Photograph by © WWF-Canon/William F. RODENBURG






Tropical Rainforests

The Southeast Asian rainforests are the oldest, consistent rainforests on Earth, dating back to the Pleistocene Epoch 70 million years ago. It has a biological richness and diversity unequaled by that of the Amazon or African rainforests. Yet Southeast Asia is losing its rainforests faster than any equatorial region, and has the fewest remaining primary rainforests. It is projected that most of the primary rainforests of Southeast Asia will be destroyed in the next 10 years.

Southeast Asia is a 3,100 mile long chain of about 20,000 islands strung between Asia and Australia. It covers and area of 1,112,000 square miles, almost twice the size of Alaska. The area lies from latitude 20° north and 16° south, and longitude 95° to 105° east. The average daily temperature varies from 70°F to 90°F. Humidity is always high.

Millions of years ago, as the rest of the world went through cooling and warming periods, the climate of the Southeast Asian region remained more or less the same. This was due mainly because of its location on the equator and being surrounded by water. Because the climate on the equator doesn't change much and the surrounding oceans provide plenty of moisture in the form of rain, the region was able to have consistent forests over very long periods of time. As sea levels rose and fell through warming and icing cycles, small pockets of forests survived as "forest refugia", or reservoirs of wildlife from which various species could reestablish themselves. Malaysia and the islands of Borneo, Sumatra and Java were all part of the same landmass during the last ice age. When the glaciers melted and sea levels rose many of these reservoirs were cut off from each other. This forced species to developed their own distinctive evolutionary paths in response to local environments, leading to an amazing diversity of species of every kind.

One interesting feature of the lowland rainforests of Malaysia, Borneo and Sumatra is the dominance of one family of trees, the Dipterocarpaceae. Dipterocarp are emergent trees and can reach heights of 120 feet. Their crowns are supported on large straight trunks. Many epiphytes, like orchids and ferns grow on the trees. Lianas, vines and strangler figs cling to the trees as they grow towards the sunlight. The emergent species is the tualang (Koompassia excelsa)which can reach heights of 280 feet. It is the 3rd tallest tree species in the world, and is almost never cut down because of its hard wood and massive buttresses. But most importantly it is home to large honey bees (Apis dorsata)whose honeycombs hang like enormous wedges from the underside of its branches. These trees are worth more money when left standing.

Trees and shrubs in the lower canopy have elongated crowns as the leaves reach for light. Shade tolerant species flourish here. Leaves are set at the best angle to receive light. Special swollen joints at the base of the stem, called pulvinus, rotate the leaf to follow the sun.

On the forest floor the soil is shallow with most nutrients close to the surface. Leaf litter and dead trees are quickly consumed and broken down by fungi, insects, and other decomposers. The nutrients decomposition creates are immediately taken up by the biomass of the forest. Because the nutrients are close to the surface, roots don't grow down very far, and trees have adapted by growing buttressed roots up to 30 ft high, or stilt roots which hang down from their trunks and branches.

There are many mutualistic relationships within the ecosystem of the rainforest. Dawn Bats are the prime pollinators of the durian tree. Each of the hundreds of fig species have their own species of pollinating wasp (Agaoninae spp), without which they would quickly fade into extinction and vise versa. Silvery Gibbons (Hylobates moloch)live their entire lives in the high canopies of the dipterocarp forests, never descending to the ground. Their survival depends on the dipterocarp and fig trees which house and feed them throughout the year. Links within the tropical rainforest ecosystem extend to thousands of plants which support mammals and birds. If a keystone species is eliminated, additional losses will be triggered and create a dominoes effect of extinctions.

Trees don't flower and come into fruit at the same time in the Southeast Asian rainforest. Some trees only fruit once every three years, some only every ten year. The short nutrient cycle makes it difficult for trees to produce large amounts of fruit at regular intervals. Many trees complete the flowering cycle in only one day, and are only receptive for a few hours during the day or night. Very few trees depend on the wind for pollination since there is little air under the dense canopy. These trees depend on animals and insects to pollinate and disperse their seeds. Emergent trees like the Kapok (Ceiba pentandra), dipterocarps, or Tualang which can grow to heights of 240 ft, can afford to have air-borne seeds. Their crowns grow high above the canopy and are exposed to the winds that blow there.

