Dormancy migration in time

An organism gains in fitness by dispersing its progeny as long as the progeny are more likely to leave descendants than if they remained undispersed. Similarly, an organism gains in fitness by delaying its arrival on the scene, so long as the delay increases its chances of leaving descendants. This will often be the case when conditions in the future are likely to be better than those in the present. Thus, a delay in the recruitment of an individual to a population may be regarded as 'migration in time'.

Organisms generally spend their period of delay in a state of dormancy. This relatively inactive state has the benefit of conserving energy, which can then be used during the period following the delay. In addition, the dormant phase of an organism is often more tolerant of the adverse environmental conditions prevailing during the delay (i.e. tolerant of drought, extremes of temperature, lack of light and so on). Dormancy can be either predictive or consequential (Müller, 1970). Predictive dormancy is initiated in advance of the adverse conditions, and is most often found in predictable, seasonal environments. It is generally referred to as 'diapause' in animals, and in plants as 'innate' or 'primary' dormancy (Harper, 1977). Consequential (or 'secondary') dormancy, on the other hand, is initiated in response to the adverse conditions themselves.

6.5.1 Dormancy in animals: diapause

Diapause has been most intensively studied in insects, where examples occur in all developmental stages. The common field grasshopper Chorthippus brunneus is a fairly typical example. This annual species passes through an obligatory diapause in its egg stage, where, in a state of arrested development, it is resistant to the cold winter conditions that would quickly kill the nymphs and adults. In fact, the eggs require a long cold period before development can start again (around 5 weeks at 0°C, or rather longer at a slightly higher temperature) (Richards & Waloff, 1954). This ensures that the eggs are not affected by a short, freak period of warm winter weather that might then be followed by normal, dangerous, cold conditions. It also means that there is an enhanced synchronization of subsequent development in the population as a whole. The grasshoppers 'migrate in time' from late summer to the following spring.

Diapause is also common in species with more than one generation per year. For instance, the fruit-fly Droso-phila obscura passes through four generations per year in England, but enters diapause during only one of them (Begon, 1976). This facultative diapause shares important features with obligatory diapause: it enhances survivorship during a predictably adverse winter period, and it is experienced by resistant diapause adults with arrested gonadal development and large reserves of stored abdominal fat. In this case, synchronization is achieved not only during diapause but also prior to it. Emerging adults react to the short daylengths of the fall by laying down fat and entering the diapause state; they recommence development in response to the longer days of spring. Thus, by relying, like many species, on the utterly predictable photoperiod as a cue for seasonal development, D. obscura enters a state of predictive diapause that is confined to those generations that inevitably pass through the adverse conditions.

Consequential dormancy may be expected to evolve in environments that are relatively unpredictable. In such circumstances, there will be a disadvantage in responding to adverse conditions only after they have appeared, but this may be outweighed by the advantages of: (i) responding to favorable conditions immediately after they reappear; and (ii) entering a dormant state only if adverse conditions do appear. Thus, when many mammals enter hibernation, they do so (after an obligatory preparatory phase) in direct response to the adverse conditions. Having achieved 'resistance' by virtue of the energy they conserve at a lowered body temperature, and having periodically emerged and monitored their environment, they eventually cease hibernation whenever the adversity disappears.

6.5.2 Dormancy in plants

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. For example, as the sand sedge Carex arenaria grows, it tends to accumulate dormant buds along the length of its predominantly linear rhizome. These may remain alive but dormant long after the importance of photoperiod the shoots with which they were produced have died, and they have been found in numbers of up to 400-500 m-2 (Noble et al., 1979). They play a role analogous to the bank of dormant seeds produced by other species.

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 Three types of dormancy have induced dormancy 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 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 6.8). 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 archeological excavations have been shown to be viable when 1700 years old (0dum, 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

(a)

60 m2

15,000

(b)

150 m2

7500

(c)

350 m2

20,000

(d)

170 m2

3000

(e)

40m2

3700

(f)

150 m2

12,000

(g)

150m2

12,000

(h)

150m2

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100 0

100 0

100 0

100 0

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Figure 6.8 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 a Trirhabda canadensis eruption 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. (After Morrow & Olfelt, 2003.)

1990

Year

1995

2000

Figure 6.8 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 a Trirhabda canadensis eruption 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. (After Morrow & Olfelt, 2003.)

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.

Seed dormancy may be induced by radiation that contains a relatively high ratio of far-red (730 nm) to near-red (approximately 660 nm) wavelengths, a spectral composition characteristic of light that has filtered through a leafy canopy. In nature, this must have the effect of holding sensitive seeds in the dormant state when they land on the ground under a canopy, whilst releasing them into germination only when the overtopping plants have died away.

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 1 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.

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