[ Jungle sounds ]
>> We've learned that there is a globally predictable gradient
in seasonality caused by the tilt of the earth on its axis
in relation to its path around the sun.
As you move toward the tropics from the poles,
abiotic conditions become relatively constant,
and on average conditions are warmer and wetter.
These extremely broad global patterns largely explain the
latitudinal distribution of the boreal, temperate,
and tropical forest biomes.
Remember in the introduction I mentioned some lesser-known
forest types, including the tropical deciduous forest.
These seasonal forests are characterized
by a long dry season and tend to occur at intermediate latitudes
between about 10 and 25 degrees.
To understand the distribution of this forest we need
to discuss global rainfall seasonality.
This graph shows where the sun is directly overhead
for each month of the year.
Where the sun is directly overhead it heats the earth
below, clouds are formed and participation follows.
Now look at the latitudes
where we find tropical deciduous forests
in the northern hemisphere,
from March to September the sun passes
over these latitudes bringing precipitation.
Then from September through March these forests enter a dry
season while the sun is over the southern hemisphere.
The opposite is true for the tropical deciduous forests
in the southern hemisphere.
Another big factor that influences the distribution
of forest biomes is wind.
The spin of the earth from west to east
on its axis creates a Coriolis force
which deflects free moving masses such as air and water.
The earth's surface moves much faster at the equator
than at the poles because the circumference is much larger,
so there's a deflection of air masses to the right
in the northern hemisphere and to the left
in the southern hemisphere.
These winds are known as the trade winds,
because they powered merchant ships
from Europe to the New World.
At higher latitudes air masses moving pole-ward are deflected
in the same direction right in the northern hemisphere and left
in the southern producing the Westerlies.
The impact of the trade winds on climate
and thus vegetation can be seen
in this precipitation map for South America.
Notice that the Amazon basin has higher rainfall
in the West than in the East.
That's because the trade winds push moisture-laden air
from the east resulting in more rainfall in the western Amazon.
The Coriolis force is also evident in major ocean currents.
Surface water pushed by wind is deflected to the right
in the northern hemisphere setting up a clockwise rotation
and to the left in the southern hemisphere resulting
in a counterclockwise flow.
Look at the Atlantic Ocean.
Warm water from the tropics, represented by the red arrows,
is pushed westward by the trade winds but is deflected
to the right in the northern hemisphere,
forming a warm current known as the Gulf Stream
that runs along the eastern United States.
As the Gulf Stream reaches temperate latitudes it cools
and is pushed eastward by the westerly winds continuing
to be deflected to the right, the result is a cold current
that flows back toward the equator off the West Coast
of Europe and North Africa.
This circulation pattern is the same in all the world's oceans.
It results in warmer waters off the East Coasts
of all the major confidence
and colder waters off the West Coasts.
Which side of the continents, then,
have wetter vegetation types?
Check out North America: there's forests in the East fed
by that moist air of the Gulf Stream, and desert and scrub
in the West, kept dry by West winds off the cold
California current.
South America, too, forest and grassland in the East,
but in the West, the Atacama Desert, the driest place
on earth, adjacent to the cold humbled current off the
continent's West Coast.
Australia is the same, rain forests
and temperate forests only occur in the East,
and dryer ecosystems are in the West.
Lastly, I need to mention elevation.
When an air mass hits some topography
like a mountain range it is forced up and over and pushed
by the wind behind it.
As this air rises, it cools at a fairly predictable rate
of around six degrees Celsius for every 1000 meters.
This is called the adiabatic lapse rate.
As you know cooling air loses its water.
So a couple of interesting phenomena come out of this.
One is that the windward side of a mountain is usually rainier
than the leeward side.
The second, is that the other side of the mountain is dryer,
a phenomenon known as the rain shadow effect.
You can see the rain shadow effect very clearly
in the precipitation map of South America.
The Andes Mountains block moisture blowing
in from the east on the trade winds.
The eastern side of the Andes thus receives tons of rain,
but dry forests and deserts occur on the western side.
You should now have a well-rounded knowledge
of the complex but predictable climate factors
that shape global vegetation.
Now when you look at any biome anywhere on earth,
you should be able to explain why it occurs there
in terms of global climate.
Start with the biome you live in, why is it found there?
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