Tropical Zone

I

Introduction

Tropical Zone, one of the world’s major climato-vegetational zones lying predominantly between the tropic of Cancer (23°27′ north) and the tropic of Capricorn (23°27′ south) and characterized by a minimum mean annual temperature, at sea level, of 18° C (64.4° F) in the coldest month. It covers around 40 per cent of the Earth’s surface, taking in large parts of South East Asia (including India and China), Australia, Africa, and Central and South America. The tropics receive large amounts of solar radiation (sunshine) throughout the year, so seasonal fluctuations in temperature are minimal. The amount and timing of rainfall, however, often varies considerably and it is rainfall that is commonly used to subdivide the tropics. Rainfall directly and indirectly affects vegetation, soils, landform processes, agriculture, and even economic activity.

Based on rainfall and vegetation, three major subdivisions of the tropical zone can be distinguished. The humid tropics have at least 1,000 mm (39 in) of rainfall per annum and no distinct dry season. They are typically covered with forest vegetation, notably rainforest. The wet and dry (wet-dry) tropics receive between 250 and 2,000 mm (10 and 79 in) of rain annually and have one or two distinct dry seasons. They are typically covered with grassland (savannah) vegetation. The dry, or arid, tropics receive less than 250 mm of rainfall per annum and have only a short wet season. They are typically covered with sparse, shrubby xerophilous vegetation; that is, plants adapted to survive very dry conditions. This article deals primarily with the humid tropics. The wet-dry tropics and dry tropics are described in the Arid Zone and Savannah articles.

II

Climate

A

Circulation Systems

The humid tropics lie under the rising, or equatorial, limbs of the Hadley cells—the thermally driven atmospheric circulation systems that exist between latitude 0° and 30° in each hemisphere. Near the equator heated, moist air rises, cooling and condensing as it does so to release its moisture as heavy showers. As this equatorial air rises it leaves a surface low-pressure area known as the Intertropical Convergence Zone (ITCZ). Once it reaches the tropopause—the boundary between the troposphere and stratosphere—the equatorial air begins to move poleward and, as a result of radiation, to cool further. It reaches around latitude 30° before sinking, this time creating an area of high pressure at the surface, the subtropical high pressure zone. As it sinks it warms by compression, thus inhibiting cloud growth and helping to create the world’s tropical deserts. To complete the Hadley cell most of the dry air begins moving back towards the equator. Because of the effect of the Earth’s rotation, these equatorward-moving air currents are deflected towards the west and become the trade winds. Some of the air continues to move poleward, and is deflected towards the east to become the middle-latitude westerly winds.

The position of the ITCZ varies seasonally. It sweeps north during the Northern hemisphere summer reaching around latitude 25^o north in July over continental south-eastern Asia and around latitude 15^o north over Africa. During the Northern hemisphere winter it migrates south, reaching a maximum of around latitude 15^o south over land areas in January. The migrating ITCZ brings the wet season to the seasonal parts of the tropics. In continental areas of the Northern hemisphere tropics, the migration north of the ITCZ allows dry, offshore south-easterly “trade wind” air to be replaced by moist, onshore, south-westerly winds, bringing about the monsoon.

B

Weather Disturbances

Even in the wettest parts of the tropics it does not rain all the time. Rainfall occurs as weather disturbances pass overhead. Individual cumulus clouds, 1,000 m to 10 km (3,281 ft to 6 mi) in diameter, bring isolated showers, especially when air is forced to rise, and thus cool, in order to cross mountains. More prolonged periods of rain occur during the passage of larger-scale weather disturbances, the most powerful of which are tropical cyclones. Tropical cyclones, known as hurricanes or typhoons when at their most intense, derive their power from energy obtained from the evaporation of warm ocean water. They decay over land, but often not before causing considerable damage. Along low-lying coastal regions tidal surges of up to 15 m (50 ft) may accompany the strong winds and heavy rain that characterize tropical cyclones. The 80 or so tropical cyclones that occur each year are responsible for, on average, 20,000 deaths and millions of dollars’ worth of damage.

III

Vegetation

There are many different types of tropical vegetation and not all parts of the humid tropics are, or ever have been, covered by forest. However, lowland forest, or rainforest, is the most extensive vegetation type. It covers large parts of South America, notably the Amazon basin, Indonesia, and central Africa, for example, the Congo basin. Most humid tropical lowlands are part of ancient (Precambrian) shield areas, whose soils are typically also old. Soil nutrients have been comprehensively leached by thousands of years of tropical rainfall. The luxuriance of the forest is instead maintained by the efficient cycling of nutrients between the growing vegetation and the decomposing litter. If the vegetation is removed the nutrient cycle is broken and the forest soils quickly become degraded and unsuitable for agriculture. In mountainous areas of the humid tropics, such as Central America, the Caribbean region, and parts of south-eastern Asia, montane forests and tropialpine vegetation are found. The geology and soils of these areas are much younger, and soil nutrients are often still “locked up” in the bedrock.

Lowland forests are characterized by evergreen, woody vegetation and generally have three or four strata, or layers, of vegetation. An understorey of low-growing plants, a closed canopy formed by the tops of the trees, and emergents, isolated trees protuding above the canopy, are the strata that are almost always recognizable. Most trees are broad-leaved evergreens that form a canopy at 30 to 50 m (98 to 164 ft) above the ground; emergent trees attain heights of 60 to 70 m (197 to 230 ft). There are typically 60 to 250 species of plant per square km (247 acres) and woody climbers (lianas) and epiphytes are abundant. Where the soil becomes saturated mangrove and freshwater swamps occur. As altitude increases premontane and then lower montane forests take over. These are lower in height and have fewer species—40 to 100 per square km and 25 to 50 per square km respectively. On poor soils single-species forests may be found. Lianas become rare although epiphytes remain common. True montane tropical forests occur in the next altitudinal zone and are frequently almost continually covered with cloud—which is why they are often called cloud forest. Their canopies are always below 20 m (65 ft) and are often only a few metres high. Single species forests are common, woody climbers are absent but epiphytes are often present. At the tops of the highest tropical mountains are heathlands and grasslands known as tropialpine communities. Many of these contain giant species of flora commonly found in temperate latitudes. Another variety of tropical forest is the monsoon rainforest. Most common in south-eastern Asia, they are characterized by well-drained soils and a distinct dry season. Trees are shorter than in the true equatorial forest and some are deciduous. Because of this more light can reach the understorey, which shows denser growth than in equatorial forests.

Elsewhere at the margins of the humid tropics, as the dry season becomes more prominent and more intense, closed-canopy forests become broken and grass species more important. These savannah regions vary from open-canopied forests with a grass layer below, to areas where trees are very rare and the ground surface is covered with hummocky grasses and some shrubs. Not all savannahs, however, are climatically determined. For example, in eastern Africa grasses are maintained by periodic burning and the constant grazing of large herds of mammals. Edaphic, or soil-related, controls are also important. For example, soils in the wet-dry tropics often develop an impenetrable, iron-rich lateritic crust just below the surface. In such areas grasses survive better than trees in the shallow overlying topsoil.