for sailing ships carrying goods for trading), except
near the equator at the ITC, an area called the doldrums
(fig. 2- 4). In the doldrums the winds are usually
light (often becalming sailing ships during historic
times). From the equator to 30° N, the easterly trade
winds blow steadily from the northeast, a direction
determined by the constant rotation of Earth from west
to east. South of the equator to 30° S, the easterly trades
blow from the southeast, again due to the rotational
motion of the planet. As Earth, tilted at its axis, moves
in its orbit around the sun, its direct angle to the sun’s
radiation varies with latitude, causing seasonal change.
Such change is manifested in the tropics essentially by
changing heat patterns of air masses around the ITC,
resulting in seasonal variation in rainfall.
In the Western Hemisphere, severe wind with
accompanying anticyclonic rainstorms, called
hurricanes (see “The Impact of Hurricanes,” below),
often occurs from July throughout October in parts of
the Neotropics. Similar kinds of storms are referred to
as monsoons in the Old World tropics.
The Biome Concept
Ecologists have long understood that the terrestrial
world is organized into belts of major ecosystem
types termed biomes. The tropical rain forest biome
is equatorial, within the Torrid Zone. At the planet’s
northern extreme is the arctic tundra biome, a vast
polar region of treeless, windswept plains composed
of mosses, lichens, and numerous perennial prostrate
plants. South of the tundra is the northern boreal forest
biome, often called the “spruce- moose” biome, a dense
latitudinal belt extending around the world composed
of mostly evergreen needle- leaved trees such as spruces,
firs, and various pines. South of the boreal forest is the
deciduous forest biome, a forest numerically dominated
by broad- leaved tree species such as sycamores, oaks,
maples, hickories, and numerous others, all of which
drop leaves relatively synchronously in autumn and
endure the cold of winter in a leafless state. There
are also grassland and desert biomes, where water is
variously limited. Most of the Great Plains of North
America is natural grassland. The huge Mojave Desert
west of the Rocky Mountains is cold desert, with
winter snow. The Chihuahuan and Sonoran Deserts
of the American Southwest and northern Mexico are
considered hot deserts, where snow is rare.
Length of the growing season is key to understanding
how biomes vary. For example, the growing season
in Ontario, Canada, which falls in the boreal forest
biome, is a mere 140 days per year (obviously during
the summer months). In comparison, North Carolina
enjoys a growing season of about 200 days per year.
This is why as you move south in March from, say,
central New England to Georgia, the landscape
becomes progressively greener. The growing season is
longer in the South.
In the tropics the growing season is generally
year- round. This fact is extremely important in
understanding the nature of ecosystems such as rain
forest. The only factor limiting growing season in the
lowland tropics is distribution of moisture. In areas of
extreme dry season, the growing season is confined
mostly to times when rainfall is adequate to support
continued plant growth.Direction of rotation
North
poleSouth
pole
Pressure and wind belts of the worldEquatorPrevailing westerliesPolar easterlies
Subpolar calm 60 NSubpolar calmNortheast trade windsHorse latitudesEquatorial calms — DoldrumsHorse latitudesPrevailing westerlies30 S60 SSoutheast trade windsPolar easterlies30 NFigure 2– 4. As planet Earth rotates from west to east it
influences the movement of the various heat belts of the
planet, creating trade winds north and south of the equator
and the doldrums at the equator. Reprinted with permission
from Kricher, John. Tropical Ecology. Princeton, NJ: Princeton
University Press, 2011.32 chapter 2 why it is hot, humid, and rainy in the tropics