5: Climates and Soils & 6: Biomes

Between the fall equinox in September and the spring equinox in March, the situation reverses and the Southern Hemisphere has longer days and receives more direct solar energy than the Northern Hemisphere

The latitude that receives the most direct rays of the Sun, known as the solar equator, shifts throughout the year—from 23.5° N latitude in June to 23.5° S latitude in December. These are the warmest latitudes on Earth and are known as the tropical latitudes.

lakes can be subdivided into several ecological zones, each with distinct physical conditions

The littoral zone is the shallow area around the edge of a lake or pond containing rooted vegetation, such as water lilies and pickerelweed. The open water beyond the littoral zone is the limnetic (or pelagic) zone, where the dominant photosynthetic organisms are floating algae, or phytoplankton. Very deep lakes also have a profundal zone that does not receive sunlight because of its depth. The absence of photosynthesis, as well as the presence of bacteria that decompose the detritus at the bottom of the lake, causes the profundal zone to have very low concentrations of oxygen. The sediments at the bottoms of lakes and ponds constitute the benthic zone, which provides habitat for burrowing animals and microorganisms.

The weathering of granite illustrates some basic processes of soil formation.

The minerals responsible for the grainy texture of granite— feldspar, mica, and quartz—consist of various combinations of oxides of aluminum, iron, silicon, magnesium, calcium, and potassium.

Growing season

The months in a location that are warm enough to allow plant growth

The neritic and the oceanic zones may be subdivided vertically into a photic zone and an aphotic zone.

The photic zone is the area of the neritic and oceanic zones that contains sufficient light for photosynthesis by algae. The aphotic zone is the area of the neritic and oceanic zones where the water is so deep that sunlight cannot penetrate. bacteria in the aphotic zone use chemosynthesis to convert inorganic carbon into simple sugars. Other organisms in the aphotic zone depend on the organic material that falls from the photic zone. One of the fascinating adaptations of many organisms in the aphotic zone is the ability to generate their own source of light, known as bioluminescence, to help them find and consume prey. A number of jellyfish, crustaceans, squid, and fish species have independently evolved this ability.

The Great Basin Desert of the western United States, for example, lies in the rain shadow of the Sierra Nevadas and the Cascade Mountains, and covers a large area that includes nearly all of Nevada and much of western Utah.

The processes involved in creating rain shadows have much in common with the processes we saw occurring in Hadley cells, including adiabatic cooling, adiabatic heating, and release of latent heat

Gyres redistribute energy by transporting both warm and cold ocean water around the globe

The proximity of these ocean waters to continents can make the continents considerably warmer or colder and therefore influence terrestrial climates. You can see this impact on the coastal patterns of the plant hardiness zones. For example, England and Newfoundland, Canada, are at similar latitudes. However, England is adjacent to a warm-water current, the Gulf Stream, that comes out of the Gulf of Mexico, whereas Newfoundland is adjacent to a cold-water current that comes down from the west side of Greenland and pushes the warmer Gulf Stream water offshore. As a result, England experiences winter temperatures that are, on average, 20°C warmer than Newfoundland.

The spring and fall turnover are typical of lakes that exist in temperate climates because they experience cold winters and warm summers.

The seasonality of vertical mixing is much less dramatic in lakes that are not exposed to such dramatic temperature changes. In warmer climates, water temperatures do not fall below 4°C. As a result, such lakes do not stratify in the winter, and many have only one mixing event each year following summer stratification.

The vertical mixing of the lake water that occurs in early spring and is assisted by winds that drive the surface currents is known as the spring turnover.

The spring turnover brings nutrients from sediments on the bottom to the surface and oxygen from the surface to the depths. This mixing results in the rapid growth of phytoplankton, the algae the float throughout the water column and serve as a major food source for herbivores.

In most lakes and ponds in temperate and polar regions, the temperature of the water forms layers.

The surface water, known as the epilimnion, can have a different temperature than the deeper water, known as the hypolimnion. Between these two temperature regions is the thermocline, which is a middle depth of water that experiences a rapid change in temperature over a relatively short distance in depth. The thermocline serves as a barrier to a mixing between the epilimnion and hypolimnion.

The fraction of solar energy reflected by an object is its

albedo

Warm tropical water circulates up through the western reaches of ocean basins toward the poles

cold polar water circulates down the eastern edges of ocean basins toward the tropics.

The atmospheric convection currents that move air between 60° and 90° latitudes are called

polar cells.

The intertidal zone is a biome consisting of the narrow band of coastline between the levels of high tide and low tide.

. As the tides come in and go out, the intertidal zone experiences widely fluctuating temperatures and salt concentrations. The species living in this biome—including crabs, barnacles, sponges, mussels, and algae—must therefore possess adaptations that allow them to tolerate such harsh conditions. Intertidal zones can occur along steep rocky coastlines, as one might find in Maine, or gently sloping mudflats, as one might find in Cape Cod Bay in Massachusetts

The intense sunlight at the solar equator drives the Hadley cells and the ITCZ, causing the warmed air to rise and precipitation to be released in the form of rain

. This also means that the seasonal movement of the solar equator influences seasonal patterns of rainfall

Rain shadow

A region with dry conditions found on the leeward side of a mountain range as a result of humid winds from the ocean, causing precipitation on the windward side.

Subtropical deserts, are characterized by hot temperatures, scarce rainfall, long growing seasons, and sparse vegetation.

