December 2012 Number 1 - Utah Native Plant Society

Utah Native Plant Society
(primarily on the southern and western sides of the
Peaks) and also the vicinity of Kendrick Mountain
(Figure 1). Collections from areas to the south, including
Oak Creek Canyon and Yavapai County are of uncertain
validity. It is ranked G3 (vulnerable) by Nature-
Serve (2009) and is on the U.S. Forest Service sensitive
species list for Region 3 (Southwestern Region).
A. rusbyi has been placed in the section Strigulosi,
which contains approximately 35 species found mainly
in the Mexican highlands north to Arizona and New
Mexico and generally associated with oak and pine forests
(Barneby 1964, Spellenberg 1974). Section Strigulosi
is thought by some authors to be the most primitive
group of Astragalus species in North America. The majority
of the evidence, including research on chromosome
numbers and more recent molecular phylogenetic
data, currently points to an Old World origin for Astragalus,
presumably in the steppes and mountains of
southwestern and south-central Asia and the Himalayan
Plateau (Spellenberg 1976, Wojciechowski 2005).
Astragalus rusbyi has a chromosome number of 11
(2n=22) (Spellenberg 1974). Morphologically, it appears
to be most closely related to two other members of
section Strigulosi, A. egglestonii and A. longissimus,
with which it shares technical features (Barneby 1964).
Like most members of Strigulosi, A. rusbyi flowers
from mid-summer onward into the fall, varying in abundance
in response to the amount and timing of summer
rains. Astragalus rusbyi is differentiated from these
species of Astragalus by minor differences in characters
of its pendulous, stipitate, bilocular, trigonously compressed
pods (Barneby 1964). However, the ranges of
these three species do not overlap and they maintain
geographic isolation from one another with no observed
intermediate populations.
ECOLOGY OF ENDEMIC WESTERN SPECIES
Astragalus is a very large genus with little ecological
information available for the vast majority of species.
However, many uncommon and rare species share characteristics
with each other that may directly affect monitoring
and conservation planning for A. rusbyi. For example,
some species exhibit prolonged vegetative dormancy,
such as A. scaphoides (Bitterroot milkvetch) and
A. sinuatus (Whited’s milkvetch) of the sagebrush
steppe of the Pacific Northwest, or dormancy during dry
years, as demonstrated by A. schmolliae (Schmoll’s
milkvetch) of Mesa Verde National Park, in southwestern
Colorado (Anderson 2004, Gamon 1995, Lesica and
Steele 1994). A. scaphoides plants may utilize prolonged
vegetative dormancy as a bet-hedging strategy in
an effort to conserve resources while avoiding the risk
inherent in funneling resources into aboveground
growth (Gremer et al. 2012). Lesica (1995) found that
A. scaphoides plants may remain dormant belowground
for as long as five years before reappearing. Dormancy
may be inferred in other species, such as A. ripleyi, due
to an increased number of visible plants in years of
above average precipitation (Ladyman 2003). Barneby
(1964) notes that A. rusbyi, like most members of section
Strigulosi varies in “vigor and abundance in proportion
to amount and timing of summer rains,” but prolonged
vegetative dormancy has not been established.
Many Astragalus species exhibit large underground
storage organs (A. scaphoides), a vigorous creeping root
system (A. cicer – chickpea milkvetch), or long taproots
(A. ampullarioides – Shivwits milkvetch) (Jennifer Gremer,
unpublished data; Mark Miller, personal observation;
Horvath 2002). These extensive rooting systems
exhibited in the genus may be linked to observed patterns
of vegetative dormancy. A. ripleyi (Ripley’s milk-
Figure 1. Known range of Astragalus rusbyi in northern Arizona based on herbarium collections and research studies
(figure compiled by J.E. Crouse and D. L. Crisp).
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