Biomes and the Physical Environment

•Temperature and precipitation determine ranges of species and biomes•Atmospheric circulation patterns determine climatic zonation•Whittaker and Walter plots summarize climatic data•Zonation in freshwater and marine environments

Plant geographers noticed areas with similar climates had similar looking plants, though often from unrelated families

A tree-forming cactus from Mexico (Cactaceae)

An East African spurge (Euphorbiaceae)

Leafless, thick, fleshy branches with spines have evolved to minimize water loss in unrelated plants—convergence. Cactus spines are leaf petioles and euphorb spines are stem spurs.

Climate is the major determinant of plant distribution

The distribution of sugar maple—this area has the temperature and precipitation regime that the species needs

Other maple species have different ecological tolerances and their ranges differ somewhat

Heinrich Walter’s biome classification was based on climate zones

Whittaker plotted temperature and precipitation data for each biome

At high temperatures, precipitation varies greatly

Variation in precipitation decreases as temperature decreases

Warm moist air rises in the tropics—constant rain

Hadley cells—a circulation pattern resulting from differential heating

Convergence zone—constant downwelling of cool dry air

Earth’s rotation deflects the surface flows in the Hadley cells because of the Coreolis force—rotational speed of the earth is highest at the equator

Air flowing toward the equator will be deflected west—it will be spinning more slowly than the earth and lag behind

poleward flow will be deflected east—it will be spining faster than the earth and surge forward

Clockwise circulation

Counter-clockwise circulation

Direction of spin40,000 km/d

35,000 km/d

20,000 km/d

Hadley cell pattern establishes the major deserts and rain forest zones of the world

Walter Climate diagramsAt higher temperatures plants require more water

Generally plants need about 20 mm of monthly ppt for every 10Cso the Precipitation scale interval is twice the temperature scale

When the blue line is above the orange line, precipitation is plentiful and temperature is the main determinant of plant growth

Walter plots allow meaningful comparisons among sites

Tropical rain forestBlue always above orange

DesertBlue always below orange

Tropical seasonal forestBlue below orange during dry season

Global distribution of the world’s biomes

The lake environment is structured around light and temperature

Light reaches the bottom—rooted plants

Phytoplankton are the major primary producers

little light—few rooted plants

Little wave disturbanceSediments accumulate

Climate doesn’t work nearly as well for classifying aquatic environments

The dimictic mixing pattern of temperate zone lakes

In the summer termperate lakes are stratified

Epilimnion—warm mixed layer,

Hypolimnion—cold lower layer

In the winter the lake is inversely stratified

The spring and fall are mixing periods, water column 4oC

Thermal stratification allows both warm and cold water species to coexist

Ecological zones in the ocean

Light, the continental shelf, and tides contribute to marine zonation

The intertidal zone is exposed to air regularly when the tide recedes

Coral reef are found in shallow well lit seas—highly productivesystems

The major ocean currents are driven by winds and the earth’s rotation

Upwelling zones occur mainly on the western sides of continentsSurface waters are drawn towards the equator and away from the continent by the coreolis force—this draws deep nutrient-rich water upward

The major ocean currents “gyres” generate zonation in temperature and productivity

A satellite image showing the gulf stream, a clockwise current in the North Atlantic