2.4. 2: Why are Biomes where they are?

June 14th, 2010

The above animation was made by S Thomas

Ecologist Robert Whittaker plotted records of annual precipitation against annual temperature for locations around the planet and then grouped them within the biomes generally found in those places. The result was the graph on the left. This helps to illustrate that biomes that form anywhere in the world are mainly the result of the combination of rainfall and temperature found in those areas. The graph also illustrates that the concept of biome may not be as precise and clear cut as it at first appears. Marginal areas exist where a continuum of climatic conditions can give rise to a gradient of ecosystems as one biome is slowly replaced by another. Large areas of Boreal forest in Northern Europe slowly change as you move South into areas that support predominantly Deciduous forest.

This is because the climatic conditions across the planet are not distinct but themselves show a gradients. From the equators out both North and South temperature drops until the permanently frozen regions at the poles is encountered. The result of incoming solar radiation being spread over greater and greater area with the curvature of the Earth.

The maximum incoming solar radiation  at the equator gives rise to high temperatures which in turn lead to maximum evaporation of water from the large expanses of ocean found here. As the moisture laden warm air at the equator rises in the atmosphere it the water condenses out as clouds and falls back to Earth as exceptionally high rainfall. the rainfall which when combined with high temperatures and maximum sunlight creates the perfect conditions for maximum plant growth. The result equatorial or tropical rainforest.

This rapidly rising warm air sucks in air from both Southern and Northern latitudes along the planets surface. In the atmosphere the still warm but now dry air moves away from the equator. These two air currents set up an atmospheric cell with descending warm dry air at around 30° north and south. This leads to the establishment of desert biomes at these latitudes.

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