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Convention on biological diversity


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Tropical forests

Distribution and structure


In the tropical forest biome, three major regions are recognized: American, African and Indo-Malaysian-Australian (Whitmore, 1984). Tropical forests may be broadly classified as moist or dry, and further broken down into rain forest (some 66% of the total tropical moist forests), cloud forest, evergreen seasonal forest, dry evergreen seasonal forest, semi-evergreen tropical forest, moist deciduous forest (monsoon forest), dry deciduous forest, and mangrove. Rain forests occur in Central and South America, Africa, the Indo-Malayan region and in Queensland, Australia. Where several dry months (60 mm rainfall or less) occur regularly in the tropics, monsoon or seasonal forests are found. Both rain and monsoon/seasonal forests (closed forests) have together been termed ‘tropical moist forest’ (Sommer, 1976). Cloud forests situated at middle to high altitudes derive a significant part of their water supply from cloud and fog and support a rich abundance of vascular and nonvascular epiphytes. The evergreen seasonal forests are found in regions where every month is wet (100 mm rainfall or more) and in areas with only short dry periods (Whitmore, 1990). Dry tropical rain forests were originally described by Schimper (1903) as ‘evergreen, hygrophilous in character, at least thirty metres high, rich in thick-stemmed lianes, and in woody as well as herbaceous epiphytes.’ Mangroves are the characteristic littoral formations of tropical and subtropical sheltered coastlines, they have been variously described as ‘coastal woodland’, ‘tidal forest’ and ‘mangrove forest’

Basing his work on previous classifications, Whitmore (1990) has, for convenience, grouped the formations within the tropical rain forest according to the main physical characteristics of their habitats, noting that the naming of vegetation types is always problematic. In this arbitrary arrangement, the first division is between climates with a dry season and those that are perhumid (for moist forest), the second division (for the rain forest) is a crude measure of soil water availability and distinguishes swamp from drier land forests. The third division is based on soils and, within dryland forests, distinguishes those on parent materials with atypical properties – peat, quartz sand, limestone, and ultrabasic rocks – from the widespread ‘zonal’ soils, mainly ultisols and oxisols. Finally there is a division of the forests on zonal soils by altitude. In the Indo-Malaysian region the tropical rain forest lies as a belt of evergreen vegetation extending through the Malay Archipelago from Sumatra in the west to New Guinea in the east (Whitmore, 1984). This is the non-seasonal humid zone of the Southeast Asian dipterocarp forests. Patches of rain forest, or outliers, are found in southern Thailand, in Sri Lanka, India, northern Queensland in Australia and on the Melanesian islands of the Pacific. Box 3 describes the tropical forest types in Malaysia, as an example of the complexity of tropical forests in general.




Box 3. The rain forest types of Malaysia (adapted from Symington 1943 and Wyatt-Smith 1964)


Malaysia’s forests have been categorised into types that have been influenced by either climatic or edaphic factors. Climatic climax forest types traverse an altitudinal gradient, whereas the edaphic forest types are found in the lowlands.


Climatic climax forest
Lowland dipterocarp forest

Hill dipterocarp forest

Upper dipterocarp forest

Montane oak forest

Lower ericaceous forest

Montane subalpine vegetation




Edaphic climax forest
Heath forest

Forest over limestone

Forest over ultramafic outcrops

Beach stand vegetation

Mangrove forest

Brackish-water forest

Peat swamp forest

Fresh water swamp forest

Seasonal swamp forest

The lowland, hill and upper dipterocarp forests are stratified into three storeys of trees. The Dipterocarpaceae is the main tree family in the forests of Southeast Asia and forms a high proportion of the upper strata of the forest. At least three-quarters of the forests of Southeast Asia are dipterocarp forests, and in Malaysia, they form over 86% of the forested areas. Dipterocarps do not grow at higher altitudes so are not found in the montane oak forest, which consists of two storeys of trees, or the ericaceous forest, which has a single storey of trees. The montane subalpine vegetation is low shrub vegetation on mountain peaks. The edaphic climax forest includes the low-lying swamp forests and forests on sites with extreme drainage and deficiency in available moisture due to violent winds. In Malaysia, swamp forests, including mangroves, form over 12% of the forested land.


Significant conversion of lowland forests to other land use began with the increase in tin mining activities in the western parts of Peninsular Malaysia in the middle of the nineteenth century and the beginning of rubber (Hevea brasiliensis) plantations at the start of the twentieth century and, later, the cultivation of oil palm (Elaeis guineensis), which was planted when rubber prices dropped sharply in the world markets in the 1960s. Forest conversion to cash crop agriculture, mainly oil palm, intensified from 1971 to 1990. Over the period 1970 to 1989, forested land in the whole of Malaysia was reduced by 23 per cent. (Manokaran 1992). Almost all the forests cleared were lowland forests, the largest reservoir of genetic variation of the dipterocarps and a major storehouse of biological diversity. Species richness in these forests is exemplified by the results of enumeration of woody trees of 1 cm dbh and greater in a 50-ha plot in Pasoh in Peninsular Malaysia. A total of 820 species in 294 genera and 78 families were recorded, this being almost one third of the total number of tree species found in Peninsular Malaysia, indicating that small areas include a surprising large percentage of a region’s tree and shrub flora (Kochummen et al., 1990).



