Geoscience Reference
In-Depth Information
and lead to the extinction of many plant and animal species. The development of
the Andes, about 15 mya, greatly influenced the South American subregion.
Climate
In the Neotropical seasonal forest, temperatures remain warm throughout the year.
Average temperatures range from 73
C). Rainfall occurs during the
summer and fall with total annual precipitation below 80 in (2,000 mm). These areas
experience a short dry period (one to two months) during the summer and a longer
dry period (two to six months) during the winter when little rain falls. Dry months
vary in duration on the Atlantic and Pacific sides. Prevailing airmasses in the Neo-
tropics flow from the oceans to the continents in both hemispheres. Seasonal forests
are found on the leeward side and in the rainshadow of volcanoes and mountains, or
in the interior of the continent. The shifting Intertropical Conversion Zone (ITCZ)
plays a major role in the influx of rainfall in these seasonal forests (see Chapter 2).
During the summer, the sun progresses toward the subtropics along with the ITCZ,
and an increase in cloud cover and intense rainfall. In the winter, as the ITCZ moves
away, a zone of subtropical high pressure with little precipitation develops. In Central
America, tropical hurricanes can produce strong seasonal rainfall. Seasonal variations
in moisture availability create a variety of seasonal forest types in the Neotropics.
During glacial and interglacial periods of the Pleistocene Epoch, tropical sea-
sonal forests expanded and contracted. Some researchers suggest that during drier
times, much of the Amazon rainforest reverted to seasonal forest and savanna, and
when the Ice Ages ended, the rainforest expanded again, contracting and fragment-
ing the seasonal forests.
-80
F(23
-26
Soils
Soils are varied within the tropical seasonal forests of the Neotropical region.
In most areas, soils tend to be very old and nutrient-poor. In other areas, soils are
younger and fertile, a product of more recent volcanic activity. Underlying a large
part of the South American subregion is one of the oldest rock formations in the
world, the Precambrian Guyanan and Brazilian shields, and many of the soils in
the region reflect that ancient history.
Soils within the Neotropics are classified into three main types: oxisols, ultisols,
and inceptisols. Oxisols are deep red or yellow infertile soils, making up about 50
percent of all Neotropical soils. They occur primarily in areas influenced by the
Precambrian basement rock. Ultisols are other weathered soils in the Neotropics.
They have a high clay content and are slippery when wet and susceptible to ero-
sion. Inceptisols occur on older alluvial plains along major rivers or are derived
from volcanic activity and tend to be more fertile. Many of the forested areas where
