Arctic Light Limitation

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Fig. 3.9 Change in stomatal density with altitude for V. myrtillus. (a) In situ observations. (b) Transplants from 200—900 m. (Reproduced with permission from Woodward et al., 2002.)

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Fig. 3.9 Change in stomatal density with altitude for V. myrtillus. (a) In situ observations. (b) Transplants from 200—900 m. (Reproduced with permission from Woodward et al., 2002.)

Vaccinium myrtillus show an increase in adaxial stomatal density with increasing altitude (Figs. 3.8-3.9). When individual plants were transplanted from 200 m to 905 m there was an even more marked response demonstrating the ability to respond phenotypically to the effects of increased altitude. Similarly, plants of this species growing in snow patches had mean adaxial stomatal densities of 18 mm~2 while those in exposed areas had mean values of 55 mm~2. The signal for altering the development of the leaf in relation to stomatal density appears to be sensed by mature leaves, which then influence the stomatal density in the leaves of the developing bud (Woodward et al., 2002).

Resource foraging is another aspect of functional adjustment which is seen in the ability of some plants to actively search for resources. This is most readily apparent in stoloniferous plants where surface stems or underground stolons can place shoots in locations favourable for light capture and where subsurface stolons can aid roots in their foraging for mineral nutrients. The purple saxifrage (Saxifraga oppositifolia) has two forms (Sections 2.2.4 and 3.2.4), one of which has tufted shoots and a taproot and does not forage, and lives in drier habitats with a longer growing season. The other form adopts a creeping habit which allows it to forage for resources and exploit cracks in rocks and other places for shelter, nutrients and water, and has a greater metabolic activity both in respiration and photosynthesis which compensates for the shorter growing season associated with wet hollows where snow lies late into the growing season. The common bog cotton sedges are an example of two closely related species that share adjacent habitats, with one, Eriophorum angustifolium, being a stoloniferous foraging species while E. vaginatum is a tussock-forming non-foraging species. The latter is more tolerant of changing depths in the water table while the former is more successful in wetter areas through which the plants spread by means of their flood-tolerant stolons (Figs. 3.10-3.11).

The larger the organism the greater, and frequently disproportionately greater, is the share of resources that it can command to the detriment of competitors. Thus, large trees remove much of the light from any terrain they occupy. In scientific terms, competition for light is size asymmetric, in that a large plant can potentially dominate a competitive relationship and therefore the light resources obtained by the taller plant are disproportionate to its size (Blair, 2001).

In marginal habitats the environment is often uncertain and episodes of drought, flooding, storm exposure or cold, plus physical disturbance, can undermine the tendency of large plants to dominate the landscape. In these situations, size, at least above ground, can be incompatible with long-term survival, and organisms with reduced exposure and lower demands for resources may prevail. It is therefore important to consider the dual nature of plant existence with one part in the air and the other rooted in the ground or

Arctic Light Limitation
Fig. 3.10 Bog cotton (Eriophorum vaginatum and E. angustifolium) colonizing a bog from which peat has been recently extracted industrially in Caithness, Scotland.
Fig. 3.11 Divergent strategies in bog cotton growth. (Left) The tufted non-foraging species E. vaginatum which favours drier portions of the bog. (Right) The foraging species E. angustifolium which grows in the wetter parts of the bog.

submerged in water. In contrast to aerial shoots the below-ground parts of plants live in a potentially more heterogeneous habitat where resources are rarely evenly dispersed. Therefore accessing resources from soil may be more size symmetric as the amount of soil nutrients obtained will be in direct proportion to the size of the foraging organs. Unfortunately, most studies examining below-ground competition use homogeneously distributed nutrient resources and soil homogeneity is not often found in nature.

3.2.3 Adverse aspects of capacity adaptation

Every adaptation to a specific environment has its limitations which arise from an increased dependence on a particular set of environmental conditions that is

Mertensia Maritima Range
Fig. 3.12 European distribution of two coastal arctic species that reach their southern limits of distribution in the British Isles and Scandinavia. (Left) Scot's lovage (Ligusticum scoticum). (Right) The oyster plant (Mertensia maritima). (Reproduced with permission from Hulten & Fries, 1986).

created in the adapted plant. In this sense 'adaptation is the first step on the route to extinction' (Crawford, 1989). Thus the use of capacity adaptation by plants of cold climates to compensate for the limitations of low temperatures can place such adapted plants at a disadvantage in warmer environments. Examples of this are found in the carbon deficits that arise in some arctic species as a result of warm environments. The southern distribution of the arctic coastal herbs Scot's lovage (Ligusticum scoticum) and the oyster plant (Mertensia maritima) are examples of this condition. Both these species occur in the Arctic (Figs. 3.12-3.15) and reach the southern limits of their distribution on the shores of Scotland. They are also remarkable for the speed with which they extend their foliage when growth recommences after winter. Such rapid growth, exploiting the reserves of previous carbon gains, requires high respiration rates. A typical response of L. scoticum to temperature in its respiration rate is shown in Fig. 3.14. The rapid increase in dark respiration rate with temperature is also found in the oyster plant (Mertensia maritima) and contrasts with the lower respiration rates of two comparable coastal species of more southern distribution. The depletion of carbohydrate reserves by high respiratory activity at warm temperatures has often been suggested as a limiting factor in the southward extension of northern species (Crawford & Palin, 1981).

An examination of the carbohydrate content of various Vaccinium species growing in Scotland (Bannister, 1981) showed that the species which suffered the severest depletion of carbohydrate reserves at warm temperatures, V. uliginosum, was the most restricted in its southern range. Thus for some perennial herbs and woody heath species the consequence of having high respiration rates to exploit carbohydrate reserves in short cool growing seasons places the plants at a disadvantage in warmer climates (see also Figs. 3.12-3.15).

Many studies have sought to determine whether or not the total carbon balance of woody plants can be

Ligusticum Scoticum Planting
Fig. 3.13 Coastal species with predominantly arctic and subarctic distribution growing on a shingle beach in Orkney. (Left) Scot's lovage (Ligusticum scoticum). (Right) The oyster plant (Mertensia maritima).

the converse, namely that tree growth near the timberline is not limited by carbon supply (Fig. 3.16) and that it is more probable that it is sink activity and its direct control by the environment that restricts biomass production of trees under current ambient carbon dioxide concentrations (Hoch & Körner, 2005).

Although carbon limitation as measured in the total biomass of woody plants may not be a feature that relates directly to distribution of woody species with temperature, it is nevertheless possible that certain vital organs, such as root tips and buds, and cambial tissue, rather than the whole plant can be seriously carbon deficient under specific conditions (Section 3.2.4).

3.2.4 Climatic warming and the vulnerability of specific tissues

The meristematic region of roots is always anaerobic. Consequently, any diminution of oxygen supply to the roots will increase the amount of root that is hyp-oxic, which accelerates the drawdown of carbohydrate levels through anaerobic respiration. Translocation of carbohydrates does not appear to be able to replenish the distal regions of roots in mid-winter. Carbohydrate depletion of these tissues reduces antioxidant content and thus renders the roots vulnerable to post-anoxic injury when water tables fall and aeration is restored in

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