Book Summary: “Tropical Forest Biomes” by Barbara Holzman

Title: Tropical Forest Biomes
Author: Barbara Holzman
Scope: 3 stars
Readability: 3 stars
My personal rating: 4 stars
See more on my book rating system.

Topic of Book

Holzman overviews the key characteristics of Tropical Forest biomes.


I believe that biomes are a neglected factor in accounting for differing levels of economic development. This book has nothing to do with human history or progress, but is an excellent overview of the Tropical Forest biome. This book is part of a an excellent series of books on biomes.

Key Take-aways

  • While Tropical Forests are outstanding for plant and animal diversity, they offer some serious disadvantages for human habitation. In particular, the near impossibility of implementing plow-based agriculture created limits to producing a food surplus.

Important Points from Book

The Tropical Rainforest Biome occurs closer to the Equator where day length, temperature, and rainfall are consistent throughout the year. The Tropical Seasonal Forest Biome is present away from the equatorial zone, toward the tropical margins. In a few areas, these tropical forests stretch beyond the tropics, due to specific oceanic or climatic influences. Tropical rainforests make up about 86 percent of tropical forest; the remaining 14 percent is composed of Tropical Seasonal Forest.

Tropical rainforests are broadleaved evergreen forests found at elevations below 3,300 ft (1,000 m); temperatures are warm year-round and precipitation is high. Tropical seasonal forests are broadleaved deciduous forests and evergreen dry forests found in regions that are both warm year-round and strongly water limited with several months of drought.

Some characteristics of the Tropical Rainforest Biome include more than 100 ft (30 m) tall emergent trees, with buttressed bases, understory plants with large evergreen leaves with rain-adapted ‘‘drip tips,’’ and hundreds of species of epiphytes and woody vines that hang on trees throughout the forest canopy. On the margins of the tropics, drier or xeric plant communities such as thorn forest have short trees less than 15 ft (5 m) tall and lacking buttresses.

Species are abundant in the Tropical Forest Biomes; however, in many cases their populations are small. Many species within tropical forests have restricted distribution areas. There may be as many as 100–300 different trees within 2.4 ac (1 ha) of tropical rainforest, but few will be found in other parts of the forest. Significantly more species than genera are found in the Tropical Forests; in other words, the species to genera ratio is high and many congeners—species in the same genus—are found. Nonetheless, the numbers of both species and genera far exceed the numbers in other biomes.

Equal day length, constant heat and moisture, and the effects of large water bodies surrounding most of the tropics provide the Tropical Forest Biomes with temperatures that are constant throughout the year. In general, the Tropical Rainforest Biome’s average annual temperature ranges from 79–81 F (26–27C). Daily temperatures are more varied than annual temperatures, shifting as much as 8 F (4.5C) in some areas, depending on cloud cover and rainfall.

In the rainforest, the dry season is not truly dry. During this time, the rain falls intermittently, with an occasional week or two with no rain. Precipitation amounts decrease as you move away from the Equator. In monsoonal and seasonal forests, there is a true dry season, usually for four to seven months, but that is compensated for by abundant precipitation throughout the rest of the year.

In Tropical Forests, day length (photoperiod) and the angle at which sunlight strikes the Earth’s surface varies little throughout the year.

Two major global circulation systems affect the Tropical Forests of the world. They are the Intertropical Convergence Zone (ITCZ) and the Trade Winds. These two systems regulate the climate and weather patterns in the tropics.

Soils within the tropical zone are the product of high heat and moisture over millions of years. They are primarily highly weathered ancient soils with high acidity and low nutrient and organic matter content. They tend to be red or yellow in color. Other younger soils do occur within the tropics, and they tend to be more fertile and have a higher nutrient and mineral content than those developed on ancient surfaces. They are brown or black in color due to the higher levels of organic matter in the soil and their volcanic origins.

