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Fig. 3.14 Overwintering root respiration rate at a range of temperatures for four coastal species. (a) Ligusticum scoticum, a northern species, and Crithmum maritimum, a southern species. (b) Mertensia maritima, a northern species, and Limonium vulgare, a southern species. (Data from Crawford & Palin, 1981.)

related to their latitudinal or altitudinal distribution. Intuitively, it might be expected that woody plants which devote a considerable part of their resources to the formation of non-productive trunks and stems may be unable to support such a growth strategy when growing seasons are cool and short. However, an extensive worldwide study of the carbon balance in trees at their upper altitudinal boundaries has shown

Fig. 3.15 Arrhenius plots of root respiration rate (oxygen uptake — see above) in relation to temperature. (a) Ligusticum scoticum. (b) Crithmum maritimum. (Data from Crawford & Palin, 1981.)

spring (Crawford, 2003). Observation of Sitka spruce roots (Picea sitchensis) has shown that winter warming when combined with flooding can lead to a dieback of the root system when the water table drops in spring and oxygen is suddenly restored to tissues that have endured a prolonged period of oxygen deprivation. This damage is not immediately fatal or even damaging to the tree, but repeated dieback of the deeper roots makes the trees liable to windthrow, which causes the greatest loss of trees in oceanic climates.

The damage caused by warm winter temperatures is unlike heat injury in that no visible signs or tissue lesions are observed. Arctic plants transplanted to the south of their natural range typically grow for a season or two until they have exhausted their carbohydrate reserves. Spring starvation then prevents the resumption of growth and the plants merely disappear.

Limitation in southern distribution will be caused by competition in many species, but there is a significant number of examples where this explanation is not adequate. Coastal habitats with their open communities which frequently have to be renewed after storms give ample opportunity for the establishment of fresh colonists. The continuous nature of coastlines also provides a habitat which extends through changing climatic zones without imposing any topographical barrier to plant dispersal. As these examples show, starting on the shores of Greenland and Spitsbergen (Fig. 3.17, left panel), there are several species which enjoy an uninterrupted distribution as far south as the British Isles, and in the case of sea lyme grass (Leymus arenarius; Fig. 3.17, right panel) even to the coast of northern France, but cannot be found further south.

A striking example of a species confined to high latitudes, even in the apparent absence of competition, is the whiplash saxifrage or spider plant (Saxifraga flagellaris). This species which does not occur south of 75° N in either Greenland or Spitsbergen is related to the high altitude whiplash saxifrage species of the Himalaya (e.g. S. stenophyltta) of Pakistan and Nepal. In both Greenland and Spitsbergen this saxifrage grows in open shingle habitats and there are no grounds for believing that it is competition with other more aggressive species that limits its southern spread. When visiting the plant at 75° N in Spitsbergen (Fig. 3.17) it is difficult to imagine what there might be in more southern climates that is inimical to the survival of this species.

Fig. 3.16 Estimated concentration (% dry matter) of low-molecular weight sugars and starch (NSC) and lipids in the total above-ground biomass of pines along altitudinal transects as compared with a temperate lowland reference site near Basel, Switzerland (LR), at three dates during the local growing seasons. Values are the means (+SE) of 5—15 trees. F, closed forest; I, intermediate stand between closed forest and treeline; TL, treeline stand. (Reproduced with permission from Hoch et al, 2003.)

Fig. 3.16 Estimated concentration (% dry matter) of low-molecular weight sugars and starch (NSC) and lipids in the total above-ground biomass of pines along altitudinal transects as compared with a temperate lowland reference site near Basel, Switzerland (LR), at three dates during the local growing seasons. Values are the means (+SE) of 5—15 trees. F, closed forest; I, intermediate stand between closed forest and treeline; TL, treeline stand. (Reproduced with permission from Hoch et al, 2003.)

A regime of continuous polar light and minimal risk of heat injury allows arctic plants to adopt a number of other low temperature compensating mechanisms which would not be viable in warmer climates. Dark colours absorb heat and the predominant hue of much of the vegetation cover of the Arctic is brown not green. This is particularly the case in the ultra-short growing season habitat of some shore communities (Fig. 3.18). In addition, a number of species have the ability to keep their flowers and sometimes also their leaves oriented towards the sun (sun tracking) and by having parabolic flower structures that concentrate the sun's rays on their reproductive organs, which not only hastens their development but also increases their attractiveness to insects (Fig. 3.19; see also Fig. 10.3). A number of woolly species even produce their own mini-greenhouses. Female plants of the arctic willow (Salix arctica) can become covered in down which can raise their leaf temperature to 11 °C above the air temperature (see Fig. 4.1).

So important is adjustment of metabolic rate to temperature that even within the High Arctic there are specialized forms of the same species for living in cold or warm microhabitats which may be only a few metres from each other. One striking example of such differing functional adaptation in ecotypes of a widespread arctic species is found in the purple saxifrage (Saxifraga oppositifolia), one of the hardiest plants of polar regions. On dry, exposed ridges with warmer temperatures and a longer growing season than the adjacent low shore, the purple saxifrage has a tufted form that starts growing before the snow melts in adjacent low-lying shore habitats. By contrast, on the late, cold shore sites this species is found usually as a trailing prostrate plant that does not give an impression of being particularly robust. However, appearances are misleading and this frail-looking prostrate saxifrage has a capacity adaptation which allows it to outperform the more robust type on the beach ridge in gross photosynthetic capacity, respiration and shoot growth (Crawford et al., 1993). Physiologically, the tufted form on the beach ridge is much more drought tolerant, and conserves carbohydrate for periods of stress. The plants of the shore habitat are by contrast spenders and use a much greater portion of their energy gains immediately for rapid growth (Figs. 3.20-3.21). The two forms have developed opposing strategies which aid their

Fig. 3.17 Examples of species of cold climate coastal plants with southern limits to their distribution in Europe. (Left) The spider plant Saxifraga flagellaris ssp. platysepela on an arctic shore in Spitsbergen at 75° 30' N. This species occupies open habitat with apparently no physical barriers to its southern migration yet is not found south of 73° N. (Right) Sea lyme grass (Leymus arenarius), a coastal species in dunes or gravel from the Arctic to north-west Spain.

Fig. 3.17 Examples of species of cold climate coastal plants with southern limits to their distribution in Europe. (Left) The spider plant Saxifraga flagellaris ssp. platysepela on an arctic shore in Spitsbergen at 75° 30' N. This species occupies open habitat with apparently no physical barriers to its southern migration yet is not found south of 73° N. (Right) Sea lyme grass (Leymus arenarius), a coastal species in dunes or gravel from the Arctic to north-west Spain.

survival in their particular microhabitat but which would disadvantage them if conditions were to change. Plants of the tufted and creeping ecotypes of purple saxifrage maintain their distinctive growth forms in cultivation and although interfertile have been recognized as distinct types (Lid & Lid, 1994). This same phenomenon is found in the mountain avens with different ecotypes occupying exposed ridges and snow hollows (Section 1.3.3).

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