Tundra ecosystems

A circumpolar band of roughly 5 million acres of treeless country called the arctic tundra lies between the forests to the south and the Arctic Ocean and the polar ice caps to the north. Smaller yet ecologically similar regions found above the timberline on high mountains are called alpine tundra. The Russian word tundra came from the term tunturi, meaning “treeless plain.” This name gives a brief but effective description of the plant species that grow on the northern flanks of the latitudinal tree line in the Northern Hemisphere, and above the altitude of the vertical tree line. Based on the temperature and environment, tundra can usually be categorized as short-term (hours, days, or a half month), seasonal (a half month to several months), and permafrost (years to thousands of years). Geographically, short-term, seasonal frozen soil and permafrost areas occupy about 50 percent of land area on Earth, while the permafrost area can take up to 25 percent of the land area. Nearly half of the territory of the former Soviet Union and Canada is permafrost, and 85 percent of the land in Alaska is permafrost. In addition, even near the equator, permafrost is found on the peak of Mount Kilimanjaro.

Among many factors that affect the distribution of ecosystem types, the temperature is a very important one. The higher the temperature is, the more energy is available for plants to use in photosynthesis and to be stored in the biosphere. When we go forward north from the equator, the temperature generally decreases. Also, as altitude increases, the temperature declines. So there is a general similarity between latitude and altitude in determining the distribution of ecosystems. Bearing this in mind, arctic tundra and alpine tundra are described and compared below.

In tundra, cold is the number-one limiting factor. Temperature often ranges from 60 degrees F (15.5 degrees C) in the summer to minus 130 degrees F (minus 90 degrees C) in the winter. Precipitation is also low, but water is not a limiting factor because of the very low evaporation rate in the tundra region. The growth of woody plants is essentially restricted due to the lack of warm summer at high latitude and altitude. Therefore, vascular plants that can survive in these cold environments must complete their annual growth rather quickly during the short but cool summer, and need to have the ability to endure the long yet extremely cold winter. Consequently, the species diversity in the tundra is lower compared to most other biomes. In fact, tundra can be considered as a cold marsh or a wet grassland that is frozen for a portion or even most of the year.

Tundra Temperature and Soil

The tundra regions at high latitude areas are dominated by cold, dry, polar wind for most of the year. Air with limited moisture only enters the tundra region in summer when the sea ice begins to melt down. The typical summer temperature is usually below 50 degrees F (below 10 degrees C), and the temperature stays above the freezing point for only two to six months in a year. In the short summer, the sun remains above the horizon for most of the day, and does not even set for several weeks at extreme latitudes. But in the winter, the sun rises only briefly or not at all. Alpine tundra is similar to arctic tundra in terms of having a rather short growing season but a long and cold winter. Otherwise, the climates are quite different. Alpine tundra has less extreme photoperiods, more precipitation (mainly as snow), stronger winds, and greater day-night temperature fluctuations. Therefore, in contrast to arctic tundra, the weather in alpine is very variable. In addition, alpine tundra differs from arctic tundra in having higher ultraviolet radiation and lower oxygen concentration.

In addition to temperature, soil is an important environmental factor for the formation and function of ecosystems. Many types of soil are present in tundra areas, ranging from moist deep peats to coarse rocky materials where only mosses and lichens are able to grow. Usually only the very top layer of soils can thaw every year. The length and depth of thawing depends primarily on the amount of solar radiation received. However, factors that affect surface thermal properties, including soil color and moisture, vegetation cover, snow depth, and so on, will also play a role. The thawing layer can range from about three feet (one meter) to only a few centimeters in depth. In most areas, lower layers can remain frozen throughout the year. Consequently, drainage is usually poor and many soils remain waterlogged in summer. In contrast, alpine tundra soils generally lack permafrost but are thinner and immature, mostly classified as cryic (cold) entisols or inceptisols. Thus, they are better drained than arctic tundra soils. Because of the good drainage, late-summer drought can occur, limiting plant growth. Therefore, rocky sites with little plant cover are fairly common in alpine tundra. Solifluction (also known as soil fluction), a type of mass wasting in which waterlogged sediment moves slowly downslope over impermeable material (in tundra's case, a deeper frozen layer or bedrock), occurs in both arctic and alpine tundra, especially in alpine tundra because of the slope. Solifluction tends to produce an undulating landscape.

Tundra Fauna and Flora

To survive in extremely cold environments, plants must have special morphological and physiological characteristics. In arctic tundra, the dominant plants are dwarf shrubs and perennial herbs. These plants grow very close to the ground surface from which they get more energy during the rather short growing season. But in extreme environments, other than lichens and mosses, very few plants can survive. These plants usually have the ability to go dormant during severe drought or cold and resume growth when conditions become better. Some plants can even survive under continued snow cover for years. In addition, plants must be able to tolerate short and cool summers when temperatures are only slightly above the physiological limit for plant growth. They have to grow fast because the summer is very short in tundra areas. Also, many tundra plants have perennating organs underground to help them survive in extreme habitats.

