December 2012 Number 1 - Utah Native Plant Society

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Utah Native Plant Society Studying the Seed Bank Dynamics of Rare Plants Susan E. Meyer, USDA Forest Service Shrub Lab, Provo, UT Abstract. Seed bank studies are important for understanding the population biology of rare plants, especially shortlived species from unpredictable environments. Seed retrieval studies are used to determine how long seeds persist in the soil and how seed dormancy changes through time. Even short-term studies (three to five years) can help determine whether the seed bank is transient or persistent and characterize the shape of the seed depletion trajectory. Laboratory germination studies can provide clues about seed bank persistence, and in situ seed bank studies can quantify seed bank size, but only retrieval studies provide the information needed to inform population viability analysis. In order to assess the status of a rare plant population, botanists need some way to measure and quantify demographic (life history) variables. This can be as straightforward as quantifying the number of plants present in an area using a simple monitoring protocol, or it can involve classifying the individuals present by age, size or stage (for example, seedling, juvenile, vegetative adult, reproductive adult). It can also include an evaluation of the potential contribution of an individual to the next generation, that is, its reproductive output in terms of numbers of seeds or vegetative ramets produced and the quality of those seeds or ramets. If the plants are permanently marked and their attributes quantified on multiple occasions, it becomes possible to perform quantitative life history analysis (Brigham and Schwartz 2003). One of the goals of life history analysis for plants of conservation concern is the formal inclusion of life history information into mathematical models that predict the probability of population persistence under different scenarios. The term used for the creation, execution, and interpretation of such models is population viability analysis (PVA; Beissinger and McCullough 2002). Population viability analysis for plants is a relatively new area of research (Doak et al. 2002). One feature of plant life history that makes PVA difficult is the presence of a largely invisible life cycle stage, namely seeds in the soil seed bank. This paper deals both with assessing the need for including soil seed bank data in life history analysis for a particular plant species and tackling the problem of obtaining seed bank data when necessary. WHEN ARE SOIL SEED BANKS IMPORTANT? Doak and others (2002) provide an excellent discussion of problems associated with assumptions about seed banks in the context of PVA for plants. They first present the two extremes for plant life history strategy. At one extreme is a life history much like that of the vertebrates for which PVA was first developed. In these organisms, for example, giant sequoias, newborns are subject to high and variable mortality, the juvenile phase lasts a long time and is characterized by slow growth and increasing annual survival, and adults are longlived. Plants with this life history usually produce seeds that have a very short tenure in the seed bank, less than a year. At the other extreme are plants that have short and risky juvenile stages and short life spans as adults, but whose seeds have high dormancy and high survivorship in the soil, resulting in a large, persistent seed bank. For plants in the former category, the seed bank can safely be ignored in PVA, because the transition from seed production to seedling takes place within a single year. But for plants that produce long-lived seeds, the dynamics of the seed bank can play a very important role in population biology, and false assumptions based on inadequate data can lead to major problems with the resulting PVA. Doak and others (2002) identify two plant life history attributes most critical for deciding how important accurate measurement of seed bank dynamics is likely to be. The first, as implied above, is plant longevity, and the second is the effect of a variable environment on adult survival and reproduction. A long and stable mature stage with multiple opportunities for reproduction is likely to minimize the importance of a persistent seed bank. On the other hand, short-lived plants which are at variable risk of mortality and/or reproductive failure as adults are likely to be associated with long-lived seeds and persistent seed banks. Different combinations of positions along these two gradients (life span and environmental risk to adults) result in different assessments of the importance of seed banks. Even relatively longlived shrubs may have persistent seed banks if there is a risk of catastrophic mortality to adult plants, for example through fire or epidemic disease. And even annuals may have relatively short-lived seeds if the chances of adult survival to seed production are high. Another source of clues about the importance of the persistent seed bank is in the germination behavior of 46
