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Seed dormancy is an extremely widespread phenomenon in flowering plants. The young embryo ceases development whilst still attached to the mother plant and enters a phase of suspended activity, usually losing much of its water and becoming dormant in a desiccated condition. In a few species of higher plants, such as some mangroves, a dormant period is absent, but this is very much the exception – almost all seeds are dormant when they are shed from the parent and require special stimuli to return them to an active state (germination).

Dormancy in plants, though, is not confined to seeds. Many species accumulate dormant bud banks analogous to the seed banks produced by other species. In one study of tallgrass prairies in north‐eastern Kansas, USA, for example, it was estimated that more than 99% of new tiller production arose from below‐ground vegetative buds rather than from seed (Benson & Hartnett, 2006); and in another prairie study, in Montana, USA, the differential responses of grass species to fire at different seasons of the year, especially their release by fire from dormancy, were crucial in driving the overall dynamics and community structure (Russell et al., 2015).

Indeed, the very widespread habit of deciduousness is a form of dormancy displayed by many perennial trees and shrubs. Established individuals pass through periods, usually of low temperatures and low light levels, in a leafless state of low metabolic activity.

innate, enforced and induced dormancy

Three types of dormancy have been distinguished:

1 Innate dormancy is a state in which there is an absolute requirement for some special external stimulus to reactivate the process of growth and development. The stimulus may be the presence of water, low temperature, light, photoperiod, fire (see previously) or an appropriate balance of near‐ and far‐red radiation. Seedlings of such species tend to appear in sudden flushes of almost simultaneous germination. Deciduousness is also an example of innate dormancy.

2 Enforced dormancy is a state imposed by external conditions (i.e. it is consequential dormancy). For example, the Missouri goldenrod Solidago missouriensis enters a dormant state when attacked by the beetle Trirhabda canadensis. Eight clones, identified by genetic markers, were followed prior to, during and after a period of severe defoliation. The clones, which varied in extent from 60 to 350 m2 and from 700 to 20 000 rhizomes, failed to produce any above‐ground growth (i.e. they were dormant) in the season following defoliation and had apparently died, but they reappeared 1–10 years after they had disappeared, and six of the eight bounced back strongly within a single season (Figure 4.8). Generally, the progeny of a single plant with enforced dormancy may be dispersed in time over years, decades or even centuries. Seeds of Chenopodium album collected from archaeological excavations have been shown to be viable when 1700 years old (Ødum, 1965).

3 Induced dormancy is a state produced in a seed during a period of enforced dormancy in which it acquires some new requirement before it can germinate. The seeds of many agricultural and horticultural weeds will germinate without a light stimulus when they are released from the parent; but after a period of enforced dormancy they require exposure to light before they will germinate. For a long time it was a puzzle that soil samples taken from the field to the laboratory would quickly generate huge crops of seedlings, although these same seeds had failed to germinate in the field. It was a simple idea of genius that prompted Wesson and Wareing (1969) to collect soil samples from the field at night and bring them to the laboratory in darkness. They obtained large crops of seedlings from the soil only when the samples were exposed to light. This type of induced dormancy is responsible for the accumulation of large populations of seeds in the soil. In nature they germinate only when they are brought to the soil surface by earthworms or other burrowing animals, or by the exposure of soil after a tree falls.

Figure 4.8 Dormancy in goldenrods is enforced by defoliation. The histories of eight Missouri goldenrod (Solidago missouriensis) clones (rows a–h). Each clone’s predefoliation area (m²) and estimated number of ramets is given on the left. The panels show a 15‐year record of the presence (shading) and absence of ramets in each clone’s territory. The arrowheads show the beginning of dormancy, initiated by eruption of the beetle Trirhabda canadensis and defoliation. Reoccupation of entire or major segments of the original clone’s territory by postdormancy ramets is expressed as the percentage of the original clone’s territory.

Source: After Morrow & Olfelt (2003).

Most of the species of plants with seeds that persist for long in the soil are annuals and biennials, and they are mainly weedy species – opportunists waiting (literally) for an opening. They largely lack features that will disperse them extensively in space. The seeds of trees, by contrast, usually have a very short expectation of life in the soil, and many are extremely difficult to store artificially for more than one year. The seeds of many tropical trees are particularly short‐lived: a matter of weeks or even days. Amongst trees, the most striking longevity is seen in those that retain the seeds in cones or pods on the tree until they are released after fire (many species of Eucalyptus and Pinus). This phenomenon of serotiny protects the seeds against risks on the ground until fire creates an environment suitable for their rapid establishment.