December 2012 Number 1 - Utah Native Plant Society

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Utah Native Plant Society PERCENTAGE (BASED ON INITIALLY VIABLE SEEDS) 100 80 60 40 20 0 100 80 60 40 20 0 100 80 60 40 20 0 A 8-COLLECTION MEAN 11/91 3/92 7/92 11/92 3/93 7/93 11/93 3/94 7/94 11/94 3/95 7/95 11/95 3/96 7/96 B PIPE SPRINGS 11/91 3/92 7/92 11/92 3/93 7/93 11/93 3/94 7/94 11/94 3/95 7/95 11/95 3/96 7/96 C NORTH OF MILFORD 11/91 3/92 7/92 11/92 3/93 7/93 11/93 3/94 7/94 11/94 3/95 7/95 11/95 3/96 7/96 RETRIEVAL DATE DORMANT NOT CHILL-RESPONSIVE DORMANT DORMANT+GERMINABLE DORMANT+GERMINABLE+FIELD-GERMINATED Figure 5. Patterns of change in the precentage of non-chill-responsive dormant seeds, chill-responsive dormant seeds, dormant plus germinable seeds and viable plus germinated seeds over a five-year period for: (A) The mean of eight seed collections, (B) the Pipe Springs seed collection, and (C) the north of Milford seed collection. Adapted from Meyer and others (1998). 52
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
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December 2012 Number 1 - Utah Native Plant Society

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