Also known as hot deserts, subtropical deserts develop at 20° to 30° north and south of the equator, in areas associated with the dry, descending air of Hadley cells. Subtropical deserts include the Mojave Desert in North America, the Sahara Desert in Africa, the Arabian Desert in Asia, and the Great Victoria Desert in Australia. Because of the low rainfall, the soils of subtropical deserts are shallow, virtually devoid of organic matter, and neutral in pH. Whereas sagebrush dominates the cold deserts of the Great Basin, creosote bush (Larrea tridentata) dominates the subtropical deserts of the Americas. Moister sites support a profusion of succulent cacti, shrubs, and small trees, such as mesquite and paloverde (Cercidium microphyllum). Most subtropical deserts receive summer rainfall. After summer rains, many herbaceous plants sprout from dormant seeds, grow quickly, and reproduce before the soils dry out again. Few plants in subtropical deserts are frost-tolerant. Species diversity is usually much higher than in temperate arid lands.

Corals are tiny animals—related to hydra and other cnidarians—that live in a mutualistic relationship with algae.

An individual coral is a hollow tube that secretes a hard exoskeleton made of calcium carbonate. It also has tentacles that sweep food particles of detritus and plankton into the tube. As it digests these particles, the coral produces CO2 that can be used by their symbiotic algae in photosynthesis. Some of the sugars and other organic compounds the algae produce leak into the coral tissues and further support coral growth. Although an individual coral is tiny, corals live in huge colonies. As an individual coral dies, the soft tissues decompose but the hard outer skeletons remain behind. Over time, these skeletons accumulate to form massive coral reefs.

The pattern of air circulation near the equator also exists near the two poles

At approximately 60° N and 60° S latitudes, air rises up into the atmosphere and drops moisture. The cold, dry air then moves toward the poles and sinks back to Earth at approximately 90° N and 90° S latitudes. This air then moves along the ground back to 60° N and 60° S latitudes where it rises again.

The tundra is found in the Arctic regions of Russia, Canada, Scandinavia, and Alaska, and in the Antarctic regions along the edge of Antarctica and nearby islands

At high elevations within temperate latitudes, even within the tropics, one finds vegetation resembling that of Arctic tundra, including some of the same species or their close relatives. These areas of alpine tundra above the tree line occur widely in the Rocky Mountains of North America, the Alps of Europe, and, especially, on the Plateau of Tibet in central Asia. In spite of their similarities, alpine and Arctic tundra have important differences. Areas of alpine tundra generally have warmer and longer growing seasons, higher precipitation, less severe winters, greater productivity, better-drained soils, and higher species diversity than Arctic tundra. However, harsh winter conditions ultimately prevent the growth of trees in both Arctic tundra and alpine tundra.

The unequal heating of Earth explains the general pattern of declining temperatures as we move from the equator to the poles.

At the equator, the Sun's rays lose less energy to the atmosphere, solar energy is spread over a smaller area, and the low albedo of dark-colored forests causes much of this energy to be absorbed. Near the poles, however, the Sun's rays lose much more of their energy to the atmosphere, solar energy is spread over a larger area, and the high albedo of the snow-covered land causes much of this solar energy to be reflected.

ocean waters near the equator generally to be warmer than ocean waters at higher latitudes

Because of the unequal heating, the tropical waters expand as they warm. This expansion causes the water near the equator to be approximately 8 cm higher in elevation than the water in mid-latitudes. Although the difference may seem small, it is enough for the force of gravity to cause movement of water away from the equator.

The woodland/shrubland biome, is characterized by hot, dry summers and mild, wet winters, a combination that favors the growth of drought-tolerant grasses and shrubs

Because this type of climate is found around much of the Mediterranean Sea, it is often referred to as a Mediterranean climate regardless of where it is actually found. The woodland/shrubland biome has many different regional names, including chaparral in southern California, matorral in southern South America, fynbos in southern Africa, and maquis in the area surrounding the Mediterranean Sea. although there is a 12-month growing season, plant growth is limited by dry conditions in the summer and by cold temperatures in the winter. This biome supports thick evergreen shrubby vegetation 1 to 3 m in height, with deep roots and drought-resistant foliage. The small, durable leaves of typical Mediterranean-climate plants have earned the label of sclerophyllous ("hard-leaved") vegetation. Fires are frequent in the woodland/shrubland biome, and most plants have either fireresistant seeds or root crowns that resprout soon after a fire. Traditional human use of this biome has been for grazing animals and growing deeprooted crops such as wine grapes, as we discussed at the beginning of the chapter.

The nine biomes fall within three temperature ranges that we refer to often throughout this book.

Boreal forest and tundra biomes have average annual temperatures that are generally below 5°C. Temperate biomes—temperate rainforest, temperate seasonal forest, woodland/shrubland, and temperate grassland/cold desert—are warmer, with average annual temperatures generally between 5°C and 20°C. Finally, tropical biomes—tropical rainforest, tropical seasonal forest/savanna, and subtropical desert—are the warmest biomes, with average annual temperatures greater than 20°C.

As granite weathers, many of the positively charged elements—such as iron (Fe 3+ ) and calcium (Ca 2+ )—are replaced by hydrogen ions to form new, insoluble materials, such as the clay particles

Clay particles are important to the water-holding capacity of soils. They accumulate negative charges on their surfaces that attract positively charged ions called cations. Cations—including calcium (Ca 2+ ), magnesium (Mg 2+ ), potassium (K+ ), and sodium (Na + )—are important nutrients for plants.

