruction because of toxic properties (Pickart et al. 1991). Extensive logging may actually harm populations (Bencie 2001). This may cause the stored soil seed bank to germinate all at once. Any

Edward Guerrant, Ph.D.
  • 01/01/2010

3 sites. Two in Humboldt Co, one in Mendocino Co. One population was discovered in 1987, one in 1999, one in 2000. The population discovered in 1999 contained over 5000 individuals (CA Natural Diversity Database). While populations may be large, they are transient and their survival is dependent on suitable habitat conditions.

Edward Guerrant, Ph.D.
  • 01/01/2010

Germination requirement study. 30 minutes scarification in sulfuric acid followed by 20 days of stratification at 4C lead to an 88.5% germination rate. Less than 5% of seeds germinated if scarification was not performed (Hiss 1990). Light requirement study. High light treatments showed greater seedling emergence, earlier leaf emergence, and produced more robust plants compared to low or moderate light treatments (Enberg 1990). Estimation of reproductive output (number of pods and number of seeds per pod) for one year (Pickart and Stauffer 1994). Genetic analysis of one population of A. agnicidus utilizing isozymes (enzymes) and the role of mating system in maintaining genetic variability were examined by Bencie (1997). Astragalus agnicidus plants analyzed showed low genetic variability. Flowers were experimentally open-pollinated or self-pollinated, and five fitness variables were measured: seed set, seed weight, germination, survival, and seedling weight. Fitness was reduced to some extent in the selfed progeny for all variables, but the difference was only significant in regards to seedling survival.

Edward Guerrant, Ph.D.
  • 01/01/2010

Listed as Endangered by the State of California. There is no federal status. Voluntary protection agreement between owner of the site discovered in 1987 and The Nature Conservancy of California (Landowner Contact and Registry). Fencing of largest concentrations of plants to exclude herbivores (Pickart, Hiss, and Enberg 1991) Recent surveys revealed two new populations on private land managed for timber production (Bencie 2001). Monitoring and mitigation of a large population discovered in 2000 (Bencie 2001).

Edward Guerrant, Ph.D.
  • 01/01/2010

Study population biology (Pickart et al. 1991). Management in concert with logging to create periodic openings in the canopy and to scarify the ground (Berg and Bittman 1988). Openings created when trees are selectively removed could be managed for A. agnicidus, and then be allowed to revert to forest while other openings are created in nearby areas (Pickart, Hiss, and Enberg 1991). Populations should be large enough to promote high outcrossing rates. If populations are too small, there is a high risk of allele fixation through drift (Bencie 1997). Study chemical properties (it may be more protected if it is found to have pharmacological value) (Berg and Bittman 1988). Other members of the genus Astragalus have been found to stimulate macrophages and aid in general immunity (Moore 1993). Protect pollinators and their habitat (Bencie 1997).

Edward Guerrant, Ph.D.
  • 01/01/2010

Collect and store seed from each of the three known populations. Collect as new plants emerge due to local disturbances. Because Astragalus agnicidus appears to have an extensive soil seed bank, it will be necessary to collect over a long period of time in order to encompass the genetic variability present in the population.

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Photos
Nomenclature
Taxon Astragalus agnicidus
Authority Barneby
Family Fabaceae
CPC Number 362
ITIS 25404
USDA ASAG
Common Names Humboldt milkvetch | Humboldt milk-vetch
Associated Scientific Names Astragalus agnicidus
Distribution CA: Outer North Coast Range
State Rank
State State Rank
California S3
Habitat

An early successional species, Astragalus agnicidus prefers disturbed sites such as logged ridges, open canopy wooded areas and scarified ground.

Ecological Relationships

Dormant seeds are able to persist in the soil seed bank for long periods of time, measured in decades. Openings in the canopy and soil scarification allow the seeds to germinate and the plants to flourish (Pickart, et al.. 1991). Observations suggest that A. agnicidus is a short-lived perennial with an estimated life span of 5-10 years (Bencie, 1997). Because of the ecology of this species, it is probable that the populations remain relatively small through a repeating sequence of 1) seedling growth as suitable habitat is opened by disturbance, followed by 2) above ground disappearance due to loss of suitable habitat through tree encroachment (Bencie, 1997). This is a dynamic system, with transient populations thriving when conditions are right in one place and then disappearing when conditions change. The extensive soil seed bank allows plants to quickly colonize an area when a localized disturbance occurs. This scenario may lead to a history of successive population """"bottlenecks"""" thereby leading to low levels of genetic variation (Bencie, 1997). Disturbances probably came in the form of fires or tree falls.Astragalus agnicidus has a mixed mating system--it is self-compatible but requires an insect mediated tripping mechanism to facilitate pollination (Bencie, 1997). This allows for both out-crossing and self-pollination as insects may bring pollen either from neighboring plants or from flowers on the same plant. The most common insect visitor is a native bumblebee (Bombus sp.) that visits A. agnicidus flowers to forage for nectar and pollen (Bencie, 1997). Observations indicate that the bees systematically visit all open flowers on one inflorescence before traveling to a nearby inflorescence (usually on the same plant). This most likely leads to a high level of self-pollination via transfer of pollen between flowers of the same plant (Bencie, 1997). But, because an insect is required to """"trip"""" the pollination mechanism, some amount of outcrossing does occur as bees bring pollen from neighboring plants. This helps reduce inbreeding and maintain low levels of heterozygosity (Bencie, 1997).

Pollinators
Common Name Name in Text Association Type Source InteractionID
Bees
Anthophorine bees Anthophora Floral Visitor Link
Bumble bees Bombus mixtus Confirmed Pollinator Link
Bumble bees Bombus sitkensis Confirmed Pollinator Link
Bumble bees Bombus vosnesenskii Confirmed Pollinator Link
Leaf-cutting bees Osmia dolerosa Confirmed Pollinator Link
Leaf-cutting bees Osmia laeta Confirmed Pollinator Link
Leaf-cutting bees Osmia tristella Confirmed Pollinator Link
Flies
Bee flies Bombylius major Floral Visitor Link
Syrphid flies Small hover fly Floral Visitor Link
Syrphid flies Syrphid flies Floral Visitor Link
Other
Wasp Floral Visitor Link

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