December 2012 Number 1 - Utah Native Plant Society

Calochortiana December 2012 Number 1
D.C. Laughlin (personal communication, 2009) collected
trait data on 137 ponderosa pine understory species,
including A. rusbyi, and found that, on average, it
has a higher specific leaf area (24 mm 2 /mg) and higher
nitrogen and phosphorous concentrations in its foliar
tissue (4.4% and 0.18%, respectively) (Figure 4). Because
of the high nitrogen content, it has a relatively
high net photosynthetic rate. On average, it also has a
lower leaf dry matter content (0.19 mg/mg) and foliar
C:N mass ratio (10.7). Combined with its high photosynthetic
rate and comparatively tall stature (with an
average height of 31 cm), it is able to compete well with
other understory species, but is not very tolerant of
stresses such as deep shade. The combined trait data
place it in the category of a competitive ruderal species
(Hodgson et al. 1999).
CONCLUSIONS
As previously mentioned, little is known of the ecology
of this locally abundant but narrowly endemic species,
and much of its known range is slated to undergo
various thinning and prescribed burning activities
in the very near future. Many Astragalus species are
long-lived, recruit slowly by seed, and maintain longlived
seeds in the soil seed bank. Whether A. rusbyi utilizes
a similar strategy is unknown but could be determined
from additional research. We currently do not
have a thorough understanding of the population dynamics
of this species over time. Rigorous long-term
demographic monitoring would be valuable in determining
population baselines and is essential for understanding
the ecology and conservation and habitat needs of
this species. Such monitoring can also reveal patterns
that might be caused by precipitation fluctuations. From
the information available, it appears to have a large taproot,
which should give some resistance to the impacts
of drought and fire, but high-intensity fire or burning at
peak growth times could be detrimental. It has shown
positive to no effects from tree thinning and prescribed
burning operations in ecological restoration research
studies, but additional research that specifically targets
this species would be useful before we can draw firm
conclusions.
Figure 3. Proportion of permanent monitoring plots through time containing Astragalus rusbyi at an ecological restoration
study area near Flagstaff, AZ. Treatments were randomly assigned within each block and included (a) no thinning,
no burning (control), (b) 1.5-3 tree replacement (high-intensity thinning), (c) 2-4 tree replacement (mediumintensity
thinning), and (d) 3-6 tree replacement (low-intensity thinning). All treatment units were thinned in 1999
and subsequently treated with prescribed fire in spring 2000 (Block 3) and spring 2001 (Blocks 1 and 2).
161