December 2012 Number 1 - Utah Native Plant Society

Calochortiana December 2012 Number 1
Two or three years of retrieval data are usually sufficient
to determine whether a species will show a negatively
exponential or a linear depletion trajectory. When
Utah ladyfinger milkvetch seeds were placed into retrieval
experiments at three contrasting sites, depletion
trajectories were clearly linear at all three sites (Figure
6). The slope of the depletion trajectory was quite similar
across habitats and showed no clear pattern as a
function of habitat, indicating that rate of loss of hardseededness
was not tightly tied to environmental conditions.
For A. utahensis, regression equations based on the
first two years of retrieval data at each site were able to
predict the approximate size of the remaining fraction in
the subsequent four years. These equations were also
used to estimate maximum longevity of this seed population
in the seed bank, which was about 14 years at the
montane site, 10 years at the foothill site, and 12 years
at the salt desert site. Including the later retrievals in
these regressions did not change them significantly,
even though the fit of the lines for these later dates was
not as good. This retrieval experiment had only two replications
per retrieval date, resulting in considerable error
in the estimate of hard-seededness, especially in later
years, when values dropped far below 100%. The regression
equation may be the best indicator of actual
rate of seed bank depletion under this scenario.
100
Rush Valley - Desert Study Site
80
60
40
Percentage of Initially Viable Seeds
20
0
100
80
60
40
20
0
100
80
Hard Seeds
Hard + Germinable Seeds
9-90 3-91 9-91 3-92 9-92 3-93 9-93 3-94 9-94 3-95 9-95 3-96 9-96
Hobble Creek - Foothill Study Site
9-90 3-91 9-91 3-92 9-92 3-93 9-93 3-94 9-94 3-95 9-95 3-96 9-96
Strawberry - Montane Study Site
60
40
20
0
9-90 3-91 9-91 3-92 9-92 3-93 9-93 3-94 9-94 3-95 9-95 3-96 9-96
Date
Figure 6. Patterns of change over a six-year period in the percentage of hard seeds and hard plus germinable seeds
(total viable seeds) for a collection of Utah ladyfinger milkvetch (Astragalus utahensis) placed in seed retrieval experiments
at three sites. Regression lines are fit to total viable seed percentage values at each site based on the first
two years of retrieval (Rush Valley site: Percentage of viable seeds = -0.022 (days)+100.4, d.f.=13, R 2 = 0.794,
P