Ecology. Michael Begon

      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.

4.6 Monitoring birth and death: life tables, survivorships curves and fecundity schedules

      We turn now to look in more detail at the patterns of birth and death in a variety of life cycles, and at how these patterns are quantified. Often, in order to monitor and examine changing patterns of mortality with age or stage, a life table may be drawn up. This allows a survivorship curve to be constructed, which traces the decline in numbers, over time, of a group of newly born or newly emerged individuals or modules. It can also be thought of as a plot of the probability, for a representative newly born individual, of surviving to various ages. Patterns of birth amongst individuals of different ages are often monitored at the same time as life tables are constructed. These patterns are displayed in age‐specific fecundity schedules.

The underlying principles are explained in Figure 4.9. There, a population is portrayed as a series of diagonal lines, each line representing the life ‘track’ of an individual. As time passes, each individual ages (moves from bottom‐left to top‐right along its track) and eventually dies (the dot at the end of the track). Here, individuals are classified by their age. In other cases it may be more appropriate to split the life of each individual into different developmental stages.

Figure 4.9 Derivation of cohort and static life tables. See text for details.

      Time is divided into successive periods: t₀, t₁, etc. In the present case, three individuals were born (started their life track) prior to the time period t₀, four during t₀, and three during t₁. To construct a cohort life table, we direct our attention to a particular cohort and monitor what happens to them subsequently. Here we focus on those born during t₀. The life table is constructed by noting the number surviving to the start of each time period. So, four were there at the beginning of t₁, two of the four survived to the beginning of t₂; only one of these was alive at the beginning of t₃; and none survived to the start of t₄. The first data column of a cohort life table for these individuals would thus comprise the series of declining numbers in the cohort: 4, 2, 1, 0.

      A different approach is necessary when we cannot follow cohorts but we know the ages of all the individuals in a population (perhaps from some clue such as the condition of the teeth in a species of deer). We can then, as the figure shows, direct our attention to the whole population during a single period (in this case, t₁) and note the numbers of survivors of different ages in the population. These may be thought of as entries in a life table if we assume that rates of birth and death are, and have previously been, constant – a very big assumption. What results is called a static life table. Here, of the 11 individuals alive during t₁, five were actually born during t₁ and are hence in the youngest age group, four were born in the previous time interval, two in the interval before that, and none in the interval before that. The first data column of the static life table thus comprises the series 5, 4, 2, 0. This amounts to saying that over these time intervals, a typical cohort will have started with five and declined over successive time intervals to four, then two, then zero.

      4.6.1