Species diversity


Where seasonality of rainfall occurs, it produces a strong temporal effect on primary production (Orians et al., 1996). Productivity varies considerably among the primary tropical forest types; Lieth and Whitaker (1975) and Murphy (1975) provide the following data: tropical rain forest: 1800-3210 g/m2; cloud forest: 2400 g/m2; dry deciduous and mixed tropical forests: 1040-1230 g/m2; for seasonal forest, a single estimate of 1340 g/m2 from west Africa, and for mangroves: 930 g/m2 from the Caribbean and 1000 g/m2 at 10 to 25 years of age at Matang, in Peninsular Malaysia. These data show a primary productivity of 2–4 times that recorded in boreal forests and correlate broadly to a general latitudinal reduction in diversity of plants and animals north from the tropical forest biome.

Tropical forests are the most species rich and diverse forests on earth, estimated to contain at least 50% of all plant and animal species (Myers 1986). This is especially true for wet tropical forests, where, for example, some 700 tree species have been recorded in 10 selected 1-hectare plots in Borneo (UNEP, 1995). Within tropical moist forests, species richness varies greatly by region and some tropical moist forests actually have relatively low tree species diversity. In the Amazon Basin, for example, less than 90 tree species per hectare have been recorded in the eastern portions compared with nearly 300 species/ha in the western areas (WCMC, 2000). Mangroves have relatively low terrestrial species richness, with counts in some river deltas of about 30 species (IUCN, 2000), although the aquatic life they support is diverse and abundant. African rain forests have fewer plant species than other tropical regions (by about 20%), with several pantropical genera and families (e.g., Lauraceae, Myrtaceae and Palmae) being either absent or poorly represented (Jacobs 1981). Lianas and epiphytes are also less abundant in African rain forests compared to other tropical regions (Jacobs, 1981).

Few tropical genera are pantropical and endemism is much higher in this biome than in the temperate or boreal forest biomes (UNEP, 1995). For example, in fourteen areas with exceptionally high species richness in the tropics, on about 300,000 km2., more than 37,000 plant species can be found (Myers, 1990). Tree species richness declines as altitude increases and as climate becomes more seasonal (Orians et al., 1996). The mixture of many tree species, with few individuals of each, in a given forest area is a key feature of tropical forests and one which distinguishes them from forests in the boreal and temperate biomes. This feature is significantly related to a predominance of dioecious species and to a seed dispersal relationship with animals in the tropics, compared to boreal and temperate forests where wind is often the medium of seed dispersal (Orians et al., 1996). Low density of individual species has particular consequences with respect to the necessity for large areas for preserving populations (Gentry, 1992). Where tropical forests with single dominants do occur (usually dry forest), there are no corresponding species among the regions. In the Americas, Eperua and Mora dominate such tropical forests, in Africa, Gilbertiodendron is a common dominant, dipterocarps dominate in areas of Southeast Asia, in Indo-Malaysia, Agathis is sometimes dominant, while in tropical Australia, Eucalyptus is the dominant genus in low-richness stands (Whitmore, 1990).

In rain forests, epiphytes, although common to all regions, are highly distinct and certain families predominate (Gentry, 1992) such as: Orchidacae in Africa; Orchidacae, Bromiliaceae and Cactacae in the Americas; and Orchidacae, Asclepiadaceae and Rubiaceae.7 in Indo-Malaysia: Lianas are another important component of the structure of tropical rain forests, absent from the other biomes. They make up 8% of the species (in Borneo 150 genera exist) and are indicators of an undisturbed state of forests (Jacobs, 1981).

Twelve genera and some 470 species of the family Dipterocarpaceae are found in the rain forests of the Indo-Malayan region, ranging from the Seychelles through Sri Lanka to the south of Peninsular India, east to India, Bangladesh, Myanmar, Thailand, Indo-China, to continental South China (Yunnan, Kwangsi, South Kwangtung, Hainan) and through Melanesia (natural botanical kingdom comprising Peninsular Malaysia, Sumatra, Java, Lesser Sunda Islands, Borneo, the Phillippines, Celebes, the Moluccas, New Guinea and the Solomons) (Ashton, 1982). With the exception perhaps of New Guinea and the eastern part of the region, the tropical rain forests of the Indo-Malasian region are characterised by family dominance of the Dipterocarpaceae (See Box 1).

With respect to fauna, the forests of South America and Asia maintain very high animal species richness compared to the African tropical forests (UNEP, 1995). The rivers of the Amazon Basin host the most diverse fish population in the world and the insect populations present in its canopy also have high species richness (WCMC, 1999). Wilson (1986) recorded 43 species of ants, belonging to 26 genera, on a single tree in Peru, about the same number of species as the entire ant fauna of the British Isles. It is not unusual for a square kilometre of forest in Central or South America to contain several hundred species of birds and many thousands of species of butterflies, beetles and other insects (Wilson, 1986). Stattersfield et al. (1998) note that, of the total world forest avifauna, 88% are endemic to tropical forests, and of those, more than half are found in wet forest types.

