December 2012 Number 1 - Utah Native Plant Society

Calochortiana December 2012 Number 1
Ecology of Rusby’s Milkvetch (Astragalus rusbyi),
a Rare Endemic of Northern Arizona Ponderosa Pine Forests
Judith D. Springer, Michael T. Stoddard, and Daniel C. Laughlin,
Ecological Restoration Institute, Northern Arizona University, Flagstaff, AZ
Debra L. Crisp and Barbara G. Phillips,
Coconino National Forest, Flagstaff, AZ
Abstract. Rusby’s milkvetch (Astragalus rusbyi Greene) is endemic to basaltic soils northwest and west of Flagstaff,
Arizona. Recent interest in this species is due in part to its addition to the U.S. Forest Service Region 3 sensitive species
list in 1999 and its occurrence in ecological restoration projects and proposed fuels reduction projects that involve
tree thinning and prescribed burning. Some of its habitat has been subjected to large wildfires over the last few
decades, and other areas have undergone ecological restoration treatments, while much of its range in ponderosa pine
forest is slated to undergo such treatments in the near future. In a ponderosa pine restoration study area northwest of
Flagstaff, A. rusbyi was an indicator species of remnant grass patches and increased following tree thinning and prescribed
burning. However, in an area less than 3 km away, there appeared to be no relationship to restoration treatments,
trees per ha, pine basal area, or canopy cover, but A. rusbyi did appear to be sensitive to an extreme drought
event in 2002 and may have remained dormant in that year, a pattern that has been observed in other rare Astragalus
species. A. rusbyi has a foliar nitrogen content of 4.4% and a foliar C:N mass ratio of 11. It is classified as a competitive
ruderal species, meaning it is able to compete well with other understory species, but is not very tolerant of
stresses, such as deep shade. We currently do not have a thorough understanding of the ecology of this species, or the
effects of ecological restoration or fuels reduction treatments. In this paper we will discuss ecology of other members
of the genus Astragalus and explore the relationships of A. rusbyi to moisture, vegetation treatments and overstory
mortality.
Astragalus is believed to be the largest genus of
flowering plants in the world, with over 2500 species
worldwide and over 400 species in North America
alone, primarily in arid regions of the western U.S. The
highest diversity in North America is centered in the
Great Basin and on the Colorado Plateau (Barneby
1989, Sanderson 1991). Astragalus species are often
found in marginal habitats or on specialized soil types,
and their geographic ranges are strongly skewed towards
narrow endemism (Barneby 1964, 1989; Sanderson
1991). Along with a limited dispersal ability possessed
by many members of the genus, there is widespread
local differentiation and geographic speciation,
particularly in the areas of the western U.S. where it
achieves the highest levels of diversity (Sanderson
1991, Lesica et al. 2006). However, due to restricted
ranges and habitats, some Astragalus species may exhibit
low genetic variability and reduced fitness from
inbreeding depression (Karron et al. 1988, Allphin et al.
2005, Breinholt et al. 2009). Neoendemism is common
in the intermountain regions of North America where
there are large numbers of both widespread, recently
evolved species, as well as narrowly endemic species,
which are often associated with extreme edaphic conditions
and reduced competition from dominant species
(Lesica et al. 2006). Lesica and his co-authors suggest
that restricted ranges and high local abundances of
neoendemic species may be due more to patterns and
processes of speciation than to ecological tolerance. The
small ranges exhibited by many Astragalus species in
the western U.S. may be due to recent speciation and an
insufficient amount of time for these species to have
increased their ranges significantly (Lesica et al. 2006).
Reticulate evolution may not be widespread in the Astragalus
genus, for many members of the genus appear
to exhibit allopatry (geographic isolation) along with
high levels of local endemism and little hybridization
(Sanderson 1991).
The type specimen of A. rusbyi was collected by
Henry Hurd Rusby on July 2, 1883 on Mt. Humphreys,
near Flagstaff Arizona (Welsh 2007) and was first described
by Edward Lee Greene in 1884 (Greene 1884).
A. rusbyi is a slender perennial averaging 15-40 cm in
height. It also has a fairly deep taproot (D.C. Laughlin,
personal communication, 2008). It grows primarily in
meadows in ponderosa pine forests and in aspen groves
(Barneby 1964, Welsh 2007), but it also may be found
in moderately dense ponderosa pine forests. Populations
are mainly concentrated on basaltic soils in two areas in
northern Arizona: around the San Francisco Peaks
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