December 2012 Number 1 - Utah Native Plant Society

Utah Native Plant Society
A Taxonomic Revision of Astragalus lentiginosus var. maricopae
and Astragalus lentiginosus var. ursinus
Two Taxa Endemic to the Southwestern United States
Jason A. Alexander, curator
Utah Valley University Herbarium, Utah Valley University, Orem, UT
and research associate, Wesley E. Niles Herbarium, University of Nevada, Las Vegas
Abstract. Two taxa in the Astragalus lentiginosus complex of Section Diphysi, Astragalus lentiginosus var. maricopae
and A. lentiginosus var. ursinus, have been historically overlooked by taxonomists and have had an uncertain
taxonomic status. Astragalus lentiginosus var. maricopae is a highly endangered endemic (likely totaling less than
5,000 individuals primarily due to habitat loss from development) and confined to a small region of igneous and granitic
alluvial fans in the vicinity of Scottsdale and the Verde River drainage in northern Maricopa Co., Arizona. The
second variety, A. lentiginosus var. ursinus, is a highly restricted limestone talus endemic (totaling less than 5,000
individuals) and is confined to a small region of the Beaver Dam Mountains in Mohave Co., Arizona and Washington
Co., Utah. Two morphological principal coordinates analyses (PCoA) were used on vouchers of these two varieties
and nearly 150 specimens from related taxa in Section Diphysi. The results of the first PCoA showed that the floral
and pod morphology of A. lentiginosus var. maricopae contributed highly to its distinctiveness when compared to
other varieties, especially A. lentiginosus var. wilsonii (its geographically closest relative ). These results combined
with field observations indicate that A. lentiginosus var. maricopae is a morphologically unique and highly endangered
taxon that is threatened by disturbance and development throughout its known range. Based on the second
PCoA, A. lentiginosus var. ursinus trends toward smaller pods and flowers than its geographically nearest relative (A.
lentiginosus var. mokiacensis) and is herein recognized at the varietal level. Astragalus lentiginosus var. ursinus is
more ecologically specialized than A. lentiginosus var. maricopae. However, most of the population is in a wilderness
area and is threatened by recreational activities, not extirpation by suburban development.
Marcus E. Jones (1923) was one of the first taxonomists
to attempt to write a comprehensive treatment of
species previously considered related to Astragalus lentiginosus
Douglas ex Hook., a species described almost
a century earlier by Hooker (1831). After Hooker's publication,
new species were added to this complex by Asa
Gray (1849, 1865) and Sereno Watson (1871), but the
complex remained poorly known until the late 19th century.
In 1898, Jones proposed a set of new combinations,
placing some species from Section Diphysi A.
Gray (sensu Gray 1863) as varieties within a greatly
expanded concept of Astragalus lentiginosus (Jones
1898). Jones' concept of A. lentiginosus remained relatively
unchanged and culminated in his Revision of
North-American Species of Astragalus (Jones 1923)
which was ignored by taxonomists for two decades
(Barneby 1964). Jones' core varietal concepts in Astragalus
lentiginosus are largely accepted today, mainly due
to the eloquence and precision of Rupert C. Barneby in
his 1945 and 1964 monographs.
Barneby (1945) was the first to make explicit and
unambiguous the relationships between the 40 varieties
of Astragalus lentiginosus and their placement into Astragalus
Section Diphysi. Although many new varieties
would be described and old ones further refined over the
next 60 years (see Barneby 1956, 1989, Isely 1998,
134
Kearney & Peebles 1960, Munz & Keck 1959, Welsh
1978, 1993, 2003), the recognition of 42 varieties remains
even in the latest monograph (Welsh 2007). Despite
the many revisions of this complex, the best description
of the extremes of the diversity within the A.
lentiginosus complex was published by Barneby in his
first monograph:
"The varieties of A. lentiginosus, as known at
present, are not of equal stature: some, indeed,
are doubtfully distinct, while others appear to
be isolated and might, in another group of the
genus, pass as species of the first rank. It is noticeable,
however, that every example of the latter
type is comparatively little known, whereas
all those represented by extensive collections
are found to intergrade at some point with a
related variety" (1945: 70).
The Astragalus lentiginosus complex can be divided
into two major groups based on pod morphology. The
first group, comprising the majority of the varieties of
this complex, is distinguished by the presence of a deciduous,
bladdery inflated, biloculate, ovoid to orbicular
pod. These characters were viewed as the most representative
by Barneby (1964) and used to distinguish