An underlying question throughout this book has been whether or not marginal areas will be sensitive to climate change. Although treelines may be slow to advance, just as they are slow to retreat, there are many other species which have a capacity to invade and which may therefore readily migrate with a changing climate. Changes in both location and species composition of some marginal communities are one of the possible consequences of climatic change. A threat that cannot be ignored is that rapid climatic change may outpace the ability of plants either to migrate or adapt (Jump & Penuelas, 2005).
Even if the threat of extinction is absent, frequent species advances and retreats might impoverish biodiversity. This has been discussed mainly in relation to animal species (Hewitt, 1996). During periods of rapid climatic change population extinctions might be presumed to outnumber immigrations. In addition, leading edge colonization during a rapid expansion could be expected to be leptokurtic (similar to a normal distribution, but with a higher frequency of values near the mean) and thus lead to increased homozygosity (Hewitt, 1996).
Both past history and current climatic trends mark out the Arctic as an area that has endured an unending series of climatic fluctuations over the past 2-3 million years. It might therefore be expected that repeated mass extinctions and immigrations have reduced polar plant population heterozygosity. Although there are examples ofsome species in arctic and subarctic regions with low levels of genetic diversity it is not a universal phenomenon and many examples exist of plant species at high latitudes which are just as diverse as those at lower latitudes (Crawford, 2005). This may be due to the ability of many plant species to maintain viable populations in favoured refugia during times of climatic change. Plants have a number of long-term survival strategies that are denied to most animals (see Section 4.1). Being able to dispense with sexual reproduction in favour of asexual reproduction is a major benefit in times of climatic adversity. There are viable aspen populations in Scotland that are now beginning to produce seed, possibly for the first time since the onset of the Little Ice Age (see Section 4.13, Fig. 4.40). Many plant species have reserves of biodiversity in their seed banks and this together with clonal longevity, which can be measured in thousands of years (Steinger et al., 1996), can do much to ensure survival through periods of climatic adversity.
The application of molecular techniques to the biogeography of high-latitude populations is providing more and more evidence to refute the oversimplistic concept of a tabula rasa at high latitudes as a result of the Pleistocene ice ages and in particular the Last Glacial Maximum. Studies of high-latitude populations of Saxifraga oppositifolia, particularly in Greenland and Beringia (Abbott & Comes, 2004), and white spruce (Picea glauca) in Alaska (Anderson et al., 2006) have demonstrated the Pleistocene survival of arctic populations north of the major ice sheets. Such a feat is a striking demonstration of the ability of marginal plant populations to withstand extreme and repeated climatic fluctuations. Having survived substantial climatic oscillations in the past, which sometimes took place with great rapidity, it is not unreasonable to assume that they may be able to adapt to future changes.
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