Geoscience Reference
In-Depth Information
dioxide in the rainforest biomass. When the forest is destroyed, the sink they once
provided is destroyed.
Soils
Soils within the Neotropical rainforest are varied. They are classified into three
main groups: oxisols, ultisols, and inceptisols. Oxisols make up about 50 percent of
all Neotropical soils. They are deep red or yellow infertile soils. Two-thirds of all
tropical oxisols are found in South America. They occur primarily in areas influ-
enced by the Guyanan and Brazilian shield (remnants of the Precambrian basement
rock). Oxisols are found throughout the Amazon and in Colombia along the Pacific
Coast. The other deeply weathered soils in the Neotropics, the ultisols, occur in the
Amazon Basin and along the eastern coast of Central America and Brazil. These
tropical soils tend to have a high clay content, making them slippery when wet with
high erosion potential. About 32 percent of Neotropical soils are ultisols.
The other soil types are inceptisols, entisols, and alfisols. Inceptisols occupy the
largest portion of those soils. Half of the inceptisols occurring in the Neotropics
occur on older alluvial plains along major rivers, others are of volcanic origin.
Inceptisols are more fertile than oxisols or ultisols.
A unique white, sandy soil exists in parts of the Amazon Basin. This soil derives
from the eroding and weathering of the ancient Brazilian and Guyanan shields
and from former coastal beaches. These soils are extremely poor in mineral content
and are infertile, having been weathered for hundreds of millions of years. They are
extremely well drained and nutrient poor. A unique forest called a caatinga grows
on these soils, but if it is removed, the forest does not return. The caatinga is described
in further detail in Chapter 5, Regional Expressions of Tropical Seasonal Forests.
All soils of the Neotropical rainforest tend to be rather infertile and low in
nutrients and minerals. Interestingly, recent studies document that sands from the
Sahara travel across the Atlantic and enrich the soils of the Neotropical rainforest.
Vegetation in the Neotropical Rainforest
The Neotropical rainforest throughout its distribution remains similar in general
plant structure and appearance to other rainforests (see Chapter 2). However, the
particular plant and animal species within the forest are distinct. The vertical strati-
fication of three canopy layers and two understory layers is typical. Along with
these tree layers, are an accompanying array of woody vines, orchids, bromeliads,
and other epiphytes (see Figure 3.2).
Forest Structure
Neotropical rainforests hold some of the tallest rainforest trees in the world. The
emergents are widely spaced trees ranging on average from 100-165 ft (30-50 m)