Coral reefs are a marine biome found in warm, shallow waters that remain above 20°C year-round.

Coral reefs often surround volcanic islands, where they are fed by nutrients eroding from the rich volcanic soil and by deepwater currents forced upward by the profile of the island.

The differences in temperature around the globe are the result of how much solar radiation strikes the surface of Earth at a given location.

Differences in solar radiation are determined by the angle of the Sun striking different regions of the globe, the depth of the atmosphere that the energy passes through, and seasonal changes in the position of Earth relative to the Sun

Once the thermocline is well established, the surface and deep waters no longer mix because the warmer, less dense surface water floats on the cooler, denser water below, a condition known as stratification.

During the fall, the temperature of the surface layers of the lake drops. As this water becomes denser than the underlying water, it begins to sink. The vertical mixing that occurs in the fall and is assisted by winds that drive surface currents is called fall turnover. Similar to the spring turnover, fall turnover brings oxygen to deep waters and nutrients to the surface. The infusion of nutrients into surface waters in the fall may cause a second phytoplankton bloom. This mixing persists into late fall, until the temperature at the lake surface drops below 4°C and winter stratification ensues.

The tilt of Earth as it orbits around the Sun causes the Northern Hemisphere to receive more solar energy between March and September than the Southern Hemisphere

During this time the daylight period in the Northern Hemisphere is greater than the nighttime period, and the Sun's angle is 90° somewhere over the Northern Hemisphere

The tropical seasonal forest/savanna biome occurs in Central America, the Atlantic coast of South America, sub-Saharan Africa, southern Asia, and northwestern Australia

Fire and grazing play important roles in maintaining the character of the savanna biome. Under these conditions, grasses can persist better than other forms of vegetation. When grazing and fire are prevented within a savanna habitat, dry forest often begins to develop. As in more humid tropical environments, the soils tend to hold nutrients poorly but the warm temperatures favor rapid decomposition. Rapid decomposition provides a rapid recycling of nutrients into the soil, which trees can quickly take up and use for growth and reproduction. Such a rapid cycling of nutrients also makes this biome an attractive place for agriculture, including raising cattle. On the Pacific coast of Central America and on the Atlantic coast of South America, for example, over 99 percent of this biome has been converted to agriculture.

When they pass through the atmosphere, gases absorb some of the solar energy.

Following the path of the rays, you can see that the distance traveled through the atmosphere is shorter at the equator than at the poles. This means that less solar energy is removed by the atmosphere before it strikes Earth at the equator

Finally, some surfaces of the globe reflect solar energy more than others. Light-colored objects reflect a higher percentage of solar energy than darkcolored objects, which absorb most incoming solar energy

For example, asphalt absorbs 90 to 95 percent of the total solar energy that strikes its surface, which explains why asphalt pavement becomes so hot on a sunny summer afternoon. On the other hand, cropland reflects 10 to 25 percent of the total solar energy that strikes its surface and fresh snow reflects 80 to 95 percent.

The two circulation cells of air between the equator and 30° N and 30° S latitudes are known as

Hadley cells. The area where the two Hadley cells converge and cause large amounts of precipitation is known as the intertropical convergence zone (ITCZ).

The region between Hadley cells and polar cells can have dramatic changes in wind direction and therefore can experience large fluctuations in temperature and precipitation

However, winds generally move from west to east. This wind direction contributes to warmer conditions on the west coast of North America than on the east coast, as you can see from the plant hardiness zones

Because air and water currents are responsible for distributing energy throughout the world, the effects of an ENSO event extend over much of the world.

In North America, ENSO events bring cooler, wetter, and often stormy weather to the southern United States and northern Mexico, and warm, dry conditions to the northern United States and southern Canada. Some ENSO events have been particularly strong. For example, a strong ENSO event in 1982‒83 disrupted fisheries and destroyed kelp beds in California, caused reproductive failure of seabirds in the central Pacific Ocean, and killed off large areas of coral in Panama. Precipitation was also dramatically affected in many terrestrial ecosystems. Another ENSO event from 1991‒92 was accompanied by the worst drought of the twentieth century in Africa, which caused poor crop production and widespread starvation. The event also brought extreme dryness to many areas of tropical South America, Australia, and the islands of the South Pacific. Heat and drought in Australia reduced populations of red kangaroos to less than half their pre-ENSO levels.

This means that in any month in which the precipitation line goes below the temperature line, plant growth is constrained by a lack of sufficient precipitation.

In contrast, any month in which the temperature line goes below the precipitation line, plant growth is constrained by a lack of sufficient temperature. Since climate is the primary force determining the plant forms of different biomes, locations around the world that are from a particular biome have similar climate diagrams

Every 3 to 7 years, however, this series of events changes. In the atmosphere, the normal difference in air pressures reverses and the equatorial winds weaken

In some years, these winds can even reverse direction. This change in air pressures in the Southern Hemisphere is the Southern Oscillation element of the ENSO With either weakened or reversed equatorial winds, the warm surface waters of the western Pacific move east toward South America. As a result, the upwelling of nutrients shuts down and the normally productive fisheries in the area become much less productive. The accumulating warm water also serves as a source of increased precipitation for this region. The unusually warm water is the El Niño ("the baby boy") element of the ENSO, so named because it typically occurs around Christmas time.

