Biosphere- I. Terrestrial biomes

Learning goals:

1) learn the distinctive climatic features for each biome - e.g. seasonality of temperatures and precipitation

2) the characteristic plant growth forms of  the biome

3) the important disturbance agents, if any (e.g. fire)

4) how and how much humans have altered the biome

Living things are found on every part of Earth, from the highest mountains to the deepest oceans.  Bacteria and archaea are found everywhere, even on dust high in the atmosphere.

Most organisms occur within a thin veneer of Earth’s surface, from the tops of trees to the surface soil layers, and within 200 meters of the surface of the oceans = Biosphere

Biomes are large-scale terrestrial biological communities shaped by the physical environment, particularly climatic variation and often natural disturbances such as fire and grazing.  Biomes are characterized by the dominant growth forms of plants, which integrate the physical environment; growth form includes height, plant form (e.g. herbs, shrubs, trees), type of leaves (needles, broad leaves) and leaf structure (e.g. thickness).

Convergence: Evolution of similar growth forms among distantly related species in response to similar selection pressures.

Biomes reflect the major climatic zones, including both the average and seasonal variation in temperature and precipitation.  Actual distribution of biomes is greatly influenced by human activities - land-use change – especially agriculture, grazing, forestry, and urbanization- approximately 60% of Earth surface has been altered by land-use-change.

READ THE TEXT TO LEARN ABOUT THE SPECIFIC BIOMES, INCLUDING THE MAJOR CLIMATIC CHARACTERISTICS, VEGETATION (GROWTH FORMS), AND ANTHROPOGENIC FACTORS THAT HAVE INFLUENCED THEM

Tropical Rainforests:
* Between 10° N and S.
* Annual precipitation > 200 cm.
* High biomass, high diversity—about 50% of Earth’s species.
* Broadleaved evergreen and deciduous trees.
Tropical rainforests are disappearing rapidly due to logging and conversion to pasture and croplands.  About half of the tropical rainforest biome has been altered.

Tropical Seasonal Forests and Savannas:
* From 10° to 23.5°S and N
* Wet and dry seasons associated with movement of the ITCZ.
* Shorter trees, deciduous in dry seasons, more grasses and shrubs
* Influenced by fire and grazing
* large expanses converted to agriculture
Includes a complex of tree-dominated systems:
Tropical dry forests – many trees lose leaves in the dry season
Thorn woodlands—trees have heavy thorns to protect from herbivores
Tropical savannas—grasses with intermixed trees and shrubs

Hot Subtropical Deserts
* Associated with high pressure zones (subsidence from Hadley cells) around 30° N and S
* High temperatures, low water availability
* Sparse vegetation and animal populations.
* Many plants exhibit stem succulence—e.g. cacti in the Western Hemisphere, euphorbs in the Eastern Hemisphere, as well as drought-deciduous shrubs, grasses, and short-lived annual plants that are active only after a rain

Temperate Grasslands:
* Between 30° and 50° latitude
* Seasonal temperature variation—warm, moist summers and cold, dry winters
* Grasses dominate; often maintained by frequent fires and large herbivores
Most of the fertile grasslands of central North America and Eurasia have been converted to agriculture
Some marginal grasslands have been overgrazed and converted to desert

Temperate Shrublands and Woodlands:
* Wet season in winter; hot, dry summers.
* Mediterranean-type climates—west coasts of the Americas, Africa, Australia, and Europe, between 30°–40° N and S
* Vegetation characterized by evergreen shrubs and trees
* Fire is a common feature in some shrublands
Mediterranean shrublands have been subject to land-use change for centuries, sometimes with poor results- soils and climate not well suited for agriculture (with important exceptions)
Temperate shrublands and woodlands often associated with rain shadows

Temperate Deciduous Forests:
* Occur at 30° to 50° N, on continental edges, in areas with rainfall to support tree growth.
* Leaves are dropped during winter.
* Oaks, maples, and beeches are common in both E and W hemisphere in this biome.
Chesnuts formerly important component of North American deciduous forests, but Asian fungus (blight) wiped most of them out.
Regrowth of deciduous forests following a long period of agricultural use is common in NA and Eurasia, as agriculture has shifted to tropical and sub-tropical biomes

Temperate Evergreen Forests:
* At 30° to 50° N and S, in coastal and maritime zones
* Lower diversity than tropical and deciduous forests
* Leaves tend to be acidic, and soils nutrient-poor
* Temperate rainforests receive 50–400 cm rain per year
* Low nutrient availability, acidic soils, and fire are common features

Boreal Forests (Taiga):
* Between 50° and 65° N.
* Long, severe winters.
* Permafrost (subsurface soil that remains frozen year-round) impedes drainage and causes soils to be saturated.
* Dominated by coniferous trees—pines, spruces, larches, except for birch in parts of Scandinavia
* Fires possible during dry summers
* In low-lying areas, extensive peat bogs form

Tundra:
* Above 65° latitude, mostly in the Arctic
* Cold temperatures, low precipitation
* Short summer with long days
* Vegetation is sedges, forbs, grasses, low-growing shrubs, lichens, and mosses
* Permafrost is widespread
* Biome most affected by global climate change

Mountain biological zonation reflects similar changes in latitudinal distribution of biomes, except that some biological communities lack a biome analog (e.g. tropical mountains)