Tropical Rain Forests in Lowland Borneo

As many government agencies organized the Asia Forest Partnership, the sustainable management of forests is urgently needed in Asian region. Dr. Nishimura will introduce the current situation of Forests in Asia and their characteristics.

Takashi Nishimura ( Researcher, Ecosystem Change Research Program, FRSGC )


In early 1990s, 15.4 million hectares of tropical rain forests were lost per year, and 4.2 million hectares were disappeared in South East Asia. Carbon dioxide fixation and vapor emission of tropical rain forests affect global climate and cycle of matters. High solar radiation provides high productivity for forests in low latitude. Thus, tropical rain forests have a great influence on global environment.

Many types of tropical forests exist from lowlands to mountain areas in South East Asia. Trees in Fagaceae are found in montane forests. Fagaceae, for example beech and oak, is very familiar in the temperate regions. Dipterocarp forests expand over lowlands in central and south Borneo, where annual rainfall is around 3000 mm. Dipterocarpaceae is commonly distributed in South East Asia, and over 260 species are found in Borneo. Some dipterocarp trees grow up to 70 meters tall. Shorea, one of the largest genuses in Dipterocarpaceae, is called "meranti" in Malaysian. In Japan, they are famous as "rawan" from a Tagalog word in Philippine. Shorea trees were common material for plywood.

Meanwhile in lowland Borneo, its altitude is 20-30 meters, even the areas are a few hundred kilometers away from the seacoast. Two distinct vegetation types found in lowland Bornea are heath forests and peat-swamp forests. They cover over 10% of lowlands Borneo respectively. Heath forests develop over white sandy soil. It is called "Kerangas" after an Iban word that means "land where hill rice does not grow." Similar forests on white sand are found in Neotropical areas. Periodical low water retention of sandy soil causes severe desiccation to heath forests. For this reason, saplings have deep root systems to endure seasonal droughts. Root depth of large heath trees is a few meters because nutrients are concentrated on upper stratum of soil. In dry season, some trees are dead because of water shortage.


 
The Kahayan River in Palangka Raya city,
the provincial capital of Central Kalimantan, Indonesia

In contrast, peat-swamp forests develop over waterlogged low areas along rivers. High water table in rainy season prevents dead trees from decomposition, and then produces "peat strata." In high latitude regions, peat is mainly made of undecomposed herbaceous plants, due to excessive water and low temperature. Tropical peat is called "woody peat" because it contains much woody debris unlike high latitude peat. Thousands-year peat accumulation produces deep strata of 20 meters in some areas. In peat-swamp forests, because peat contains toxic components and water table is high, trees have shallow root systems. Peat-swamp trees maintain stilt-roots and kneeroots for water-logging in rainy season. Although peat-swamp trees rarely suffer from droughts, floods cause uprooting of trees and form undulated micro-topography with many small hummocks of a few meters in height. Water-logging for a few weeks does not kill peatswamp trees, but frequent floods increase their mortality. Small trees are contagiously distributed on tall hummock where they are not disturbed by flooding water. Sumac trees (Anacardiaceae), which are very common in riparian swamp forests, prefer tall hummocks without any disturbances by flooding. Black sap (lacquer) drops from large sumac trees, and makes rashes on your skin if you stay in a peat-swamp forest for a long time. They poison your skin less seriously than Japanese sumac trees.

Both heath and peat-swamp forests have poor species diversity, compared to mixed dipterocarp forests, and they are around 30 meters in stature. A few hundreds tree species are found in 1-ha dipterocarp forests, and some species have only 1-3 individual trees in a forest. Heath and peatswamp forests consist of a hundred species at most, and are dominated by particular species.


Heath forest on white sandy soil

Some commercial trees are logged in riparian peat-swamp forests because flooding in rainy season makes it easy to carry them out to rivers. Because peat-swamp had no values for farmland, the area was protected from developments. Large cities are mostly located along sea coasts, and people sparsely dwell near rivers in inland Borneo. However during the Soeharto regime, many channels were constructed for drainage of inland swamp, and farmland was developed. This project is abandoned nowadays because of many difficulties for soil improvement and cultivation in peat-swamp areas. Many peatswamp trees were logged during the project. To make matters worse, peat is being lost. Since excessive water from rainfall and flooding promotes peat accumulation, peat cannot be produced in swamp drained by channels. Dehydrated peat is decomposed by microbial activities. A large amount of carbon is emitted to the atmosphere from decomposing peat stratum near ground surface. During the El Nino Southern Oscillation in 1997, combustible dry peat accelerated the forest fire for a long period. Although it is very difficult to compute the precise amount of carbon emitted from underground peat, carbon from peat is estimated to be 4 to 5 times greater than that from burned forests during 1997 forest fire in Borneo. A great deal of peat must be lost every year as tropical lowlands are drained and developed all over the world. It is one of the key factors to know the structure and roles of tropical forests, in order to consider future of our global environment, as well as to plan for the protection strategies.


Riparian peat-swamp forest at low water in dry season


References
Whitmore,  T. C., 1998, An introductionto tropical rain forests (second edition), Oxford University Press, Oxford, 282.
Richards,  P. W., 1996, The tropical rain forest (second edition), Cambridge University Press, Cambridge, 575.


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