Environmental Limits To Reproduction

Limits to plant distribution can arise either from a failure to grow or an inability to reproduce. In many cases a failure to reproduce may be a more common response to environmental limitations than a failure to grow, probably because reproductive success requires more than just the development of viable seed. Reproduction is accomplished only when there is successful establishment of a new generation of reproductive individuals. The continued existence of viable populations in marginal areas is therefore dependent on accomplishment of flowering, fertilization, viable seed production, germination, and the establishment of new individuals in regions where the environment is uncertain and variable. These basic requirements for completion of the reproductive cycle illustrate the appropriateness of measuring genetic fitness in the Darwinian sense as the ability of an individual to contribute genetically to the next generation.

Fortunately, at least for some perennial plants, the arrival of the next generation is not as urgent as it is in animals with their genetically fixed and discrete lifespans. For plants in marginal situations failure to reproduce sexually due to climatic deterioration does not necessarily result in abandonment of the habitat.

'Adapt or migrate' is sometimes presented as the only possible choice open to organisms that live in fluctuating environments where adversity may make sexual reproduction not possible for long periods. Reasonably regular sexual reproduction may be necessary to perpetuate populations of most animals, but many plant species are able to survive for centuries and even millennia without reproducing sexually. There are even situations where plants have survived considerable climatic change without migrating. Examination of the disjunct distribution of some relict plant populations frequently finds populations with a long history of inhabiting marginal habitats with very low rates of reproductive success. The Norwegian mugwort (Artemisia norvegica) is an example of a species which in both Norway and Scotland survives in isolated populations. In Norway the colonies manage some reproduction by seed while in Scotland regeneration appears to be mainly vegetative (Fig. 4.2). A detailed examination using specimens from Scotland and southern and central Norway showed that when grown in the Bergen Botanical Garden these populations on either side of the North Sea differed from each other in lipid composition, as well as in leaf and flower morphology. From the general similarity of the populations, it was argued that the differences that have arisen between the Scottish and Norwegian populations may have evolved over post-glacial time (Ovstedal & Mjaavatten, 1992). This and other examples already discussed in Chapter 1 illustrate the well-developed capacity of flowering plants to survive and evolve under seemingly adverse conditions.

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