When seeds drop to the ground they almost always need to germinate in shady conditions. The forest floor is a difficult place to begin life, and many seeds surround themselves with fleshy, aromatic pulp as an immediate source of food. Smell plays an important part of a plant's life cycle, and many plants will have strong smelling flowers and fruits. The Rafflesia smells like a rotting corps, and the durian fruit smells almost too bad to eat, although it is known as the King of Fruit and tastes delicious. The powerful smell attract animals and insects that eat and disperse the seeds far from their parent tree.

Hundreds of animal and plant species are on the brink of extinction in Southeast Asia. The critically endangered two-horned Sumatran rhinoceros survives in small forest pockets of Sumatra and Borneo. Their entire population is thought to be only 300 to 500 individuals. The Javan rhinoceros has already slipped into extinction. The Sumatran tiger, like its cousin the Javan tiger will soon be extinct as well. The Asian elephant is another large forest herbivore which needs large amounts of forest to survive. Human encroachment and logging are shrinking their habitat to the extent that they can no longer support the elephants. The Malayan tapir is the largest of the 4 species of tapir still alive and no more than 50 animals still live in the wild. Another animal found only on Sumatra and Borneo is the orangutan, or "man of the forest". They were once found on mainland Asia from Thailand to southern China. They feed mostly on fruit and move through the forest following the fruiting trees. There are thirteen separate species of primates in Borneo's lowland forests alone. Most have overlapping home ranges but have different diets and foraging methods.

The climate of Southeast Asia is classified as a tropical wet climate in the Köppen climate zone system. The climate is influenced by maritime wind systems which originate in the Indian Ocean and the South China Sea. It has two monsoon seasons. The northeastern monsoon occurs from October to February and brings heavy rains to the eastern side of the islands. These storms carry the same punch as Atlantic hurricanes but spend much of their energy over the Phillipines. The southwestern monsoon is more powerful and occurs from April to August. Heavy rains saturate the western side of the island chain. Rain shadow effects create dryer but windy conditions on the opposite sides of the islands and the Malaysian peninsula during monsoon seasons. There are two inter-monsoon seasons between the two main monsoon seasons. Southeast Asian rainforests get an average of 79 inches of rain annually.

Any change in the monsoon cycle can have devastating results. In 1992-1993 one of the largest fires ever burned in Kalimantan. Widespread logging had degraded the primary forest and made it prone to fire. The drought brought on by the El Niño of that year created a catastrophe when agricultural fires got out of control. Twentyseven thousand square km burned out of control.

The same events unfolded in 1998. The El Niño of that year created a very weak monsoon season. Thousands of forest fires burned over Malaysia and the Indonesian archipelago, destroying rainforests and the plants and animals within them. A haze of smoke spread for thousands of miles across the region. Untold mutualistic relationships may have been destroyed, keystone species eliminated. It is still unclear what effects the events of 1998 had on the ecosystem of the rainforests. Unfortunately, in the year 2002 another strong El Niño is developing over the Pacific.

Political instability in the Indonesian archipelago has resulted in little law enforcement within protected wildlife areas. In 1992, feeling betrayed by the government of President Suharto, local people took control of the land and began indiscriminate logging and farming in the dipterocarp rainforests. Little regard has been given for the long-term environmental effects, and at the present rate of destruction there will be no primary lowland rainforests remaining in Indonesian Borneo by the next decade.

In Indonesia illegal logging has led to a "biological catastrophe" affecting thousands of plant and animal species and upsetting the natural biologic equilibrium that keeps a rainforest healthy and stable. The mutualism that sustains numerous species has been destabilized and could lead to massive extinctions. For the plants and animals and the myriad species that inhabit the rainforests of Southeast Asia it may be too late and there is no "forest refugia" left from which to replenish their species. Fragmentation of habitats will cause more interaction of animals with humans, and many animals will be killed or captured for the pet trade. Huge numbers of species will become extinct before their role in the rainforest will have become known, and the rainforest ecosystem of Southeast Asia will collapse.

E. Benders-Hyde 2002


"Rainforests",, (3/18/02).

"[Biome - Living Worlds] :: Rainforest :: Plants",, (3/18/02).

"[Biome - Living Worlds] :: Rainforest",, (3/18/02).

[Biome - Living Worlds] :: Impact :: Rainforest :: Social Impact",, (3/18/02).

[Biome - Living Worlds] :: Rainforest :: Climate",, (3/18/02).

"Rainforest Plants",, (3/18/02).

"Earth Floor: Biomes",, (2/8/02).


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