Lotic systems are extremely sensitive to modification of their water flow by dams.

In the United States, tens of thousands of dams—built to control flooding, to provide water for irrigation, or to generate electricity—interrupt stream flow. Dams also alter water temperature and rates of sedimentation. Typically, water behind dams becomes warmer, and the original stream bottoms become filled with silt that destroys habitat for fish and other aquatic organisms. Water released downstream from large dams often has low concentrations of dissolved oxygen. Using dams for flood control changes the natural seasonal cycles of flooding that are necessary for maintaining many kinds of riparian habitats on floodplains. Dams also disrupt the natural movement of aquatic organisms upstream and downstream, fragmenting river systems and isolating populations.

Even farther west in North America, annual precipitation drops below 250 mm and the temperate grasslands grade into cold deserts, also known as temperate deserts.

In the United States, the cold desert extends across most of the Great Basin, which sits in the rain shadow of the Sierra Nevada and Cascade Mountains. In the northern part of the region, the dominant plant is sagebrush, whereas toward the south and on somewhat moister soils, widely spaced juniper and piñon trees predominate, forming open woodlands with trees less than 10 m in stature and sparse coverings of grass. In these cold deserts, evaporation and transpiration exceed precipitation during most of the year, resulting in dry soils. Fires are infrequent in cold deserts because the habitat produces so little plant material to burn. However, because of the low productivity of the plant community, grazing can exert strong pressure on the vegetation and may even favor the persistence of shrubs, which are not good forage. Indeed, many dry grasslands in the western United States and elsewhere in the world have been converted to deserts by overgrazing.

Successful survival strategies vary with climate.

In the world's deserts, for example, we find plants that are well adapted to scarce water availability. In North American deserts, many species of cacti have thick, waxy outer layers covered with hairs and spines to help reduce water loss. In Africa, we find a group of plants called euphorbs that are not closely related to the cacti of North America yet have many similar features

The most recent ENSO event happened in 2015‒16 and was one of the strongest ENSO events in recent decades.

It caused 1 to 5 °C warmer winter temperatures in Canada and increased precipitation in Northern California. The impacts around the Pacific were even larger, including record-setting heat and drought in Thailand, Malaysia, and India. The El Niño portion of an ENSO event is typically followed by a La Niña portion in which conditions in the southern Pacific Ocean oscillate strongly in the opposite direction. During La Niña, equatorial winds blow much stronger to the west and all the effects of El Niño event are reversed. Regions that become hotter and drier during the El Niño become cooler and wetter during La Niña. After the cycle of El Niño and La Niña, we typically experience several years of more normal weather conditions.

Freshwater wetlands are aquatic biomes that contain standing fresh water, or soils saturated with fresh water for at least part of the year, and are shallow enough to have emergent vegetation throughout all depths.

Most of the plants that grow in wetlands can tolerate low oxygen concentrations in the soil; many are specialized for these anoxic conditions and grow nowhere else. Freshwater wetlands include swamps, marshes, and bogs. Swamps contain emergent trees. Some of the best-known swamps are the Okefenokee Swamp in Georgia and Florida and the Great Dismal Swamp in Virginia and North Carolina. Marshes contain emergent nonwoody vegetation such as cattails. Some of the largest marshes in the world include the Everglades in Florida and the Pantanal of Brazil, Bolivia, and Paraguay. In contrast to swamps and marshes, bogs are characterized by acidic waters and contain a variety of plants, including sphagnum mosses and stunted trees that are specially adapted to these conditions. Some of the largest bogs are found in Canada, northern Europe, and Russia.

Small streams are often shaded and nutrient poor, which limits the productivity of algae and other photosynthetic organisms.

Much of the organic content of stream ecosystems depends on allochthonous inputs of organic matter, such as leaves, that come from outside the ecosystem. In large rivers, a higher proportion of the organic inputs are autochthonous, meaning that they are produced from inside the ecosystem by algae and aquatic plants. As rivers progress from their source, they typically become wider, slower-moving, more heavily laden with nutrients, and more exposed to direct sunlight. They also accumulate sediments that are washed in from the land and carried downstream. High turbidity caused by suspended sediments in the lower reaches of silt-laden rivers can block light and reduce production.

Around the globe, ocean circulation patterns are also affected by the dominant wind directions and Coriolis effects

North of the equator, for example, the northeast trade winds push surface water from the northeast to the southwest. At the same time, the Coriolis forces deflect ocean currents to the right. The combination of the two forces causes tropical water above the equator to move from east to west. The topography of ocean basins, particularly the locations of continents, forces these currents to change their direction. At mid-latitudes, the westerlies push surface waters to the northeast. As this occurs, Coriolis forces deflect the ocean currents to the right, which causes the ocean currents to move from west to east at midlatitudes in the Northern Hemisphere.

The hydrogen ions involved in weathering are derived from two sources.

One source is the carbonic acid that forms when carbon dioxide dissolves in rainwater. The other source of hydrogen ions is the decomposition of organic material in the soil itself. The metabolism of carbohydrates, for example, produces carbon dioxide. This carbon dioxide is converted into carbonic acid in water, which generates additional hydrogen ions.

