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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. |
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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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