There is an alternative hypothesis to consider in relation to marginal plant populations and their responses to climatic change. As climatic fluctuations are frequent in marginal areas regions the vegetation may be pre-adapted to climatic change. If this is the case the plant communities have to be robust in maintaining their identity and biodiversity and if they do migrate there may be a considerable lag phase before this becomes noticeable.
The habitat tenacity of the arctic flora, and the ability to withstand genetic depauperization, raises the question of how plant populations in marginal areas are able to withstand climatic change. Such a phenomenon has been discussed in relation to both plants and birds in comparisons of populations at the core and the periphery of species distributions (Safriel et al., 1994). The argument asserts that as environmental conditions outside the periphery of a species' distribution prevent population persistence, the peripheral populations must live under conditions different from those of core populations. Peripheral areas are therefore characterized by variable and unstable conditions, relative to core areas. Populations in marginal areas can therefore be expected to be genetically more diverse, since the variable conditions induce fluctuating selection, which maintains high genetic diversity. It is also likely that peripheral populations evolve resistance to extreme conditions. It has even been suggested that peripheral populations rather than core ones may be resistant to environmental extremes and changes, such as global climate change induced by the anthropogenically emitted 'greenhouse gases', and that they should be treated as a biogenetic resource used for rehabilitation and restoration of damaged ecosystems. In addition, it has been claimed that as climatic transition zones are characterized by a high incidence of species represented by peripheral populations, they should be conserved as repositories of these resources, to be used in the future for mitigating undesirable effects of global climate change (Safriel et al., 1994).
Evidence for the support of this argument can also be found at the upper limits for tree growth. A distinct feature in many treelines is the degree of variation that exists in populations as the upper limit for tree survival is approached. In New Zealand mountain beech (Nothofagus solandri) forms hybrids with other Notho-fagus species more commonly at the treeline than in the forest core. In the European Alps, the variation found in the treeline krummholz pine (Pinus mugo) is particularly noticeable (Section 5.2.2, Fig. 5.12). Phenological plasticity is also a feature of plants at the limit of their distribution. Cliff-top populations of plantains (e.g. Plantago maritima and P. lanceolata) show great variation in size as well a high degree of pubescence that is not found away from the edge of the cliff (Crawford, 1989). In the Himalaya the genus Saussurea (the alpine saw worts - see Fig. 10.3) are so variable that species identification can be difficult (Mani, 1978). It might also be concluded that populations that have adapted to survive periods of climatic adversity in marginal areas provide a degree of collective homeostasis to the vegetation of peripheral areas. Plants in these regions where extinction and recolonization are frequent occurrences can be conceived as existing in stable metapopulations.
As already argued in Section 2.2.2, divergent adaptations do not normally exist in the same individual, e.g. it is rare for an individual plant to be both flood and drought tolerant. Divergent adaptations can however exist in a metapopulation with the proportion showing any one particular adaptation fluctuating from year to year in response to climatic fluctuations. In wild sunflower (Helianthus annuus) populations the isoenzyme complement for alcohol dehydrogenase is able to vary depending on whether at the time of sampling there has been a series of wet or dry years (Torres & Diedenhofen, 1979). Such genetic variation confers a wider degree of environmental tolerance on the metapopulation than that found in any one individual.
This same argument is made for the concept of suspended speciation as it applied to arctic species (Murray, 1995). In this concept wide-ranging polymorphic species with only low levels of polyploidy and readily capable of hybridizing confer a physiological plasticity in metapopulations that enables the species as a whole to withstand the fluctuations of polar climates. This phenomenon is well illustrated in arctic populations of the purple saxifrage (see Section 2.5, Fig. 2.24).
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