The fauna as well as flora of Tropical Forest Biomes are highly diverse. The length of time these regions have remained isolated with abundant heat and moisture have provided ample opportunity for animals to develop unique niches and strategies for survival. Continuous plant diversification through evolution leads to increasingly diverse habitats and abundant opportunities for the diversification of animal species. These millions of years of isolation with little floral or faunal exchange have created opportunities for speciation through adaptive radiation, the evolution of many species from a single ancestral species. This often happens in concert with isolation.

Another evolutionary phenomenon displayed in the tropical rainforests of the world is convergent evolution.

The Tropical Forest Biomes are the most ancient, diverse, and ecologically complex biomes on the planet and support more than half of the Earth’s species. At least 3 million species are known to inhabit the tropical forests of the world.

See page 10 for characteristics of Rain Forests and Seasonal Forests.

The closed canopy forests we see in today’s Tropical Rainforest Biome became widespread around 65–50 mya, in the early Tertiary Period.

The Earth receives 99 percent of its energy from the sun. The tropical zone between 23 N and 23 S latitude receives the highest amount of this energy because of the Earth’s revolution, rotation, and axial tilt. A constant input of the sun’s energy, along with the Earth’s axial tilt and revolution around the sun, results in minimal seasonal variation.

In the Tropical Rainforest, the transfer of energy from the sun to the forest primarily occurs in the forest canopy. Less than 10 percent of the incoming solar radiation actually reaches the ground due to the density and height of canopy trees.

Very little of the solar radiation entering the forest is reradiated or returned to the atmosphere.

It is both the amount and continuity of rainfall that allows tropical rainforests to remain evergreen. Evergreen plants in the tropics can only exist where annual rainfall amounts, in combination with water storage capacity, are sufficient to maintain a positive water balance throughout the year. This means that the forest encounters minimal water stress. An annual rainfall of at least 100 in (2,500 mm) has been proposed as the minimum amount needed to maintain a tropical rainforest. However, this rainfall must be distributed evenly throughout the year. A negative water balance for more than a month has been shown to have a devastating effect on the tropical rainforest.


Tropical soils are primarily Oxisols….  Oxisols have little to no organic or humus soil layer. The constant heat, abundant moisture, and rapid bacterial decay prevent the accumulation of humus. Because of intense chemical weathering and abundant rainfall, all soluble minerals, and even some that are relatively insoluble, are leached.

Oxisols are very porous and have very low fertility.

Soils from andesitic ash are rich in organic matter and have a high water-holding capacity. Because they are rich in organic matter, they tend to be black or brown in color.

Inceptisols play an important part in the vegetation of the humid tropics of the Philippines, Papua New Guinea, and Indonesia. This soil also occurs in parts of Central America, the Caribbean, and Ecuador, as well as in the highlands of Central Africa where volcanism has dominated earlier landscapes. As these soils are quite fertile and easy to till, many of the areas with these soils have been converted to agriculture, growing coffee, tea, and cacao in most regions; coca in South America; and rice in the Asian Pacific.

Wet inceptisols (also called aquepts or gleysols by FAO) occur on older alluvial plains and along rivers and swamps in tropical Central and South America, Africa, and Asia. These permanently wet soils have been used for rice cultivation for centuries in Asia.

Intense organic activity decays material dropped from plants and dead organisms. This decomposing layer can be 6–8 in (15–20 cm) in depth. It is the lifeblood of the Tropical Rainforest Biome. The rapid decomposition of dead plants and animals is undertaken by many organisms, from insects to smaller bacteria and fungi.

Rainforest vegetation has adapted to soils with little fertility by rapidly recycling nutrients from decomposing material in the forest litter. Beneficial aerobic and anaerobic bacteria convert unusable compounds into usable minerals and nutrients that are necessary for plant growth… The roots of tropical plants tend to be shallow and often above the ground to allow for the greatest use of these nutrients.

This ancient biome occurs under optimal growing conditions: abundant precipitation and year-round warmth. With no annual rhythm to the forest, each species has evolved its own flowering and fruiting season.


The diversity within the forest allows great numbers of species to exist. Though species numbers are high, the actual population numbers are low.

If you would like to learn more about the role of geography and biomes in human history, read my book From Poverty to Progress.

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