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Fig. 3.5 Iceland purslane (Koenigia islandica), a species limited by time for successful reproduction, growing in patchy moss communities in Iceland. This species, approximately 3 cm high (see 1 cm scale), is the only annual species to survive in the Arctic and for the size of the plant succeeds in producing relatively large seeds in a very short growing season.

Fig. 3.5 Iceland purslane (Koenigia islandica), a species limited by time for successful reproduction, growing in patchy moss communities in Iceland. This species, approximately 3 cm high (see 1 cm scale), is the only annual species to survive in the Arctic and for the size of the plant succeeds in producing relatively large seeds in a very short growing season.

in deciduous shade-tolerant trees, is prolonged survival in the non-reproductive state while available light is close to the illumination compensation point (the intensity of illumination at which the oxygen evolved by photosynthesis is equivalent to the oxygen consumed by respiration - usually about 1% of full daylight). At such low light intensities, where energy acquisition from photosynthesis is only just adequate to meet the needs of maintenance respiration, growth is almost negligible, yet many saplings succeed in surviving for long periods in this suppressed state.

Most animals require conditions that permit sexual reproduction if their populations are to remain viable. This is not always the case for plants. In the Arctic and Boreal regions numerous species can survive as vegetative clones for centuries and even millennia. Aspens (Populus tremuloides), bracken (Pteridium aquilinum) and various arctic sedges (Carex spp.) are but three examples where clonal age has been estimated in millennia. In the Siberian Arctic estimates of over 3000 years have been made for the longevity of clones of Carex ensifolia ssp. arctisiberica (Jonsdottir etal., 2000). More examples are coming to light as molecular markers present new and remarkable insights into the longevity of many clonal species. The ability to lie dormant as seeds, or for perennating plants to miss several growing seasons while buried under mud or ice, together with the capacity of many established plants to withstand starvation, demonstrates that plants are often endowed with a certain degree of indifference to a constant and uninterrupted provision of resources, a characteristic which facilitates their survival in marginal habitats (see Sections 1.3.2 and 3.6.2).

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