CPC Plant Profile: Sentry Milkvetch
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Plant Profile

Sentry Milkvetch (Astragalus cremnophylax var. cremnophylax)

Full shot of small, prostrate plant w/ penny for scale Photo Credit: Sue Rutman
Description
  • Global Rank: T1 - Critically Imperiled
  • Legal Status: Federally Endangered
  • Family: Fabaceae
  • State: AZ
  • Nature Serve ID: 150633
  • Date Inducted in National Collection: 03/08/1989

Sentry milk-vetch is a rock-hugging plant that literally grows in sites that overlook the Grand Canyon. It's scientific name means 'gorge watchman'. The population where this plant was first collected was threatened by trampling from the millions of visitors to Grand Canyon National Park. In 1990, the Park Service erected a fence to re-route traffic around the sentry milk-vetch. Since protection, this population has thrived, increasing in numbers 4-fold. New locations where plants grow along the rim have been discovered; one site has only 2 plants growing on a Kaibab limestone platform about the size of a dinner table. A second site, on the north rim of Grand Canyon, is fairly remote and free from trampling pressure. (Maschinski & Rutman 1993)

Participating Institutions
Updates
Center for Plant Conservation
  • 11/25/2021
  • Reintroduction

In 1991, the Arboretum at Flagstaff conducted a second sentry milk-vetch seed augmentation study. Using observations of where natural seed germination occurred, 230 seeds collected from mature plants in 1991 were sown. Sites for seed placement included soil at least 5 cm deep, soil in cracks in limestone, and near nurse plants or sticks to provide temporary shelter. No germination occurred in reintroduction plots.

Center for Plant Conservation
  • 11/25/2021
  • Reintroduction

In July 1990, the Arboretum at Flagstaff augmented the largest known population of sentry milkvetch with 196 seeds that had been collected in 1989. We sowed seeds 7 replicates of 7 seeds into each of 4 microhabitats: 1) unshaded powdered limestone/fine gravel in pockets in the bedrock; 2) unshaded cracks in the bedrock; 3) 2-3 cm of existing soil on the east side of a tree or shrub; and 4) 2-3 cm of existing soil on the southwest side of a tree or shrub. Despite good rains in the summer of 1990, by Sept 1990 only 10% of seeds had germinated and only 2 seedlings (1 %) produced true leaves. By April 1991, only one seedling survived in a crack in limestone.

  • 08/21/2020
  • Propagation Research

The Center for Research of Endangered Wildlife is investigating using tissue culture techniques to propagate sentry milk-vetch. Reproductive success and the possibility of inbreeding depression in the wild populations is being investigated by Allphin et al. (in prep).

  • 08/21/2020
  • Genetic Research

The genetic diversity of the live sentry milk-vetch plants in the National Collection is being compared to the genetic diversity of the wild population (Maschinski and Travis, in prep).

  • 08/21/2020
  • Living Collection

The genetic diversity of the live sentry milk-vetch plants in the National Collection is being compared to the genetic diversity of the wild population (Maschinski and Travis, in prep).

  • 08/21/2020
  • Propagation Research

There is a great need to investigate ways to propagate large numbers of individuals if reintroduction is to ever be a possibility. Tissue culture is one of the areas being investigated.

Nature Serve Biotics
  • 05/02/2017

Endemic to Arizona. Known with certainty from 1 populations on the South Rim of the Grand Canyon and 1 on the North Rim. The South Rim population has been exhibiting severe reproductive problems with very little recruitment since 2004. Because the plants grow on the edge of the rim - where there are spectacular views - several subpopulations had been severely trampled by Park visitors, but the area has been fenced and these long-lived plants seem to be thriving, even though reproduction is sporadic and reduced due to inbreeding depression believed to be caused by the earlier trampling. ?

Joyce Maschinski, Ph.D.
  • 01/01/2010

Threats include: Foot traffic Low genetic variation Poor seed production (NatureServe Explorer 2002)

Joyce Maschinski, Ph.D.
  • 01/01/2010

The three populations of this species are located within Grand Canyon National Park. Two known populations occur on the South Rim of the Grand Canyon, where one site has 2 living individuals and the other site has approximately 1000. On the north rim on several Kaibab limestone fingers jutting into the canyon, there are approximately 1500 individuals. The north rim populations are undergoing taxonomic scrutiny to determine if they are in fact the same variety. (Maschinski & Rutman 1993)

Joyce Maschinski, Ph.D.
  • 01/01/2010

The genetic diversity of the live sentry milk-vetch plants in the National Collection is being compared to the genetic diversity of the wild population (Maschinski and Travis, in prep). The Center for Research of Endangered Wildlife is investigating using tissue culture techniques to propagate sentry milk-vetch. Reproductive success and the possibility of inbreeding depression in the wild populations is being investigated by Allphin et al. (in prep).

Joyce Maschinski, Ph.D.
  • 01/01/2010

Grand Canyon National Park personnel monitor the populations of sentry milk-vetch to determine whether populations are stable, growing or declining. Fencing the largest south rim population has been a great and inexpensive conservation success story. (Maschinski & Rutman 1993)

Joyce Maschinski, Ph.D.
  • 01/01/2010

The number of secure populations in the wild is so few that the recovery plan (in prep as of 5/15/01) suggests introductions to new habitats as possible safeguards against extinction. Generating enough propagules for introduction is a great research need.

Joyce Maschinski, Ph.D.
  • 01/01/2010

There is a great need to investigate ways to propagate large numbers of individuals if reintroduction is to ever be a possibility. Tissue culture is one of the areas being investigated.

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Nomenclature
Taxon Astragalus cremnophylax var. cremnophylax
Authority Barneby
Family Fabaceae
CPC Number 391
ITIS 192438
USDA ASCRC4
Common Names sentry milkvetch
Associated Scientific Names Astragalus cremnophylax var. cremnophylax
Distribution Known only from the south and north rim of Grand Canyon within the National Park (Maschinski & Rutman 1993).
State Rank
State State Rank
Arizona S1
Habitat

This species grows on a white layer of Kaibab limestone with little or no soil in unshaded openings in the pinyon-juniper-cliffrose plant community (Maschinski & Rutman 1993).Sentry milk-vetch grows in association with rock mat (Petrophytum caespitosum). (Maschinski & Rutman 1993)

Ecological Relationships

These plants occupy a unique niche - tiny cracks in limestone pavement. Plants flower in response to moisture in the spring and fall. There are two bouts of seed set. (Maschinski & Rutman 1993)

Pollinators
Common Name Name in Text Association Type Source InteractionID

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