December 2012 Number 1 - Utah Native Plant Society

Calochortiana December 2012 Number 1
vetch), a species of ponderosa pine forests in northcentral
New Mexico and south-central Colorado, reproduces
by seed, but plants tend to allocate resources toward
survival of individual plants, and it is believed to
build up root stock reserves when aboveground parts are
consumed (Ladyman 2003). Direct evidence supports an
ability to lie dormant for two years, but monitoring has
not yet been used to establish if it can remain dormant
for a longer period.
Although a number of Astragalus species contain
toxins (such as miserotoxin or swainsonine) or accumulate
selenium, thus making them poisonous to livestock,
there are also many species that are highly desirable to
herbivores. Lesica (1995) found fecundity losses in A.
scaphoides due to livestock and insect herbivory ranging
from 14-90% at two sites. Observations have confirmed
that inflorescences are consumed by ants and
moth larvae. Loss of seeds to weevil predation ranged
from 0-33%. Sugars such as those found in flower nectar
may increase palatability. Lesica also found very low
recruitment, accounting for less than 17% of population
growth. However, despite heavy losses in reproductive
output and low recruitment, populations can continue to
persist and increase in size. He suspects that persistence
of many populations of long-lived plants may be more
reliant on growth and survival of established plants than
on recruitment from seed. Herbivory by cattle and game
has also been observed in A. terminalis (railhead milkvetch),
and seed predation in A. ripleyi may be the cause
of significant seed loss (Heidel and Vanderhorst 1996,
Ladyman 2003). Apparently, like A. scaphoides, this
species has low recruitment rates and allocates a significant
amount of resources toward maintenance of the
root system. A. ripleyi is also consumed by a number of
arthropods (aphids, treehoppers, carpenter ants), rodents
and large mammals, including cattle, elk, deer, sheep
and goats. A ninety percent reduction in fruit production
due to herbivory was observed in A. ampullarioides
(Shivwits milkvetch), which the authors suggest could
have a significant impact on reproductive output (Miller
et al. 2007). The toxicity of A. rusbyi is unknown.
The effects of disturbances, such as tree thinning or
burning, on Astragalus species vary widely. A. ripleyi is
thought to be a “fire evader” rather than a stress tolerator
(Ladyman 2003). Following fire, plants have been
observed in areas where they have not been detected
before, presumably emerging from dormant root systems
underground. However, the stress-tolerator category
may be appropriate, for a pattern of rapid colonization
following fire and drought has also been observed
in this species (Ladyman 2003).
Thinning activities in pinyon-juniper woodlands at
Mesa Verde National Park appeared to cause an increase
in Poa fendleriana (muttongrass) that could result in
undesirable competition impacts on A. schmolliae
(Anderson 2004). Grass seeding post-fire also has the
potential to cause negative impacts on this species.
Drought is deleterious, but it is likely tolerant of fire
because of a deep taproot. However, monitoring indicates
that while fire may confer short-term benefits, it
may also have long-term detrimental impacts (Anderson
2004).
OBSERVATIONS FROM FIELD STUDIES
A. rusbyi has a very small range in northern Arizona,
with the bulk of its population limited to a band approximately
18 x 7 km (11 x 4.5 mi) in size to the west
and north of the San Francisco Peaks and a few scattered
populations to the west (Figure 1). Some of its
habitat has been subjected to large wildfires over the last
few decades; other areas have undergone ecological restoration
treatments (tree thinning and prescribed burning);
and much of its range in ponderosa pine forest is
slated to undergo such treatments in the near future. Increasing
tree densities of ponderosa pine, and a cessation
of frequent fires in ponderosa pine forests since
Euro-American settlement of this area of the Southwest
have been well documented (Covington and Moore
1994, Fulé et al. 1997).
We currently do not have a thorough understanding
of the basic ecology of this species. Additionally, we
have insufficient knowledge of the effects of increased
tree densities, tree thinning, or fire on the population
dynamics. However, some limited information is available
from large landscape scale studies within its range.
Fisher and Fulé (2004) installed 121 20x50 m permanent
monitoring plots on the south side of the San Francisco
Peaks (specifically Agassiz Peak). Plots were established
in five forest types: ponderosa pine, mixed
conifer, aspen, spruce/fir and bristlecone pine. Overstory
measurements and plant community data were
collected between 2000 and 2003. A. rusbyi was found
to be an indicator species for ponderosa pine forest, with
an indicator value of 36.5 (p