Introduction
It is important to have a knowledge of the distribution of ecosystem types around the world, and also, those factors that have been shown to be important in determining which plants and animals can survive in each of these areas. Towards that end, the next few lectures will be devoted to documenting the various types of biomes (or ecosystems, or ecoregions, according to a new book by R. Bailey, Ecoregions: The Ecosystem Geography of the Oceans and Continents, Springer-Verlag, 1998), and the environmental factors most closely associated with the distribution of the vegetation and animals contained therein.

There being no particular order in which to start, I have decided to proceed from the far north, and work towards the equator, with a few digressions into aquatic systems, even though this course is aimed primarily at terrestrial systems.
 

Arctic tundra represents one of the newest communities or ecosystems in the world, because it has only been a few thousand years since this area was covered over by glaciers. It was about 9-11,000 years ago that the last glaciation (the Wisconsin) ended. After the glaciers retreated, they left behind stony, infertile soils. Thus, the soils have not had the opportunity to undergo as much weathering as have temperate or tropical areas. Many of the soils lack a discernible profile, typical of more older soils, and they are often very acidic (pH around 3.5).

Tundra Soils
A distinctive feature is a layer of permafrost, or permanently frozen soil, or as arctic ecologists call it "perennially frozen ground", usually about 1-2 meters below the surface. The term was coined by S.W. Muller in 1943, who wrote (quoted in Bolen, pg. 20):

                                            "...a thickness of soil or other surficial deposit or even of bedrock, at a variable
                                            depth beneath the surface of the earth in which a temperature below freezing
                                            has existed continuously for a long time (from two to tens of thousands of years)."

 See Figure 2-1 in Bolen for distribution of tundra worldwide.

Permafrost may be very deep, reaching depths of 1450 m (4800 feet!) in Siberia. It is not quite as deep in North America, reaching only about 1000 m (3300 ft). Interestingly, permafrost is deepest in areas that were devoid of glaciers - the weight of glaciers compressed the ground, raising temperatures above freezing. But in open areas, the ground could radiate its heat to the atmosphere, and it froze to deeper depths.

Permafrost has some interesting ecological implications. First off, it limits root penetration to deep soil layers, effectively insulating lower soil layers from biological activity. Materials in permafrost layers are trapped and remain stored, mostly inert, for eons. Water becomes limiting, because soils don't have much depth in which to store water, and there is only a short period during the growing season when it thaws and becomes available. Thus, water shortages can be a problem in some tundra areas. In other areas, tundra is low elevation, and water collects and makes the ground boggy during the summer months.

Plants often promote the retention of permafrost, by keeping direct solar radiation from heating the soils directly. If the vegetation is cleared, soils often thaw to deeper depths, and under the severe conditions of the north (high wind, ice blasting, etc.) erode easily.

For animals, the limited thawed depth keeps burrows near the surface. In some areas, the thaw makes the ground saturated, and animals have a tough time keeping dry. Burrowing insects as a result, are uncommon in such areas.

Tundra is often very flat, with little relief. This means that water tends to pool, and that after rains, runoff is significant (there being little space to percolate down). A common and unusual occurrence in tundra soils is polygon soils, named after the shapes these micro-topographical features resemble. See Figure 2-2 in Bolen, pg. 23. Often called patterned ground, the origins of these types of soils remained a mystery for many years, but now we know how they are formed. They range in size from 2-3 m in diameter on sites with grasses and sedges, and 20-30 m on bare ground. They are a result of shrinkage - in this case the tundra surface contracts when cold in the winter, and the resulting cracks fill in with melt water in the summer. Then, in the next winter, that water freezes, and heaves up and expands the cracks even further. This process is repeated year after year, producing the polygon features (they change with time). The cracks often fill with unique animal and plant communities (microtopographically and microclimatically different).

In slightly drier areas, frost heaving moves rocks around, and results in sorting of rocks and pebbles such that larger rocks move outward from the heaving site, and irregular patterns based on rock size appear on the tundra surface (this is true patterned ground). Again, this has implications for plants and animals.

In sites with slopes, freezing and thawing can result in slumping, and mass wasting, which rips vegetation out and creates bare ground. This may occur frequently enough that typical conceptions of succession as applied in temperate areas may not apply in the tundra biome.

Another landmark of the tundra soil is something called an esker. Eskers are long strands of rocks and debri, remnants of a stream or river when a glacier retreated. As the glaciers moved along, they scraped up rocks and other debri. When they melted, this debris was released, and flowed down with the water below the retreating glacier. Sometimes these hills of debris can be 100 m high! (See Figure 2-4 in Bolen, pg. 26). Their importance lies in the fact that they are elevated above the surrounding, soggy tundra plains, and are crucial for animal migration paths, i.e., caribou. Humans have also used these eskers for tracking animals for food, for obtaining suitable rocks for tools, and to bury their dead. Due to the severe winds and extreme cold, few plants grow on top of the eskers. Lichens are the most notable.

