Power of the

The sun is the major source of energy that powers the Earth. Although the sun's rays are relatively weak in the Arctic as they pass through the atmosphere at a more oblique angle than to the south, once the Arctic summer has started, there are 24 h of daylight per day. The sun's rays are absorbed by plants (from algae in the sea to the shrubs and krummholz of the treeline) and captured in carbohydrate molecules (sugars) in plants through the process of photosynthesis. The rates of photosynthesis in Arctic plants are lower than in plants of temperate and tropical regions, but continue unabated by nighttime throughout the summer. The captured energy, carbon dioxide, nitrogen (atmospheric and edaphic), and mineral nutrients power the trophic pyramid through herbivores to the top carnivores. In general, the amount of material produced each year in the wet meadows of Arctic systems is generally between 100 and 200 g m-2 of dry vegetative matter (net primary productivity) (carbon-based molecules, notably cellulose). The material may accumulate as standing dead matter (standing crop) to over ten times that amount, and some is incorporated into the soil. Dry tundra is less productive, and barren lands are almost nonproductive except in small amounts through bacterial activity.

The sun's energy is also captured directly by animals. In particular, butterflies, moths, and flies bask in the sun. They take up special stances to orient themselves so as to become warmed, and may follow the sun around hummocks. Butterflies are particularly noticeable as they spread or hold their wings to intercept solar rays. Flies may bask on insolated surfaces, or even take up positions within diaheliotropic flowers (which orient themselves perpendicular to the rays of the sun). Some caterpillars, by virtue of their hairiness (i.e., wooly bears, Gynaephora spp.), become warmed by the sun as they walk around plant hummocks. They pupate so as to maximize insolation and minimize wind resistance. The larvae of aquatic insects, such as mosquitoes (Aedes spp.), circumnavigate pools to remain in the warmest water or in the sunniest part. Even plants take up basking behaviors and speed their developments. Some have diaheliotropic flowers whose bowl-shaped parabolic petals turn to the sun for all or part of the day (e.g., Dryas spp., Papaver radicatum). Other plants become heated as microgreenhouses (e.g., louse-worts, Pedicularis spp., or campions Melandrium spp.), and others, like wooly bears, are hairy (e.g., willows, Salix spp.). The hollow stems of some plants warm by the greenhouse effect (e.g., Senecio conges-tus, Pedicularis spp.), and larval insects may live therein, sheltered and extra warm on sunny days. Such forms of thermoregulation may increase the animal's or plant's overall heat budget by up to 30% in a part of the world where heat is at a premium.

Behavioral and structural adaptations are part of a suite of characteristics for heat absorption. Many Arctic insects are known to be darker and hairier than their more southerly counterparts. Similarly, many Arctic plants have darker leaves, and are often pubescent. Cushion growth forms and leaves hugging the ground also allow for heat absorption from the sun and are reradiated from the ground.

Continuous daylight is not without drawbacks. Although the amount of ultraviolet (UV) radiation in Arctic daylight is less, per minute, than to the south, on a per day basis it is just as much or more. Dark pigmentation in invertebrates and plants may have a protective role as has been shown in some aquatic Crustacea: nonpigmented ones succumb rapidly to solar UV unless in tannin pigmented water. The highly UV-absorbing fur of polar bears may be related to vitamin D synthesis.

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