The tundra is commonly described as treeless. Trees growing upwards with a clearly visible trunk are only found in isolated pockets near the tundra—taiga interface (see Chapter 5). However, this does not necessarily exclude the presence of woodlands in the tundra. Whether or not trees can be considered to be present in polar regions is merely the imposition of an arbitrary human judgment. Willows that grow upwards in the temperate zone survive better in the Arctic if their trunks lie flat along the ground, and this should be recognized as a natural development for woodland survival in this particular situation. The vast mats of polar willow that cover the tundra and reach as far north as 80° N in Spitsbergen can have horizontal trunks several metres long and over 200 years old. For this achievement they have been called Spitsbergen's 'forest'. At ground level the colourful autumnal display of this prostrate woodland is as striking as any North American maple forest.
The woody flora of the Arctic comprises relatively few taxonomic groups. Birches (Betula spp.), willows (Salix spp.), alders (Alnus spp.) are all examples of tree genera that can grow in the pole form in temperate climates but can exist either in the bush form or as prostrate mats in polar regions. The phenotypic and genotypic forms of krummholz of several coniferous tree species are also found in the southern fringes of the arctic tundra where it borders with the boreal forest (see Chapter 6). In the northern hemisphere, the Ericaceae is the predominant family in providing many widespread woody arctic-alpine and heath genera such as Arctostaphylos (Fig. 9.6), Vaccinium, Erica, Calluna, Rhododendron and Kalmia.
In terms of geographic ubiquity, first place belongs to the genus Empetrum. The northern hemisphere crowberry, Empetrum nigrum, together with its almost identical, vicarious South American counterpart E. rubrum, are in their combined ranges the most widespread of all the dwarf woody plants in having a distribution range that extends from the High Arctic to the heaths of the Tierra del Fuego and the Falkland Islands. In all cases they inhabit either acid peatlands, or cold coniferous forest, and acidic rocky slopes. Crowberry also colonizes calcium-depleted sand dunes and cliff tops. In Europe there exist two subspecies: E. nigrum subsp. nigrum which is dioecious (Fig. 9.7), and E. nigrum subsp. hermaphroditum which is hermaphrodite. The hermaphrodite subspecies has a more northern and montane distribution but can occur at sea level in highly oceanic regions such as the Shetland Islands. The hermaphrodite form is easily identified from the withered stamens that persist at the base of some of the berries. Both subspecies can be found in marginal areas and are particularly successful in putting out new roots around edges of turf and spreading across open ground in areas with high wind exposure.
The preference of Empetrum nigrum for cooler moister sites is seen more clearly at lower latitudes outside the Arctic. This is seen in the East Friesian island of Spiekeroog where crowberry is dominant only on the cooler and more humid north-facing slopes (Fig. 9.8).
Maintaining the woody form even in the polar deserts of the High Arctic is a remarkable phenomenon both in terms of being able to support non-photosynthetic tissues in habitats with short cold growing seasons but also in being able to survive the stresses of winter and dangers of herbivory in unproductive habitats. In northern regions woody plants are particularly at risk as they are a principal source of forage for overwintering mammals. Domesticated sheep and reindeer, and a wide range of wild herbivores, including caribou, reindeer, red deer, musk oxen, hares, lemmings and voles, use dwarf woody plants beyond the treeline as a source of fodder. There are many examples in marginal habitats in the subarctic regions of Europe where overgrazing of dwarf woody shrubs, including Calluna vulgaris, can lead to the destruction of this vital winter resource which in subarctic regions is often replaced by unproductive acid grasslands.
Woody, non-productive stems and branches might appear as a morphological luxury in terms of photosynthetic efficiency in marginal areas where the balance between carbon surplus and carbon deficit may be problematical. A study of net primary production (NPP) in the polar willow (Salix polaris) in Spitsbergen has shown that this species is well adapted under current conditions to maximize its use of the potential growing season. Maximum values for photosynthesis rates and stomatal conductance are reached within one week after leaf emergence, which takes place immediately on snowmelt and then gradually decreases. Depending on leaf age, photosynthetic rates were found to be saturated at a photosynthetically active photon flux density (PPFD) of 200-400 imol m~ 2s~1, which is the light level usually available in this habitat. Optimum leaf temperature for photosynthesis was in the range 10-18 °C, while air temperature in the habitat varied between 8 and 20 °C. These light and temperature responses of photosynthesis permit efficient carbon gain in a natural habitat characterized by highly variable light and temperature conditions. Model-based predictions for one particular year gave values for a probable net primary productivity for the year of 26.1 gcm~2. However, this model also predicted that rising temperatures would cause a reduction of NPP due to potentially large increases in respiration (Muraoka et al., 2002).
When the respiratory activity of the below-ground parts (roots + below-ground stems) of three dominant arctic species (Salix polaris, Saxifraga oppositifolia and
Luzula confusa), were determined under laboratory conditions it was found that both the respiratory activity and the Q10 value for respiration were higher in S. polaris than in the other two species. This suggests that the polar willow will be more likely to suffer a carbon deficit with rising soil temperatures than the other species examined in this study (Nakatsubo et al., 1998).
The woody plants of the Arctic differ in their nutrient requirements and it might therefore be expected that the deposition of pollutants at high latitudes may affect the species composition of the tundra. A study in Spitsbergen comparing the arctic heather (Cassiope tetragona), mountain avens (Dryas octopetala) and the polar willow (Salixpolaris) was able to demonstrate basic differences in the carbon and mineral nutrient economies of the shrubs, related to their growth form. This was seen in the ability of the shrubs to respond to nitrogen and phosphorus treatments (Fig. 9.11). Cassiope was conservative and there were no significant treatment effects. Salix was the most responsive, showing increases in leaf nitrogen concentration, biomass and photosynthetic rate (Baddeley et al., 1994).
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