The open ocean is characterized as the part of the ocean that is away from the shoreline and coral reefs

Open oceans cover the largest portion of the surface of Earth. Beneath the surface lies an immensely complex realm with large variations in temperature, salinity, light, pressure, and currents. Ecologists recognize a number of zones in the open ocean Beyond the range of the lowest tidal level, the neritic zone extends to depths of about 200 m, which corresponds to the edge of the continental shelf. Because strong waves move nutrients to the sunlit surface layers from sediments below, the neritic zone is generally a region of high productivity. Beyond the neritic zone, the seafloor drops rapidly to the great depths of the oceanic zone. Here, nutrients are sparse, and production is strictly limited. Finally, the benthic zone consists of the seafloor underlying the neritic and oceanic zones.

Most production in a lake occurs in the epilimnion, where sunlight is most intense

Oxygen produced by photosynthesis and oxygen entering the lake at the interface of the water and atmosphere keep the epilimnion well aerated and thus suitable for animal life. Throughout a growing season, however, plants and algae often deplete the supply of dissolved mineral nutrients in the epilimnion and this curtails their growth. In the hypolimnion, which can include the lower limnetic zone and the profundal zone, bacteria continue to decompose organic material, but the reduced intensity of light causes a reduction in photosynthesis. The result is that oxygen is used up faster than it is produced, and this leads to anaerobic conditions. Oxygen is in particularly short supply deep in productive lakes that generate abundant organic matter in the epilimnion.

The temperate grassland/cold desert biome, is characterized by hot, dry summers and cold, harsh winters, and is dominated by grasses, nonwoody flowering plants, and drought-adapted shrubs

Plant growth is constrained by a lack of precipitation in the summer and by cold temperatures in the winter. The biome is also known by a variety of different names around the world, including prairies in North America, pampas in South America, and steppes in eastern Europe and central Asia.

In the Northern Hemisphere, the length of the growing season in this biome varies from 130 days at higher latitudes to 180 days at lower latitudes.

Precipitation usually exceeds evaporation and transpiration; consequently, water tends to move downward through soils and to drain from the landscape as groundwater and as surface streams and rivers. Soils are often podsolized, tend to be slightly acidic and moderately leached, and contain abundant organic matter. The vegetation often includes a layer of smaller tree species and shrubs beneath the dominant trees, as well as herbaceous plants on the forest floor. Many of these herbaceous plants complete their growth and flower in early spring before the trees have fully leafed out.

As the biome name suggests, the dominant plant forms in temperate grasslands are grasses and nonwoody flowering plants that are well adapted to the frequent fires.

Precipitation varies widely across this biome. For example, on the eastern edge of North American prairies, annual precipitation can be 1,000 mm. In such areas, grasses can grow to more than 2 m high and are referred to as tallgrass prairies. There is even enough moisture in such areas to support the growth of trees, but the frequent fires prevent the trees from becoming a dominant component of this biome. As one moves west, annual precipitation declines to 500 mm or less. In these areas, grasses generally do not grow taller than 0.5 m and are referred to as shortgrass prairies. Because precipitation is infrequent, organic detritus does not decompose rapidly, and this makes the soils rich in organic matter. In addition, the weak acidity of the soils means that they are not heavily leached, and they tend to be rich in nutrients.

Salt marshes are a saltwater biome that contains nonwoody emergent vegetation.

Salt marshes are found along the coasts of continents in temperate climates, often within estuaries, which are areas along the coast where the mouths of freshwater rivers mix with the salt water from oceans. Estuaries are unique because of their mix of fresh and salt water. In addition, they contain an abundant supply of nutrients and sediments carried downstream by rivers. The rapid exchange of nutrients between the sediments and the surface in the shallow waters of an estuary supports extremely high growth of plants and algae. Because estuaries tend to be areas of sediment deposition, they are often edged by extensive saltwater marshes at temperate latitudes and by mangrove swamps in the tropics. With a combination of high nutrient levels and freedom from water stress, tidal marshes are among the most productive habitats on earth. They contribute organic matter to estuarine ecosystems, which, in turn, support large populations of oysters, crabs, fish, and the animals that feed on them.

BOREAL FORESTS

Stretching in a broad belt centered at about 50° N in North America and about 60° N in Europe and Asia lies the boreal forest. sometimes called taiga, is a biome densely populated by evergreen needle-leaved trees, with a short growing season and severe winters. The average annual temperature is generally below 5°C and annual precipitation generally ranges between 40 and 1,000 mm. Since evaporation is low, soils are moist throughout most of the growing season. The vegetation consists of dense, seemingly endless stands of 10- to 20-m tall evergreen needle-leaved trees, mostly spruces and firs. Because of the low temperatures, plant litter decomposes very slowly and accumulates at the soil surface, forming one of the largest reservoirs of organic carbon on Earth. The needle litter produces high levels of organic acids, so the soils are acidic, strongly podsolized (as discussed in Chapter 5), and generally of low fertility. Growing seasons rarely exceed 100 days, and are often half that long. The vegetation is extremely frost-tolerant; temperatures may reach −60°C during the winter. Since few species can survive in such harsh conditions, species diversity is very low. The boreal forest is not well suited for agriculture, but it serves as a source of timber products that include lumber and paper.

These large-scale water circulation patterns between continents are called gyres

The direction of the deflections caused by Coriolis forces depends on latitude gyres move in a clockwise direction in the Northern Hemisphere and in a counterclockwise direction in the Southern Hemisphere

Climates that support tropical rainforests are always warm and receive at least 2,000 mm of precipitation throughout the year, with rarely less than 100 mm during any single month.

