Plant Diversity In The Arctic

Cold climates do not feature frequently in discussions of biodiversity and the Arctic is normally not included in global maps showing sites of high plant biodiversity

(Myers, 2003). Nevertheless, within the Arctic there is ample evidence of diversity within species and of the presence of recognizable subspecies (Table 6.4; Fig. 2.24).

As discussed above (Section 2.1), peripheral populations, provided they are not isolated into populations of minimal size and subjected to genetic drift, can be expected to be more variable, since the changeable nature of marginal conditions induces fluctuating selection, which maintains high genetic diversity (Brochmann et al., 2004; Grundt et al., 2006; Eriksen & Topel, 2006). Genetic drift is probably unlikely in both hot and polar deserts where perturbation and wind erosion can transport seeds or fragments

Fig. 2.24 The early purple saxifrage (Saxifraga oppositifolia) growing at 79° N near Ny Alesund, Spitsbergen. (Left) The tufted form which resumes growth early on dry banks and beach ridges is not flood tolerant and conserves carbohydrate in a taproot. (Right) The prostrate form which inhabits low-lying, late snowmelt habitats has a rapid growth rate and does not conserve carbohydrate.

Fig. 2.24 The early purple saxifrage (Saxifraga oppositifolia) growing at 79° N near Ny Alesund, Spitsbergen. (Left) The tufted form which resumes growth early on dry banks and beach ridges is not flood tolerant and conserves carbohydrate in a taproot. (Right) The prostrate form which inhabits low-lying, late snowmelt habitats has a rapid growth rate and does not conserve carbohydrate.

of clonal species over vast distances and at great speed over frozen surfaces.

If the environmental argument of environmental uncertainty in peripheral areas is coupled with the case for ecological release (Section 2.3) due to a lack of competition, then the High Arctic is an area where substantial subspecific variation should be expected. Evidence to support this view can be found in various studies of diversity at high latitudes. A survey of allo-zyme diversity in populations of each of the 11 species of the North American angiosperm genus Polygonella (Polygonaceae) showed that the two most widespread species had reduced within-population gene diversity with respect to their narrowly endemic congeners (Lewis & Crawford, 1995).

In bird species, comparison of variability in peripheral areas with core areas has found greater variation at the periphery (Safriel et al., 1994). A possible explanation is that the endemic species, several of which inhabit known Pleistocene refugia, have been able to maintain higher levels of diversity because of population stability during glacial cycles. As the authors point out, if this explanation is correct, an important implication for conservation is that, for many genera in eastern North America, the species richest in gene diversity may be those most in danger of extirpation in the next decade. The genus Polygonella is not alone in this respect and a number of tree species are also notable for showing the same phenomenon, namely Pseudotsuga menziesii, Abies grandis, A. concolor, Pinus moticola and Thuja plicata.

Population variation studies have progressed in recent years from classical taxonomic descriptions concentrating on morphological characters to detailed examination of many aspects of molecular genetics. Physiological characteristics are also worthy of attention in relation to metapopulations, not least for the possibility that they may link cause with effect. In any fluctuating environment there can be expected to evolve considerable physiological variation depending on factors such as water availability and length of growing season. Certain physiological variants may be evident only at times of particular stress. Thus, the remarkable capacity for arctic populations of some widespread plant species to withstand prolonged periods of total deprivation of oxygen (see Section 3.6.3) which occurs during periods of ice encasement is found only at high latitudes and does not occur in more southern populations of the same species (Crawford et al., 1994). Some characters, such as those which influence phenology, as in snow-bed and beach-ridge populations in Saxifraga oppositifolia (Teeri, 1972) and Dryas octopetala (McGraw, 1987), are also recognized as taxonomic variations as they are accompanied by associated morphological adaptations.

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