Trees are almost entirely lacking except in very limited areas near rivers, or on sheltered slopes, where the unfrozen soil can be relatively deep. The herb layer is in fact common in tundra areas and there are quite a few different species of plants, especially grasses and sedges. Surprisingly, the lichen layer covering exposed rocks is better developed than in any other biome. Alpine tundra is often similar to arctic tundra in terms of plant species. However, there are exceptions. For example, the tundra of the Sierra Nevada shares only 15 to 20 percent of its plants species with the arctic tundra.

Therefore, the ecosystems in arctic and alpine are similar, and both can be placed into the category of “tundra.” However, there are many obvious differences between them. Therefore, we must formally call them arctic tundra and alpine tundra separately to show the differences.

Although extremely cold, there are animals in the tundra area. It has been suggested that approximately 48 species of land mammals are found on arctic tundra, but surprisingly the abundance of each species is not low. Most noticeable mammals and birds in arctic tundra are arctic fox, caribou, ermine, grizzly bear, harlequin duck, musk ox, polar bear, and snowy owl. There are not many species of insects in the tundra, and the commonly found ones are black flies, deer flies, and mosquitoes. The surviving strategy of mosquitoes is to keep themselves from freezing by replacing the water in their bodies with glycerol, a chemical that works like an antifreeze and allows them to stay alive during cold winter. Also, it should be mentioned that arctic and alpine tundra share few animals.

In addition to the commonly recognized arctic tundra and alpine tundra, Antarctic tundra occurs on Antarctica and on several Antarctic and sub-Antarctic islands. Most areas in Antarctica are very cold and dry, and most of Antarctica is covered by ice fields, so plants are not supported. However, limited areas of Antarctica, particularly the Antarctic Peninsula, have coverage of rocky soil that can support vegetation. Compared to the arctic and alpine tundra, the Antarctic tundra has much less mammal population, largely because of its physical separation from other continents. Seabirds and sea mammals such as penguins and seals have their habitats nearby the shore. Some small mammals such as cats and rabbits occasionally present in Antarctic tundra, and are usually considered to be introduced by humans to some sub-Antarctic islands.

Tundra and Global Warming

Soils in tundra are considered to have the highest carbon density (21.8 per square meter) among all biomes, and the total soil carbon in tundra ecosystem is also the highest (191.8 times 10¹⁵ grams) among all specific ecosystems. In fact, tundra is recognized as one of Earth's three major carbon dioxide sinks. A carbon dioxide sink is a living entity (basically biomass) that takes in more carbon dioxide than it releases. Therefore, when discussing tundra, one thing cannot be neglected in this century: global warming. Global warming began to draw researchers' attention in the 1990s. It is a function of the greenhouse effect and caused by many kinds of greenhouse gases, such as carbon dioxide, methane, ozone, and nitrous oxides, of which carbon dioxide is thought to be the most important one. Carbon dioxide (CO₂) levels in the atmosphere are increasing, which may be mainly due to continued use of fossil fuels. It is estimated that during the 21st century, the carbon dioxide concentration could double and, consequently, simulations from general circulation models suggest that global average temperature would rise between 1.5 and 4.5 degrees C (34.7 and 40.1 degrees F). The gradual warming is accompanied by the melting of inland glaciers. One fundamental concern is that higher temperature in tundra and taiga, in which around 30 percent of the total soil-bound carbon found on our planet's surfaces are stored, will accelerate the degradation of peatlands, layers of permafrost, and forest litter, and thus lead to a large release of CO₂ and methane, exceeding the speed of biomass production. The second fear may be that a hotter, drier climate will bring more natural fires to areas in tundra where dead vegetation and peat have accumulated, which will also increase the amount of CO₂ in the atmosphere. In addition, not only the density of vegetation but also the composition of plants will be affected by climate change. Animal communities will be affected indirectly through their food plants. Ozone depletion near the earth's poles has also resulted in Arctic and Antarctic tundra having greater exposure to ultraviolet radiation, which may harm plants and wildlife. Finally, mercury that has accumulated in the Arctic tundra after being carried there from other parts of the world may leach into the ocean at a higher rate as the region warms.

From another viewpoint, the melt water from permafrost provides a direct source of water for plants in a severe drought summer, and keeps surplus water in the soil until the next summer. Global warming may disturb this permafrost system and, consequently, plants in tundra ecosystems could be negatively impacted. Also, freezing and subsequent thawing of soil in tundra may cause an increase in microbial activity, which is an important factor to keep the soil ecosystem active. Unfortunately, climate change might break the balance. However, climate warming, which brings increased availability of nitrogen and longer growing seasons, will likely increase biomass production in tundra ecosystems.

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