December 2012 Number 1 - Utah Native Plant Society

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Utah Native Plant Society VIABLE SEED PERCENTAGE 100 80 60 40 20 0 100 80 60 40 20 0 100 80 60 40 20 0 PINTO COLLECTION 9/20 10/2011/2012/20 1/20 2/20 3/20 4/20 5/20 6/20 INDIAN CANYON COLLECTION 9/20 10/2011/2012/20 1/20 2/20 3/20 4/20 5/20 6/20 STRAWBERRY COLLECTION 9/20 10/2011/2012/20 1/20 2/20 3/20 4/20 5/20 6/20 DATE WHITEROCKS DESERT SITE POINT OF THE MOUNTANI FOOTHILL SITE STRAWBERRY MONTANE SITE Figure 2. Seed bank depletion trajectories for three collections of basin wildrye (Leymus cinereus) placed in retrieval experiments at montane, foothill, and salt desert sites. All seeds germinated within a year of initiation of retrieval experiments. Adapted from Meyer and others (1995). nated the first year in all three habitats, but 20-30% remained in dormancy through the spring and carried over to the second year, generating the characteristic negatively exponential pattern. Three additional years of retrieval data for the Provo Overlook collection at these sites showed continued gradual decline in viable seed numbers each year (Meyer unpublished data). Seed bank depletion for shadscale (Atriplex confertifolia) exhibited a pattern somewhat similar to that of Lewis flax, but in this case the pattern resulted in a more persistent seed bank (Figure 5). There was no germination at all during the first year in the field, but a sizeable germination pulse occurred the second spring after retrieval initiation. During eight subsequent years in the field, the size of the remaining viable seed fraction slowly diminished until it was at or near zero for most collections (Figure 5; Meyer unpublished data). Laboratory germination experiments help to explain this pattern. Shadscale seeds are dormant at dispersal and tend to be nonresponsive to the chilling cue that triggers them to emerge at the correct time for establishment in early spring (Meyer et al. 1998). They must after-ripen in the dry state to become responsive to the chilling cue, and the rate of increase in the chilling-responsive fraction is an exponential function of temperature during dry storage (Garvin and Meyer 2003). This delays germination for at least a year, because the high summer temperatures needed for after-ripening are first experienced only after the first winter in this autumn-ripening species. Each summer another fraction becomes chilling-responsive, and this fraction is able to germinate the following spring if its chilling requirement is met. The resulting depletion trajectory is like a slow-motion version of the negative exponential pattern for species with short-lived seed banks. Once the first pulse of germination is complete, the trajectory becomes essentially linear through time. In contrast to the slow-exponential depletion trajectory seen for shadscale, species with truly cue nonresponsive dormancy tend to exhibit a linear depletion trajectory, with no large germination pulse in any one year and certainly not in the early years (Figure 1d). Hard-seeded legumes like Utah ladyfinger milkvetch (Astragalus utahensis) are typical of this group. Hardseededness refers to the inability of a seed to imbibe water because of physical barriers imposed by the seed coat or endocarp. It is often possible to trigger synchronous germination in hard-seeded species by injuring the seed coat, and it has been thought that such scarification must be a part of the natural dormancy-breaking regimen. Under field conditions, however, hard-seededness seems to be very gradually lost through time without any specific scarification mechanism. Because the seeds lose hard-seededness at different rates, this functions to spread germination over many years. 50
Calochortiana December 2012 Number 1 PERCENTAGE OF INITIALLY VIABLE SEEDS 100 75 50 25 0 7-91 10-91 1-92 4-92 7-92 RETRIEVAL DATE % DORMANT SEEDS % VIABLE UNGERMINATED SEEDS % VIABLE + GERMINATED SEEDS Figure 3. Change through time in the dormant seed fraction, the viable seed fraction, and the viable plus germinated seed fraction for blackbrush (Coleogyne ramosissima) seeds placed in a retrieval experiment in the habitat of origin at Arches National Park. All seeds germinated within a year of initiation of the experiment soon after dispersal. Adapted from Meyer and Pendleton (2004). Figure 4. Change through time during a two-year period for dormant seed percentage, viable ungerminated seed percentage, and viable plus germinated seed percentage for three collections of Linum placed in retrieval experiments at montane (Strawberry), foothill (Hobble Creek), and salt desert (Rush Valley) study sites. Adapted from Meyer and Kitchen (1994). 51
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December 2012 Number 1 - Utah Native Plant Society

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