Calochortiana December 2012 Number 1 the seeds in a laboratory setting. If the seeds are nondormant at dispersal and cannot readily be induced into dormancy, it is unlikely that they will be able to form a persistent seed bank under field conditions. Similarly, if the seeds are dormant at dispersal but lose dormancy in response to an environmental cue likely to be encountered before the optimum germination time within the year, they are also unlikely to form a persistent seed bank. This type of dormancy is called cue-responsive or predictive dormancy. It functions to time germination optimally within the year following production by allowing the seeds to sense their environment and respond appropriately. Many spring-germinating species in the temperate zone have this type of seed dormancy, with moist-chilling or cold stratification that simulates winter conditions as the cue. This prevents precocious germination the autumn following production but allows complete germination the following spring. Not all cue-responsive dormancy is associated with short-lived seed banks, however. Sometimes the cue is associated with an episodic event such as fire (Keeley 1987), or tillage that exposes weed seeds to light (Baskin and Baskin 1985). Such seeds may persist in the soil for many years, but will germinate synchronously when the cue is received. These seeds germinate readily in response to a laboratory-administered cue. The trick is in recognizing that such a cue is unlikely to be encountered under field conditions within any particular year, so that seeds with this kind of cue-responsive dormancy form a persistent seed bank. The best laboratory clue that seeds of a species are likely to form a persistent seed bank under field conditions is the presence of cue-non-responsive dormancy. These seeds will germinate to only small percentages no matter what dormancy-breaking treatment is applied. Sometimes it is possible to determine what pretreatment is needed to make the seeds become responsive to a particular cue, but more often even this is very difficult. The individual seeds are programmed to come out of dormancy at different times over a protracted period, and there seems to be no way to speed or circumvent this process. Often the only way to break cue-nonresponsive dormancy is to resort to unnatural treatments like injuring the seed, and sometimes even these draconian measures fail to trigger germination. METHODOLOGIES FOR STUDYING SEED BANKS For many people, the most obvious way to begin a study of seed bank dynamics is to attempt to quantify the in situ seed bank. This involves taking seed bank samples from the field and somehow measuring the number of seeds these samples contain. A common measuring method involves spreading the soil samples out in shallow pans, applying water, and counting and removing germinants as they emerge. Usually the soil sample is turned multiple times to encourage subsequent flushes of germination and emergence, and the seed bank is considered depleted when no further emergence occurs. Obviously, this methodology involves numerous assumptions about the dormancy status of the seeds, because seeds that do not germinate and emerge as seedlings are not included in the quantification. Sometimes the seedling emergence methodology includes multiple cycles of application of dormancy-breaking cues, for example, moist-chilling, which increases the chances of complete germination. But for truly cue-non-responsive species, these methodologies are clearly inadequate. Another commonly applied method for quantifying the in situ seed bank involves flotation, usually using a chemical such as potassium carbonate at high molarity. This method has been shown to yield more seeds on average than the emergence (germination) methodology (Ishikawa-Goto and Tsuyuzaki 2004), but the extracted seeds are no longer viable. This inability to distinguish between viable and nonviable seeds and to evaluate seed dormancy status represents a major disadvantage to this method. A third method for quantifying in situ seed banks is rather labor-intensive, but avoids some of the problems associated with the previous two methods (Meyer et al. 2007). The samples are dry-screened (or wet-screened, depending on the soil type) using sieve sizes that eliminate fine soil and large particles such as gravel and root chunks. The remaining fraction, which contains the seeds of interest, is hand-processed to remove the seeds, which can then be subjected to germination testing and/ or viability evaluation. This method works best for medium to large seeds. Its accuracy is increased by inclusion of numerous small samples rather than fewer large samples, given an equal volume of sampled material. Sampling regime is a critical aspect of in situ seed bank evaluation, because the lateral distribution of seeds in soil is usually extremely heterogeneous. This means that stratified sampling regimes and often very large sample sizes are needed to get replicable data. It is highly advisable before launching into such a study to make sure that it is well-designed and will yield the desired information. In order to quantify the persistent seed bank, it is important to sample after germination is complete for the year but before any input of seed rain from current-year production, so that only seeds at least a year old will be sampled. Sampling the in situ seed bank cannot provide any information about seed bank persistence beyond a single year, because there is no way to know the age of seeds removed from the samples. Seeds from the previous production year could be the only ones present prior to dispersal of current-year seeds, or there could be an accumulation of seeds from an unknown number of prior 47
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December 2012 Number 1 - Utah Native Plant Society

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