December 2012 Number 1 - Utah Native Plant Society

Utah Native Plant Society
Sentry Milkvetch (Astragalus cremnophylax
var. cremnophylax) Update
Janice Busco
Grand Canyon National Park, Grand Canyon, AZ
Abstract. The Grand Canyon endemic Astragalus cremnophylax var. cremnophylax (Sentry milkvetch) was listed as
an endangered species in 1990. There are 725 plants within three populations on the South Rim, all found in shallow
soils upon large flat Kaibab limestone platforms. Habitat specificity and reduced number of plants make Sentry milkvetch
vulnerable to extinction. Recovery plan actions completed in 2008 include seed collection and parking lot removal
to allow habitat restoration and population expansion. Planned recovery plan actions include establishment of
an ex situ population, seed production, and development of techniques for population augmentation and creation of
artificial populations.
The Grand Canyon National Park endemic Astragalus
cremnophylax Barneby var. cremnophylax (Sentry
milkvetch) was listed as an endangered species by the
US Fish and Wildlife Service and protected from trampling
by South Rim sightseers in 1990. There are approximately
725 individuals known in three locations,
all on the South Rim of Grand Canyon. Sentry milkvetch
occurs in shallow, well-drained soils or porous
limestone pavement in crevices and depressions in large
flat Kaibab limestone platforms in unshaded openings in
the pinyon-juniper woodland along the canyon edge.
The underlying bedrock limestone stores water and is
critical to its growth and development (USFWS 2006).
Sentry milkvetch is a small, mat-forming perennial
plant (Figure 1) and has a thick taproot and woody caudex.
Pale purple flowers appear from late April to early
May, with seed set in late May to June. Its tiny seeds
tend to fall in the mat of the plant; therefore the plant
does not spread and remains isolated.
Threats to Sentry milkvetch include small population
size, vulnerability to drought and stochastic events, digging
by ground squirrels and bighorn sheep (Figure 2),
low reproductive capacity, limited seed dispersal, limited
habitat, and reduced genetic diversity and vigor
(Allphin et al. 2005).
Figure 1. A mature Sentry milkvetch with a quarter for
scale.
RECOVERY CRITERIA AND OBJECTIVES
In order to downlist the species, the Sentry Milkvetch
Recovery Plan (USFWS 2006) requires achievement,
maintenance and long-term protection of at least
four viable Sentry milkvetch populations of at least
1,000 individuals each, for a total of at least 4,000 individuals
in the wild. Recovery will be attained when
there are eight viable Sentry milkvetch populations of
1,000 individuals each, with long-term protection.
Figure 2. Bighorn sheep damage to Sentry milkvetch.
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