2.4

• Biomes are a major association of vegetation that share similar climate characteristics and so biome distribution is largely determined by the climate.
• The tri-cellular model of atmospheric circulation explains how thermal energy is distributed around the planet and why the major biomes are where they are. It is composed of three large-scale cells that have remained fairly constant over millions of years. Weather patterns associated with the mid-latitude depression vary in the short term but their general location is controlled by the tri-cellular model and so, is predictable.
• The Hadley cell is centred on the thermal equator, that is the point of greatest heating. This may or may not be the equator we know and recognise. The seasons are determined by the tilt of the earth. Hence the thermal equator shifts with the seasons.
• Polar cell


  Another simple, thermally driven cell
• Polar cell


  • Mid latitude regions (around 60°N and S) are the starting point
  • Warm air in the region rises to the troposphere where it tracks poleward and cools
  • Cooling in this cell is extreme, at the poles the air descends and dries creating high-pressure zones
  • Air then moves out from the poles as the Polar Easterlies
  • The high pressure creates stable conditions and clear skies
  • The polar cell acts as a very effective heat sink, the coldest temperatures on earth have been recorded in Antarctica (-89°C)
  • This balances out the incoming solar radiation at the equator
• The cold drives this cell, not the heat
• The Ferrel cell

  This is a slightly odd cell in that it appears to defy the laws of physics - it has rising air in cooler regions and sinking air in the warmer latitudes. 
• Earths tilt and seasons

  The earth rotates on its own axis and it revolves around the sun, these two facts have a number of impacts:

  • The rotation creates day and night.
  • The revolution on the tilted axis create the seasons.
  • The combination of rotation and revolution gives varying day length. 

  Dictionary
• • Aquatic - which are further subdivided into:
  • Freshwater: ponds and lakes, streams and rivers and wetlands such as bogs and swamps.
  • Marine: deep ocean, coral reefs, estuaries and mangrove swamps.
  • Forest – tropical rainforest, temperate forests and boreal or taiga.
  • Grassland – savanna and temperate.
  • Desert – hot, coastal and cold.
  • Tundra – arctic and alpine.
• Biodiversity will vary between deciduous forests but in general diversity is high for plants, invertebrates and small mammals. There is also a wide variety birds, reptiles and amphibians. Variations in soil can lead to specilised plants in the herbaceous layer as well as additional invertebrates. 
• Grassland biomes:

  • Tropical grassland or savanna located between the tropical rainforest and the deserts. The major savanna areas are in Africa, India and Australia.
  • Temperate grassland located between the tropical deserts and the temperate forests. They spread throughout central Asia and Europe (Steppes) South America (Pampas) and North America (Prairies).

   
• Nutrient cycle

  Some authorities contend that the nutrient stores and flows in the desert are so small that the nutrient cycle is not applicable. 
• K-selected species tend to produce few offspring at a time, but they invest a lot of time and energy in looking after them to ensure they survive. To be able to use this strategy the environment needs to be stable. In succession stability increases with time so K-strategists are more common in the climax community. 

  Dictionary
• On the other hand r-strategists (r-selected species) focus on increased quantity of offspring at the expense of quality. With little or no parental care survival chances are low but high numbers of offspring ensures at least some survive. 
• • Succession is a change over time; zonation is a spatial change in response to changing conditions.
• • Zonation occurs without succession, the rocky shore demonstrates zonation due to tidal changes not succession.
• Zonation is the change in a vegetation community along an environmental gradient. The change may be caused by changes in altitude, depth of water, tidal level, distance from the shore etc. Zonation changes are spatial and are determined by changes in the abiotic factors. 
• Succession is the predictable change in a vegetation community over time. It starts with a pioneer community then the vegetation transitions through various intermediate communities to the final climax community.