Glacial Refugia
When the glaciers were at their maximum, it appears that a narrow band of tundra was maintained at the base of the glaciers up to where the boreal forest started. Thus, many of these plants and animals survived in these small refugia until the glaciers retreated 16,000 years ago. Many of these animals and plants are still expanding their range, including grizzly bears.
 

Plant Adaptations
The severe environment of the tundra zone has restricted which plants can survive here, and molded many of them to cope with the extreme weather and climate. Most tundra plants are short, getting no higher than bushes, even if they are trees. For example, some birches grow only a few inches tall here. Others have decumbent stems (stems that grow horizontally instead of vertically). A great number are evergreen. One might conclude that survival here depends more on coping with the microclimate and physical extremes, rather than competition.

Some herbaceous plants develop a cushion form (groups of plants grow smashed together with each other, looking like one big plant). This tends to give the plant thermal mass, and helps with heat balance.

The growing season may be as little as 6 weeks, and thus, some of these plants have very rapid flowering and seed set! Some flowers orient towards the sun, and are parabolically shaped, acting as little solar ovens. Temperatures inside the corollas have been measured 10-20 C above ambient, providing a little arctic sauna for potential pollinators.

Most plants are perennials - seed set is sporadic and chancy - too short a growing season to adequately set seed. In one survey of North Greenland, only 1% of the plants (1 species) was an annual! Perennials can also store reserves underground, so that growth can resume next spring more rapidly, and more fully utilize whatever warmth and sunlight there is.

When tundra plants so produce seeds, they must be capable of withstanding adverse conditions for long periods of time. Scientists have found 10,000 year old lupine seeds that have germinated adequately when hydrated (they had been buried in lemming burrows for all those years). Amazing!

Primary Productivity
The tundra has some of the lowest net primary productivity of any ecosystems, due mainly to the cold and short growing season, and the infertile soils. Mean productivities range from 10-400 g m^-2 yr^-1, with a mean of 140 g m^-2 yr^-1. A tropical rainforest may have productivities of 2000 g m^-2 yr^-1. Tundra makes up about 8 x 10⁶ km² worldwide, and contributes about 1.1 Gt yr^-1 worldwide, or about 0.6% of the world's total net primary productivity. A Gt equals 10x⁹ tons of organic matter. See chart in Whittaker's Book, Communities and Ecosystems (frontispiece).

Litter decomposition, like productivity, is also very slow, and organic matter decays only slightly each year. Most litter becomes incorporated into the soil, and remains as undecayed matter. This limits nutrient availability, making nutrient shortages one characteristic of the tundra.

Because the permafrost layers never thaw out, the organic matter stored in them is effectively trapped forever. But in recent years, Alaska and other northern areas have been warming up. This lowers the thaw depth, and the peat and organic matter begins decaying. As it does so, the soils sink, and the CO₂ is liberated into the atmosphere, further spurring on worries of a positive feedback effect for global warming. As the soils sink, they become inundated, and the tundra becomes flooded. Tundra soils store incredible amounts of carbon, as we will discover later in the course, and the thawing of these soils could vastly exacerbate global warming through their input of CO₂ to the atmosphere.

Major Players in the Tundra
Plants in the blueberry (or heath) family (Vacciniaceae) are common in the tundra. Perennials and evergreens also. For detailed lists, see Bolen's book.

Lemmings are common small mammals, and the stuff of legend when it comes to population cycles and suicidal jaunts to the sea (which by the way are not true!). Other animals include arctic ground squirrels, caribou, some birds, grizzly bears, and foxes. Notably absent are amphibians and reptiles, with the exception of one species of frog. No lizards, snakes, toads, or turtles.
 

Billings, W.D. 1973. Arctic and alpine vegetation: similarities, differences, and susceptibility to disturbance. Bioscience
                                23:697-704.

Bliss, L.C. 1962. Adaptations of arctic and alpine plants to environmental conditions. Arctic 15:117-144.

Bolen, E.G. 1998. Ecology of North America. John Wiley & Sons, Inc. See especially the extensive references at the end of
                                Chapter 2.

Kerfoot, D.E. 1972. Thermal contraction cracks in an arctic tundra environment. Arctic 25:142-150.

Porsild, A.E., C.R. Harrington, and G.A. Mulligan. 1967. Lupinus arcticus Wats., grown from seeds of Pleistocene age.
                            Science 158:113-114.

Shelford, Victor. 1963. The Ecology of North America. Univ. of Illinois Press, Urbana, IL.

Whittaker, Robert H. 1975. Communities and Ecosystems, 2^nd Edition, MacMillan Publishers.