The tropical rainforest climate often exhibits two peaks of rainfall centered on the equinoxes, corresponding to the periods when the intertropical convergence zone passes over the equator. Rainforest soils are typically old and deeply weathered from the high amounts of precipitation. Because they are relatively devoid of organic matter and clay, they take on the reddish color of aluminum and iron oxides and they retain nutrients poorly. Despite the poor ability of these soils to hold nutrients, the biological productivity of tropical rainforests per unit area exceeds that of any other terrestrial biome. Moreover, the standing biomass exceeds that of all other biomes except temperate rainforests. This tremendous growth is possible because the continuously high temperatures and abundant moisture cause organic matter to decompose quickly, and vegetation immediately takes up the released nutrients. While rapid nutrient cycling supports the high productivity of the rainforest, it also makes the rainforest ecosystem extremely vulnerable to disturbance. When tropical rainforests are cut and burned, many of the nutrients are carted off in logs or go up in smoke. The vulnerable soils erode rapidly and fill the streams with silt. In many cases, the environment degrades rapidly and the landscape becomes unproductive.

The temperate rainforest biome, is known for mild temperatures and abundant precipitation and is dominated by evergreen forests

These conditions are due to nearby warm ocean currents. This biome is most extensive near the Pacific coast in northwestern North America and in southern Chile, New Zealand, and Tasmania. The mild, rainy winters and foggy summers create conditions that support evergreen forests. In North America, these forests are dominated toward the south by coast redwood (Sequoia sempervirens) and toward the north by Douglas fir. These trees are typically 60 to 70 m tall and may grow to over 100 m, making them very attractive for harvesting as lumber. The fossil record shows that these plant communities are very old and they are remnants of forests that were vastly more extensive 70 million years ago. In contrast to tropical rainforests, temperate rainforests typically support few species

Tropical seasonal forests, are located mostly beyond 10° N and 10° S of the equator.

These regions experience warm temperatures and, as the intertropical convergence zone moves during the year, pronounced wet and dry seasons. Because tropical seasonal forests have a preponderance of deciduous trees that shed their leaves during the dry season, this biome is sometimes referred to as a tropical deciduous forest. In areas where the dry season is longer and more severe, the vegetation becomes shorter and develops thorns to protect leaves from grazing animals. With even longer dry periods, the vegetation grades from dry forest into thorn forest and finally into savannas, which are open landscapes containing grasses and occasional trees, including acacia and baobab trees.

These cold, dense waters then flow through the deep ocean basins and back into equatorial regions, where they eventually surface as upwelling currents.

These upwelling currents become warm and begin to make their way back to the North Atlantic. Like a giant conveyor belt, the thermohaline circulation slowly redistributes energy and nutrients among the oceans of the world in a trip that can take hundreds of years

Mangrove swamps are a biome that exists in salt water along tropical and subtropical coasts and contains salt-tolerant trees with roots submerged in water.

This biome can also occur in estuaries where fresh water and salt water mix. By living along the coasts, these salt-tolerant trees play important roles in preventing the erosion of coastal shorelines from constant incoming waves. The swamps also provide critical habitats to many species of fish and shellfish

we can see that when the water vapor condenses, it causes latent heat release, which further warms the air.

This enhances the upward motion of the air, condensation, and rainfall. As the air pressure continues to fall with rising altitude, the air temperature continues to drop. At high altitudes, the cool, dry air is pushed from below by more rising air and begins to move horizontally toward the poles.

The complex structure that corals build provides a wide variety of substrates and hiding places for algae and animals.

This helps to make coral reefs among the most diverse biomes on Earth rising sea surface temperatures in the tropics are causing the departure of the algal symbionts of corals over large areas—a phenomenon known as coral bleaching. Because the algal symbionts are critical to the survival of the coral, the stability of these biomes is now at risk.

Warmer and drier parts of the temperate seasonal forest biome, especially where soils are sandy and nutrient poor, tend to develop needle-leaved forests dominated by pines.

This includes the pine forests of the coastal plains of the Atlantic and Gulf coasts of the United States; pine forests also exist at higher elevations in the western United States. Because of the warm climate in the southeastern United States, decomposition is rapid. The rapid decomposition rates and sandy soils lead to the low availability of nutrients. The low nutrients and relatively dry conditions, in turn, favor the evergreen, needleleaved trees, which resist desiccation and give up nutrients slowly because they retain their needles for several years. Since soils in this biome tend to be dry, fires are frequent in the pine forests, although most species are able to resist fire damage. The temperate seasonal forest was one of the first biomes that European settlers in North America used for agriculture.

solar energy warms the air at the surface of Earth.

This warming causes the air to expand and rise. As air rises into regions of decreased atmospheric pressure, it expands. As the air expands, the temperature of the air cools through the mechanism of adiabatic cooling. This cooled air has a reduced capacity to contain water vapor, so the excess water vapor condenses and falls back to Earth as rain. This process, in which the surface air heats, rises, and releases excess water vapor in the form of rain, is the primary reason that latitudes near the equator experience high amounts of rainfall.

Plant communities reflect factors other than temperature and rainfall

Topography, soils, fire, seasonal variations in climate, and herbivory all affect which species can live in different plant communities.

Convergent evolution explains why we can recognize an association between the forms of organisms and the environments in which they live.

