Terrestrial biomes of the world Lecture Topics:

Biome concept Biome classification, distribution Biome climate patterns (Walters Diagrams) Biome survey/distinctive ecological

characteristics

Introduction to biome concept Classification systems

Holdridge’s Life Zone system Whittaker’s classification scheme based on annual

precipitation, temperature (nine biomes recognized)--simpler for purposes here of describing basic biomes

Robert Whittaker’s classification of biomes, based on annual precipitation, temperature

Global distribution of biomes

Walter climate diagram: generic

Effective growing season (temperature above 0 ºC)

Ecosystem water-limited if temperature line (red) exceeds precipitation line (blue)

White zone denotes frozen conditions, inhospitable for life

Temperate seasonal forest biome

Temperate forest physical environment Found at temperate latitudes, typically 40-60º latitude Climate highly seasonal:

Cool winter & warm summer, plentiful precipitation all year Cold makes water unavailabledeciduous foliage (leaf

drop), brilliant fall colors (depending on species) Spring flush of leavesabundant herbivores

(caterpillars,etc.) and migratory birds that feed on them Vernal herbs flower in spring, where forest floor warms up

before trees leaf-out (e.g., sweet white violet) Different subtypes of biome

Northern hardwoods—e.g., sugar maple, yellow birch Southern hardwoods—oaks, sweet gum, tulip poplar, &

evergreens (magnolias, live oak, and pines). Southern pinewoods on poor, sandy soils; fire important

Examples of temperate seasonal forest

Louisiana examples: Bottomland hardwoods, upland mixed-mesophytic (coniferous & deciduous) forest

Temperate rain forest

Temperate rainforest characteristics

Found at temperate latitudes, typically 40-60º (Northwestern U.S., S. Chile, E. Australia, New Zealand)

Climate moderately seasonal: Mild winter (below freezing) versus warm to hot summer, Abundant winter rain Summer fog

Plants dominated by giant needle-leaved trees (in U.S.: redwoods, western hemlock, douglas fir, western cedar) Ancient, formerly widespread forest type Why giant trees? Year-round growing season, abundant

moisture, strong potential evapotranspiration all contribute

Examples of temperate rainforest plants

Temperate grassland/desert

Physical environment of temperate grassland/desert biome

Typically 25-50º N, S of equator Climate characterized by high rate evaporation (often warm

temperatures), & periodic severe (summer) droughts Rainfall 25-70 cm annually, too little to support forest Often subject to fire—e.g., prairie , longleaf pine (essentially

prairie with scattered pine trees) Steppes are cold deserts, dominated by shrubs & grassland

Plants—1º productivity proportional to rainfall Grasses dominate (because they’re competitive over trees where

fire, grazing predominate) High degree spatial heterogeneity in plants, due to topography,

soils, fire history, animal activity (e.g., prairie dogs, bison)

Examples of temperate grassland/desert vegetation

(also known as shrub-steppe)

Woodland/shrubland

Physical environment & plants of woodland/shrubland biome

Location (30-40º N, S of equator) Mediterranean region, coastal California, Chile,

southern S. Africa, south-western Australia Climate “Mediterranean” (mild, wet winter; hot, dry

summer = drought) Plants

Fire adapted, dense evergreen shrubs Often deeply rooted, to get at scarce (summer) water Sclerophyllous (waxy, drought-resistant) leaves Aromatic compounds that dissuade insect herbivores

Examples of woodland/shrubland vegetation

Vegetation type also known as matorral (Chile)

Subtropical desert

Physical characteristics of deserts Evaporation exceeds precipitation (rainfall <30 cm) 20-30º N & S Latitude (subtrop. high pressure zone) Plants adapted to drought (= xerophytes)

Small leaves dissipate heat, large edge:area ratio Water-storage mechanisms (e.g., many cacti) Conserve H2O (e.g., CAM photosynthesis, waxy cuticle)

Protected against herbivores—spines, aromatic compounds Diverse life-forms of desert plants:

Succulents (e.g., saguaro & barrel cactus) store water Ephemerals (annuals) grow rapidly, seed after a rain Phreatophytes (e.g., mesquite, palo verde) deeply rooted Opportunistic perennials (ocotillo) flush leaves, flower after

rain

Boreal forest

Boreal (northern coniferous) biome Climate & setting

Found typically from 50-70º N. Latitude (taiga), south in mts. Cold winter, cool-warm summer, 3-5 mo. growing season

Taiga (russian for “land of little sticks”) forms continuous stretches of boreal forest, dominated by trees in species of spruce (Picea), fir (Abies); also known as “spruce-moose” biome Diversity of trees low (1-2 spp./site) Conifers dominate (tolerate cold, photosynthesize whenever

it’s warm enough, conical shape sheds snow with minimal branch breakage)

Boreal forest vegetation

Tundra biome

Tundra physical environment & plants Climate & setting

Too cold, windy for trees; short growing season (50-60 days in arctic, up to 180 days in alpine zones at higher elevation)

Precipitation generally < 25 cm annually (cold air holds little moisture), “locked up” as snow (unavailable to plants)

Only in N. hemisphere, north. edge of all northern continents Plants—grassland & mixed shrubland (some dwarf trees)

Often low to ground = warmest microclimate, out of wind (e.g., cushion plants)

Lichens often important ground (& rock) cover, important in creating soils by breaking down rocks; alders (Alnus) can be an important nitrogen-fixer

Northern bogs characteristic of the wettest spots—dominated by carnivorous plants (animal source of N)

Some plants of tundra biome

Tropical rainforest biome

Climate & setting Tropical rainforest Near equator Climate continuously favorable for intense biological

activity (abundant rainfall, >400 cm, & high temperatures) Biological interactions (ecologically, evolutionarily) impt. Biological diversity staggering (latitudinal diversity gradient)

Diverse plant life forms: trees, shrubs, epiphytes, epiphylls, vines, lianas (woody), ferns, tree ferns Layering of plants prominent: ground layer, shrubs, sub-

canopy, continuous canopy, & “emergents” (up to 60 m tall) Some plant adaptations:

Buttresses to help prop up often shallow-rooted trees (why?) Oval, waxy leaves with drip-tips (why?) Nutrient scavenging & extremely tight nutrient cycling

Some plants of tropical rainforest biome

Tropical seasonal forest biome

Tropical seasonal forest/savanna Tropical seasonal forest occurs at higher latitudes than

rainforest (10-20º N, S) Rainfall less than rainforests (240-400 cm), highly seasonal

3-6 month dry season, often with no rainfall Dry forests have almost as diverse plant and animal

communities as rainforests Trees typically drought-deciduous (facultative) Ant-acacia mutualism is characteristic of dry forests of

Central America Biome highly threatened by human activities, because of its

potential for agriculture, grazing

Some trees of tropical savannas

Acknowledgements: Some illustrations for this lecture from R.E. Ricklefs. 2001. The Economy of Nature, 5th Edition. W.H. Freeman and Company, New York.