Arctic and Antarctic tundra regions lie beyond the areas of normal tree growth, from about 60° N. and 60° S. to the poles; temperatures may be as low as —50° F. ( — 53° C) in the Arctic and —60° F. ( — 66° C) in the Antarctic. For part of the year there is continuous night with frequent high winds sweeping across the frigid landscape. During the long daylight hours of summer in the Arctic, temperatures range from an average of 35° to 55° F. (2° to 12° C). The ground below the surface is permanently frozen. There is little precipitation, at the most 6 to 10 inches (15 to 25 centimeters) in a year, including melted snow.
Low temperatures, wintry blizzards, strong winds, shallow soil and its scarce nutrients, and summer drought are reflected in the means that the plants that live in these regions have adapted to survive there. Of the plant species that live in these conditions, mosses and lichens are common, although grasses, sedges, and dwarf shrubs occur. The vegetation of the Arctic is rich compared with that of Antarctica, where temperatures are lower. In addition, most of the area in the Southern Flemisphere equivalent to that occupied by tundra in the north is ocean and the Antarctic ice cap.
Most tundra species grow close to the ground because upward growth is inhibited by limited snow cover and by harsh winds. The snow cover shields plants against wind abrasion by snow and soil particles. Cushion plants such as Saxifraga oppositifolia grow in a much warmer summer environment where plant temperatures often reach 75° to 85° F. (24° to 29° C).
Arctic shrubs grow upright where there is adequate winter snow to cover them. Along stream and river banks and below upland ridges species of willow (Salix) and birch (Be-tuia) often reach heights of 3 to 10 feet (1 to 3 meters). Roots of shrubs and some sedges may extend down to the bottom of the active layer (top of the permafrost) at a depth of 2 to 3 feet (60 to 100 centimeters) while other species have only shallow roots 3 to 6 inches (7 to 15 centimeters) such as many heath species.
The North Alaskan woolly lousewort (Pe-dicuiaris ianata) creates its microclimate by trapping air between the hairs dispersed over its stem and buds. By raising the temperature around the plant in this way, the short reproductive season can be accelerated.
To protect themselves against the desiccating winds, a number of plants, such as some saxifrages (Saxifragaceae), have a leathery cuticle covering their leaves, which retards water loss through evaporation.
Tundra plants are almost exclusively perennial. This is because the plants’ life cycle may be interrupted by the oncoming winter and the entire process suspended until the following summer; annual plants therefore stand little chance of survival.
With a growing season limited to only eight to twelve weeks, reproduction must be rapid. The green alder (Alnus crispa), for example, unfurls its catkins and leaves at the same time, rather than putting one out after the other as most other plants do.
The seeds of most tundra plants are very small; indeed, most weigh less than 1 milligram. These features not only facilitate dispersal by wind, but are also probably due to the fact that the plants need to conserve energy and cannot afford to manufacture a heavier seed. Most seeds are carried by the wind and across the snow in spring.
Some seeds lie dormant in the soil for long periods: seeds of the Arctic lupine (Lupinus arcticus), for example, have been found to germinate after having been apparently frozen for thousands of years. Most seeds, however, do manage to put down some roots and grow a few leaves before the winter cold stops all growth.
Several plants also reproduce asexually, by means of rhizomes, bulbs, or root stocks, such as some species of cotton grass (Eriophorum spp.). This method of propagation has a far higher success rate in the Arctic conditions than seed production. These organs also serve to store nutrients.
Flowering in tundra plants is sporadic; it may be early, draining the nutrient store from the previous summer, or late, using the food manufactured in the same summer. Like the seeds, most flowers are closely packed together—there can be a hundred flowers in one square yard.
Where the land is flat and boggy, a wet tundra of grass-like sedges (Carex spp.) and cotton grass proliferate. Grasses also survive in these areas, growing in tussocks and thus managing to retain warmth and moisture between their leaves. The heath tundra is dominated by several species of small, compact shrubs, mostly of the berry variety, such as crowberry (Empetrum nigrum), cranberry (Vac-cinium vitis-idaea), and bilberry (V. uligino-sum).
Mosses and lichens
The boggy areas where the frost has lifted the ground at intervals is called “paisa” mire. The hummocks are built up by successions of spongy bog mosses (Sphagnum spp.), which survive because they form low mats. Some, such as Andreaea, manage to live in exposed areas by growing rhizoids (little roots), which anchor the plant to the surface.
Around the fringes of the icecaps, lichens, mosses, and a few small flowering plants manage to survive. Lichens are particularly well adapted to low temperatures and conditions of prolonged drought. They are both fungi and algae combined to form a single structure, living symbiotically. The fungal structure (the outer layer) protects the plant and absorbs water vapor, while the alga (the inner layer) photosynthesizes and creates carbohydrates and other organic nutrients. Some lichens also fix nitrogen from the air. They are slow-growing, only about 1 millimeter per year.
Lichens such as reindeer moss (Cladonia rangiferina) are rootless and cling to other plants and rocks, especially heat-absorbing rocks in the ice-free zones. They grow when the surface they are attached to warms, and moisture is absorbed directly into the fungal cells. Growth is extremely slow because in severe weather lichens lie dormant. They reproduce vegetatively or by the dispersal of fungal spores, which join with an appropriate alga.