Tropical dry forests generally host a lower species richness, with fewer endemics than tropical moist forests, although still significantly higher than in temperate forests. The richest dry forests, found in northeast Mexico and southeast Bolivia, have an average of 90 tree species per hectare (WCMC, 2000). Dry forests are more similar in species richness to their moist counterparts in terms of mammal and insect species. Tropical dry forests are noted for their highly endemic mammal populations, especially insectivores and rodents.

An important feature of cloud forests and some other montane forests lies in their high species richness of epiphytes, shrubs, herbs and ferns (Gentry, 1992). These species increase with altitude in the humid tropics whereas in the warmer, lowland tropical forest types, they tend to be less frequent. In addition, cloud forests often contain high numbers of rare endemic plant and animal species or subspecies, such as the mountain gorilla (Gorilla gorilla beringei) in Central/East Africa, and the quetzal (Pharamachrus mocinno) of Central America (IUCN, 1995). The percentage of endemic species is even higher in cloud forests on island mountains, such as those in Hawaii and in the French overseas territories of Reunion Island and New Caledonia.

Mangroves may form very extensive and productive forests. Throughout the tropics, there are about 60 species of trees and shrubs that are exclusive to the mangrove habitat, the important genera being Avicennia, Bruguiera, Rhizophora, Sonneratia and Xylocarpus. There are also important, non-exclusive species associated with the mangroves, including the fern Acrostichum spp., and trees such as Barringtonia racemosa, Hibiscus spp. and Thespesia species.

High species richness in the tropical biome may be the result of the large range of available microhabitats and niches, the absence of mountain systems or their north-south orientation permitting ease of migration and a lengthy period without major disturbance (e.g. glaciation) (UNEP, 1995). High productivity is sustained annually, as opposed to seasonally, in many tropical areas which allows multiple breeding seasons and results in less movement away from a home range to avoid seasonality (Margalef, 1968; Richards, 1969). Further, in places such as Madagascar and the large number of tropical island habitats in Southeast Asia and the Caribbean, a high level of endemism is found because of their isolation (Margalef, 1968).


Plantations in the tropical biome


In the tropical forest biome, plantations have been reported to be a great success in terms of rapid growth due to favourable conditions, a fact that has motivated the adoption of plantations as a way to improve local forest productivity. Given the high biodiversity present in tropical ecosystems, plantations are particularly vulnerable to diseases and pests, especially when exotic species are involved. In spite of this, plantations have been promoted as manufactures have been increasingly accepting of plantation wood fibre, especially Eucalyptus. As a result, a great number of large pulpwood plantations have been established in Indonesia, South Africa and Chile and the area of plantations is still increasing in other countries such as Malaysia, Vietnam, Thailand, Uruguay, Paraguay, Argentina, Venezuela, Colombia, Mexico, Congo and Swaziland (WRM, 1999).

Several examples of plantations in tropical forests can be cited. In the Amazon region, Brazil has experienced a number of ambitious projects such as the Jari Project and the Projeto Grande Carajás, Aracruz celulose pulp plantation (Rankin, 1985; National Geographic, 1990; World Resource Institute, 1998). In Congo, the eucalyptus plantations at Pointe Noire have been successful as productivity has improved due to an imaginative and dedicated tree breeding project. On the other hand, in Senegal and in Java, Indonesia, where there are about 600,000 ha of teak (Tectonia grandis), there is some concern for site deterioration due to repeated planting, soil erosion and loss of organic matter (Evans, 1999), especially as grand scale plantations are always more vulnerable to natural disasters.


Agro-forestry


Agro-forestry can be considered to be a productive system which conserves biodiversity, mainly when it is based upon traditional knowledge, as it causes minimal land degradation and preserves biodiversity as well as native crop diversity (Toledo, 1980). It has been successfully used as a sustainable forest management practice in some parts of Central and Latin America. For instance, in Mexico and Columbia, maize cultivation is combined with rows of Swietenia macrophylla, Cedrela mexicana and Cordia alliodora, while Erythina and Cordia shade coffee and cocoa plantations (Burniske, 1993). At present, Costa Rica is developing an ambitious programme that combines scientific research with multiple-use agro-forestry projects that include fruit-tree and forest cultivation, with the social support of landowners. These projects aim to provide several environmental services that refer to carbon sequestration and biodiversity conservation [see e.g. CREP ].


  1. Scientific considerations relating to forest biological diversity

Forest biological diversity is quantified at several scales by assessing the genetic diversity within species, counting the number of species per unit area (local, regional, national, continental, global), determining areas and types of forest ecosystems, determining communities of species associated with forest ecosystems, determining numbers and arrangement of forest types and ages and through describing landscape structure. A recent shift in forest management methods in many countries, to focus on ‘ecosystem management’, is a result of the recognition of the need for a scaled-up approach. This change accompanied the further understanding that most species within systems cannot be monitored sufficiently to ensure independent long-term viability. Monitoring programmes are an important component of the sustainable management of forests to ensure that biological diversity is maintained in time and over space.
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