Trees found in tropical rainforests have the same general appearance no matter where they are located on Earth or their evolutionary lineage. The same can be said of shrubs inhabiting seasonally dry environments; they tend to have small, deciduous leaves and often have stems with spines to discourage herbivores from eating them

Our final group of biomes is found in areas of tropical temperatures and includes tropical rainforests, tropical seasonal forests/savannas, and subtropical deserts

Tropical rainforests, generally fall within 20° N and 20° S of the equator, are warm and rainy, and are characterized by multiple layers of lush vegetation. Tropical rainforests have a continuous canopy of 30 to 40 m trees with emerging trees that occasionally reach 55 m. Shorter trees and shrubs form a layer known as the understory below the canopy. The understory also contains an abundance of epiphytes and vines. Species diversity is higher in tropical rainforests than anywhere else on Earth. This biome occurs in much of Central America, the Amazon Basin, the Congo in southern West Africa, the eastern side of Madagascar, Southeast Asia, and the northeast coast of Australia. In many of these locations, however, much of the rainforest has been destroyed to harvest lumber and to make room for agriculture.

Downstream, water flows more slowly and becomes warmer and richer in nutrients

Under these conditions, ecosystems generally become more complex and more productive. In general, streams support fewer species than other aquatic biomes.

Sclerophyllous

Vegetation that has small, durable leaves.

Mountains also play a secondary role in determining climates

When winds blowing inland from the ocean encounter coastal mountains, the mountains force the air upward, which causes adiabatic cooling, condensation, and precipitation. The air, which is now dry and warmed by latent heat release, descends the other side of the mountain, warms adiabatically, and travels across the lowlands beyond, where it creates relatively warm, arid environments called rain shadows.

The temperate seasonal forest biome, occurs under moderate temperature and precipitation conditions, and is dominated by deciduous trees.

Winter temperatures can drop below freezing in this biome. The environmental conditions in this biome fluctuate much more than they do in the temperate rainforests because they do not benefit from the moderating effects of nearby warm ocean waters. In North America, the dominant plant growth form is deciduous trees, including maple, beech, and oak, which lose their leaves each fall. In North America, this biome stretches across the eastern United States and southeastern Canada; it is also widely distributed in Europe and eastern Asia. This biome is not common in the Southern Hemisphere, where the larger ratio of ocean surface to land moderates winter temperatures at many high altitudes and prevents frost.

The ability of a soil to retain these cations, called its cation exchange capacity, provides an index to the fertility of that soil.

Young soils have relatively few clay particles and little added organic material; this low cation exchange capacity leads to relatively low fertility. Older soils generally have a higher cation exchange capacity and therefore relatively high fertility. Soil fertility improves with time, up to a point. Eventually, weathering breaks down clay particles, cation exchange capacity decreases, and soil fertility drops.

Streams and some rivers are usually bordered by a riparian zone,

a band of terrestrial vegetation influenced by seasonal flooding and elevated water tables

Ocean currents are also driven by the thermohaline circulation

a global pattern of surface- and deep-water currents that flow as a result of variations in temperature and salinity that change the density of water. is responsible for the global movement of great masses of water between the major ocean basins. As wind-driven surface currents—for example, the Gulf Stream—move toward higher latitudes, the water cools and becomes denser. In the far north, toward Iceland and Greenland, the surface of the ocean freezes in winter. Because ice does not contain salts, the salt concentration of the underlying water rises, which causes the cold water to become even denser. This high-density water begins to sink and acts as the driving force behind a deep-water current in the Atlantic Ocean known as the North Atlantic Deep Water. Similar sinking currents are formed around the edges of Antarctica in the Southern Ocean.

Geographic regions that contain communities composed of organisms with similar adaptations are called

biomes Because of convergent evolution, we can categorize terrestrial ecosystems by dominant plant forms that are associated with distinct patterns of seasonal temperatures and precipitation. In aquatic ecosystems, the major producers are often not plants but algae. As a result, aquatic biomes are not easily characterized by the dominant growth forms of the producers. Instead, aquatic biomes are characterized by distinct patterns of depth, flow, and salinity.

Though the two groups of desert-adapted plants are descended from unrelated ancestors, they look similar because they have evolved under similar selective forces, a phenomenon known as

convergent evolution Convergent evolution can be observed in many organisms. For example, sharks and dolphins are not closely related to each other—one is a fish and the other is a mammal—yet both have evolved fins, powerful tails, and streamlined bodies. To perform well in an aquatic environment, natural selection has favored this set of traits because it allows both groups of animals to swim rapidly.

Sometimes ocean currents are substantially altered and this can affect climatic conditions. One of the best-known examples is the El Niño- Southern Oscillation (ENSO),

in which periodic changes in winds and ocean currents in the South Pacific cause weather changes throughout much of the world. During most years in the South Pacific Ocean, southeast trade winds and Coriolis forces push the surface waters of the Peru Current, which causes them to flow northwest along the west coast of South America, with upwelling of cold water along the coast. Equatorial winds—powered by high air pressures in the eastern Pacific and low air pressures in the western Pacific then push these surface waters offshore at Ecuador and west across the Pacific Ocean. As this water moves west, it warms. This warm water drives thunderstorm activity in the western Pacific, which results in high amounts of precipitation.

Moist continental mid-latitude climates exist at the

interior of continents and typically have warm summers, cold winters, and moderate amounts of precipitation.

