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close this bookForestry - Initial Environmental Assessment Series No. 3 (NORAD, 1994)
close this folderPart I: General account
close this folder2 The environment affected by the project
View the document(introduction...)
View the document2.1 The forest ecosystem
View the document2.2 Classification of forest ecosystems
View the document2.3 Socio-cultural conditions

2.2 Classification of forest ecosystems

For the sake of simplification, tropical forests are here divided into three main groups: a) evergreen forests, b) dense deciduous forests and c) open deciduous forests.

a) Evergreen forests (mainly rain forest) occur in regions of high rainfall, needing at least 2,000 mm per year. Such forests grow in areas with little climatic variation, with rain throughout the year and high, even temperatures with a yearly average of about 27°C. Monthly rainfall is never below 60 mm in a typical rain forest. Evergreen and semi-evergreen forests can be found in both hot and moist lowland areas and in more temperate mountain areas (sub-montane, montane and wet cloud forest). Evergreen rain forests exist in South America, Central America, West Africa, Madagascar, Asia and some in Australia. In East Africa, there are also some interesting communities of mountainous rain forest that host a great diversity of species. Due to their primeval origin, among other things, these communities represent valuable genetic material of great scientific interest.

Rain forest communities consist of several stories, or strata, of trees ranging up to 60 m high, with lianas, vines, ferns, orchids and algae covering trunks and branches. On the forest floor, there is a great variety of fungi, and complex plant communities host an unknown number of insects. There is also an abundance of wildlife. Left undisturbed by outside influences, rain forests may remain unaltered for thousands of years. On the other hand, it will be almost impossible to regain the original balance of species whenever biomass has been removed from large areas.

The soil of rain forests is often very old, poor in nutrients, and acidic. Nutrients are found almost exclusively in the biomass and in a thin layer of litter on the ground. Loss of nutrients through leaching is relatively modest, and the stream flow is fairly clean. This soil type is not well suited for cultivation. The soil of rain forests on younger volcanic ground, especially in Asia, is rich in nutrients and can be cultivated without particular danger of exhaustion.

b) Dense deciduous forests occur mainly in India and South-East Asia. The most important tree species is teak (Tectona grandis), but other commercially important species are also represented. In India along, there are about 9 million ha of teak forest. Dense deciduous forests require an average annual rainfall of about 1,500 mm, but shore is great variation. Annual rainfalls in teak forests amounting to between 500 and 5,000 mm have been recorded. Dry periods lasting from 6 to 8 months are not unusual in regions where this typo of forest occurs. In India and South-East Asia, dense deciduous forests are of great economic importance. However, clear-cutting for agricultural purposes has caused a decline in forests of this type.

c) Open deciduous forests (woodland miombo - savanna). This forest type occurs mainly in Africa, north of the equator as savanna, and south of the Equator as miombo-type forest. The climate of open deciduous forests is characterized by one or two relatively short periods of rainfall in summer (rainfall of between 400 and 1,500 mm), and a long dry period during the cold season. The end of this dry period can be very hot. Open deciduous forests consist of several sub-groups, from relatively dense woodlands to treeless savannas. These natural conditions are determined by the amount of precipitation and the soil quality. However, wildlife and human activity also play their part. Grass fires and a long, intense drought period are natural phenomena in these forests. Fire-retardant trees and plant species are endemic to this ecosystem.

The miombo forest is the most important sub-group of open deciduous forests. It is characterized by a relatively open canopy and scarce undergrowth. A plant layer on the ground, which mainly consists of grass, indicates that the canopy allows a great deal of light to reach the ground. Open deciduous forests in general, and miombo forests in particular, represent a very important natural resource for local populations. In addition to producing medicines, fibres, building materials, fuel, meat from game and domesticated animals, these forest lands are utilized for agricultural purposes. Man's use of the forest for agriculture, commercial logging, grazing and, not least, seasonal burning has resulted in exhaustion of the forests. Regular fires cause reduced regrowth and regeneration, and tree species susceptible to fire disappear. After a while, forests become more open and the soil exhausted. Eventually, this forest typo will only consist of fire-retardant tree and grass species.

d) Other special forest ecosystems

Treeless savannas (grassland - steppe) are scarce in Africa. There are grasslands above the timber line in the mountains, in regions with special soil conditions (lateritic soil and black clays - Mbuga lands), and in areas with extensive grazing (livestock and wild animals) and grass fires. South America has large areas of genuine savanna, i.e. grassland without any trees. The lack of trees or forests on grassland is often a consequence of particular soil conditions and the competition between grass and tree roots for water and nutrients. Savannas, with and without trees, may have a rich and varied wildlife. The largest wildlife reserves in Africa are situated in regions that have a mixture of savannas, trees and shrubs. In such regions, occurrences of the tsetse fly prevent habitation and utilization of the land for farming and grazing. Pastoralism is the most common form of utilization on dry savanna lands. There is some agriculture, which varies according to traditions, water supply, growth conditions, etc.

The mangrove forest is a type of rain forest which grows along many tropical coasts. Mangrove trees adapt themselves to saltwater/brackish-water and tidal conditions. In many tropical countries, the mangrove plays an important role for the coastal population, both economically and in terms of supplies. Many fish and shellfish species reproduce in this environment (cf. booklet no. 4: Fisheries). Asia has the largest communities of mangrove forest, but they also occur on the east and west coasts of Africa and Latin America. The largest separate occurrence in the world (over 4,000 km²) is situated in the Ganges delta in Bangladesh.