December 2012 Number 1 - Utah Native Plant Society

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Utah Native Plant Society production years. In situ sampling provides a quantification of the seed bank at a given point in time, but does not directly address seed bank dynamics. The method of choice for determining how long seeds can persist in the seed bank is the seed retrieval (seed burial or artificial seed bank) experiment. In this method, seeds of known age are introduced into the seed bank in a form that makes their subsequent retrieval and evaluation possible. This usually involves placing them in some sort of mesh bag that allows free passage of water and air, but not seeds. We usually use nylon mesh bags made from mosquito netting, which has a fine enough mesh size to contain even very small seeds. For larger seeds, fiberglass window screening works well. The nylon lasts for many years as long as it is covered with soil so that it cannot photo-degrade, and fiberglass screening can be placed directly on the surface. Some have criticized seed retrieval experiments because they place the seeds in an environment somewhat different from that experienced by seeds in the natural seed bank. An alternative method, or one that can be used in conjunction with a retrieval experiment, is an emergence experiment, where seeds of known age are planted in precise locations using a template or field marking system and monitored for emergence. For both retrieval and emergence experiments, it is important to include adequate replication and to use block designs to avoid localized effects that generate large experimental error. The retrieval method involves destructive sampling, so that it is necessary to include a different set of retrieval bags for each evaluation date. For rare plants, this can involve a prohibitively large number of seeds. But even yearly retrievals for three years, using a few hundred seeds, can provide valuable information that is difficult to obtain in any other way. Processing retrieval bags can be a zen experience, not suitable for those inclined to attention deficit. Particularly when the samples are wet after a germination event, it can take some time to untangle and quantify the germinants. Remaining ungerminated seeds are then incubated under controlled conditions and classified as germinable, dormant, or nonviable. With frequent sampling and adequate replication, it is possible to get a very good picture of the phenology of dormancy loss and germination, and also secondary dormancy induction, if it occurs. It is important to distinguish if possible between seeds that are lost from the seed bank through germination and those that are lost through pre-germination mortality. These two processes have potentially very different demographic consequences. Getting accurate information on field-germinated seeds requires retrieval soon after the germination event, and it can be difficult to anticipate the correct retrieval timing. Laboratory data can be helpful here. For example, if you know the seeds can germinate at near-freezing temperatures, it is reasonable to expect them to germinate during winter in places where the snow cover is deep enough to insulate the seed bed from freezing. This is a common strategy in environments where the soil dries quickly in the spring. PATTERNS OF SEED BANK DEPLETION Seed retrieval data can be examined graphically by plotting the percentage of initially viable seeds still present as viable seeds in the seed bank (actually, in the artificial seed bank) as a function of time in the field. For any given seed population, this will yield a characteristic seed depletion trajectory through time (fig. 1). For species that do not form persistent seed banks, this trajectory will essentially be a line that goes to a value of zero within a year of the initiation of the retrieval experiment (Figure 1a). Species with this type of seed depletion trajectory are said to have transient seed banks. In situ seed bank sampling after germination is complete but before current-year seed rain for species with transient seed banks is expected to yield no viable seeds. Basin wildrye (Leymus cinereus) is an example of a species with nondormant seeds and a transient seed bank (Meyer et al. 1995; Figure 2). Seed collections from three populations were placed in retrieval experiments in early fall at salt desert shrubland, sagebrush steppe, and mountain meadow sites. Germination did not take place in autumn, even though seeds were nondormant, because of their relatively long germination time. All the seeds germinated by the end of the following spring, regardless of population of origin or habitat at the seed retrieval site. Blackbrush (Coleogyne ramosissima) is another example of a species with a transient seed bank (Meyer and Pendleton 2004; Figure 3). Its seeds are dormant at dispersal in mid-summer but have cue-responsive dormancy, losing dormancy both in the dry state at high summer temperatures and during moist chilling. As a consequence, if winter rainfall is adequate, seeds germinate to high percentages during early spring. A few seeds may carry for a year over if the winter is unusually dry, but this is the exception rather than the rule. Seeds with cue-responsive dormancy that responds to a disturbance-associated cue show a similar pattern to those that form transient seed banks, except that there is an extended time delay prior to the seed bank-depleting germination event (Figure 1 b). These species are difficult to study in retrieval experiments, because the cue, such as fire or soil disturbance, is not only unpredictable but also would tend to destroy a retrieval experiment. Information on this type of seed bank depletion trajectory generally comes either from emergence experiments or from in situ seed bank studies that show abrupt 48
Calochortiana December 2012 Number 1 100 80 A 100 80 B 60 60 Viable Seed Percentage 40 20 0 0 1 2 3 4 5 6 7 8 9 10 100 80 60 40 20 C 40 20 0 0 1 2 3 4 5 6 7 8 9 10 100 80 60 40 20 D 0 0 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 Retrieval Year Figure 1. Schematic seed bank depletion trajectories for: (A) a species with a transient seed bank and a depletion trajectory that reaches zero within one year, (B) a species with cue-responsive dormancy and a seed bank that persists until a specific dormancy-breaking cue is received, (C) a species with a short-persistent seed bank, a constant loss rate, and a negatively exponential depletion trajectory, and (D) a species with cue-non-responsive seeds and a linear seed bank depletion trajectory. decreases in seed density following receipt of the germination cue. In many herbaceous perennial species, most of the seeds are programmed to germinate during the first year following production, but some possess a mechanism permitting carryover for at least a year, even when conditions for dormancy release and germination the first year are optimal. These species are said to have shortpersistent seed banks. This germination response pattern results in a seed depletion trajectory that is essentially negatively exponential (Figure 1c). If rate of loss of a cohort of seeds in the seed bank is constant across years, this is the type of seed depletion trajectory that will be generated. For example, if 80% of the seeds are lost the first year, 80% of the remaining 20% are lost the second year, and 80% of the remaining 4% are lost the third year, this would generate a negatively exponential loss trajectory. Lewis flax (Linum lewisii) is an example of a species that may exhibit a negatively exponential loss trajectory (Meyer and Kitchen 1994; Figure 4). Seed dormancy loss and germination phenology in this species and its close relative L. perenne (Euopean blue flax) also vary as a function of both population of origin and habitat at the seed retrieval site. In a two-year retrieval experiment at three sites, the “Appar” release of European blue flax had nondormant seeds that formed only a transient seed bank, regardless of retrieval site habitat. In contrast, the montane Strawberry seed collection of Lewis flax was largely dormant at the initiation of the retrieval experiment and required chilling to become nondormant (Figure 4). It lost dormancy and germinated completely by the end of the first spring at its site of origin in the mountains, exhibiting the transient seed bank pattern. It carried over a substantial fraction through the end of the second year at the foothill and especially the salt desert site. Seeds placed outside of their environmental context can show very different germination patterns than those placed in the habitat of origin. These seeds did not receive sufficient chilling to break dormancy at the drier sites and tended to form a persistent seed bank under those conditions. The foothill Provo Overlook seed collection of Lewis flax was nondormant at the initiation of the retrieval experiment, but contained a fraction that could be induced into secondary dormancy early during chilling the first year (Figure 4). This resulted in a divergence of seed sub-populations, so that a sizeable fraction germi- 49
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December 2012 Number 1 - Utah Native Plant Society

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