Weathering

is the physical and chemical alteration of rock material near Earth's surface. It occurs whenever surface water penetrates the parent material. In cold climates, for example, the repeated freezing and thawing of water in crevices cause the rock to break into smaller pieces and expose a greater surface area of the rock to chemical reactions.

Because streams and rivers are characterized by flowing fresh water, they are often referred to as ____ systems.

lotic

Ponds and lakes are aquatic biomes characterized by

nonflowing fresh water with at least some area of water that is too deep for plants to rise above the water's surface Although there is no clear-cut distinction between ponds and lakes, ponds are smaller. Many lakes and ponds were formed as glaciers retreated, gouging out basins and leaving behind glacial deposits containing blocks of ice that eventually melted. The Great Lakes of North America formed in glacial basins, overlain until 10,000 years ago by thick ice. Lakes are also formed in geologically active regions, such as the Great Rift Valley of Africa, where vertical shifting of blocks of the Earth's crust created basins in which water accumulates. Broad river valleys, such as those of the Mississippi and Amazon rivers, have oxbow lakes, which are broad bends of what was once the river, cut off by shifts in the main channel

Finally, closest to the poles, we find

polar climates that experience very cold temperatures and relatively little precipitation.

As streams flow down from their headwaters, they join together with other streams and eventually grow large enough to be considered a ___

river

Although there is no exact specification to determine classification differences between a stream and a river, in general, ____ , also called ____, are narrow channels of fast-flowing fresh water, whereas ____ are wide channels of slow-flowing fresh water

streams, creeks, rivers

At approximately 30° N and 30° S latitudes, we commonly find the dry climates

that experience a wide range of temperatures. Dry climates are not only affected by latitude, however. Many dry climates are caused by rain shadows, for example, the extensive regions that lie just east of the Andes Mountains in western South America

CH 6: TUNDRAS

the coldest biome and is characterized by a treeless expanse above permanently frozen soil, or permafrost. The soils thaw to a depth of 0.5 to 1 m during the brief summer growing season. Annual precipitation is generally less, and often much less, than 600 mm, but in low-lying areas where permafrost prevents drainage, soils may remain saturated with water throughout most of the growing season. Tundra soils contain few nutrients. They also tend to be acidic because of their high content of organic matter, which is the result of cold conditions dramatically slowing the decomposition of organic matter. In this nutrient-poor environment, plants hold their foliage for years. Most plants are dwarf, prostrate woody shrubs, which grow low to the ground to gain protection under the winter blanket of snow and ice, since anything protruding above the surface of the snow is sheared off by blowing ice crystals. For most of the year, the tundra is an exceedingly harsh environment, but during summer days with 24 hours of sunlight, there is a rush of biological activity.

The key aspect of the process of weathering is

the displacement of many of these elements by hydrogen ions, followed by the reorganization of the remaining minerals into new types of minerals.

At these times of the year, the equator receives the greatest amount of solar radiation and the poles receive the least. Three factors dictate this pattern:

the distance that sunlight must pass through Earth's atmosphere, the angle at which the Sun's rays hit Earth, and the reflectivity of Earth's surface

Once the cool, dry air is displaced horizontally toward the poles, it begins to sink back toward Earth at approximately 30° N and 30° S latitudes.

this dry air sinks toward Earth where increased pressure causes it to compress. As the air compresses, it experiences adiabatic heating. By the time the air falls back to the surface of Earth, it is hot and dry. This explains why many of the major deserts of the world—which are characterized by hot, dry air—are located at approximately 30° N and 30° S latitudes. Once this hot, dry air reaches the ground, it flows back toward the equator, completing the air circulation cycle.

Any upward movement of ocean water is referred to as upwelling.

upwelling occurs in locations along continents where surface currents move away from the coastline. As surface water moves away from land, cold water from beneath is drawn upward. Strong upwelling zones occur on the western coasts of continents where gyres move surface currents toward the equator and then veer from the continents. As surface water moves away from the continents, it is replaced by water rising from greater depths. Because deep water tends to be rich in nutrients, upwelling zones are often regions of high biological productivity. Major commercial fisheries are often located in these zones.

Tropical climates, characterized by

warm temperatures and high precipitation, occur in regions near the equator

Moist subtropical mid-latitude climates are characterized by

warm, dry summers and cold, wet winters.

The initial chemical alteration of the rock occurs when

water dissolves some of the more soluble minerals, such as sodium chloride (NaCl) and calcium sulfate (CaSO4 ). Further chemical reactions continue the soil building process.

To visualize the patterns of temperature and precipitation that are associated with particular biomes, scientists use climate diagrams

which are graphs that plot the average monthly temperature and precipitation of a specific location on Earth. Climate diagrams can also indicate whether plant growth is more limited by temperature or by precipitation. For every 10°C increase in temperature, plants require an additional 2 cm of monthly precipitation to meet the increased water needs. Climate diagrams adjust their temperature and precipitation axes such that every 10°C increase in average monthly temperature corresponds to a 2 cm increase in monthly precipitation.

The intensity of solar radiation that strikes an area also depends on the angle of the Sun's rays

you can see that when the Sun is positioned directly above the equator, the rays of the Sun strike Earth at a right angle. This causes a large quantity of solar energy to strike a small area. In contrast, near the poles the rays of the Sun strike Earth at an oblique angle, which causes the solar energy to spread over a larger area. As a result, Earth's surface receives more solar energy per square meter near the equator than near the poles