When a seed is dispersed away from its parent, it may be either dormant (in a resting state and be unable to germinate) or non-dormant (able to germinate) (Fig. 6.6). Both dormant and non-dormant seeds eventually become incorporated into the soil as part of the 'seed bank' - think of it as a repository of seeds that will be withdrawn over time. Seeds that are able to remain in the seed bank for long periods of time do so because they are dormant. In the following sections we discuss seed banks and seed dormancy. Seed germination is discussed later in this chapter.
The seed bank is referred to as 'dispersal in time' because it provides the same essential benefit as dispersal through space - it increases the chance that at least some seeds will survive to germinate under suitable environmental conditions. Unfortunately for plants, the seed bank is not a benign place and seeds cannot survive indefinitely. Seed survival decreases because of failed germination, physiological death, disease, her-bivory, pathogens, adverse soil conditions (pH and moisture) and deep burial (Simpson et al., 1989). Burial in the seed bank offers a brief respite at best for most individuals. Seeds in the seed bank are continually redistributed through secondary dispersal. Human activity is another mechanisms of seed redistribution. For example, tillage can alter the distribution (Fig. 6.7a) and density (Fig. 6.7b) of seeds in the soil, often species specifically (Clements et al., 1996).
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