Borneo rainforests

• Category: Forest Biomes.
• Geographic Location: Asia.
• Summary: The third-largest island on Earth contains five major forested ecoregions that exist because of Borneo's unique geomorphology and the climatic and biological characteristics that define the Cool Equatorial Rainforest biome.

Thick, impenetrable jungle sparsely populated by humans is a description that could have at one time summarized the island of Borneo, at least for a great many travelers and writers familiar with world geography. This depiction still circulates today, but the reality is that more than 20 million people inhabit the island, mostly along the coastline, and that almost 70 percent of the arboreal descendants of the oldest tropical rainforest in the world have been razed during the late twentieth and early twenty-first centuries for the sake of economic development. Much of the physical landscape of Borneo has become in one way or another a harvestable natural resource, and many indigenous people and wage laborers and their cultures have become tourist commodities. What remains of long-standing ecoregions that formed in the relationship between geology and climate are generally traces left mostly unaltered by human beings, save for the occasional wildlife surveillance camera set to capture a portion of the immense biodiversity that the island is still home to.

The extraordinary flora and fauna of these regions depend on a continuously warm and moist atmosphere—two climatic conditions that qualify the island as a global biome called Cool Equatorial Rainforest. The life of these ecoregions also depends on environmental policies and initiatives set forth by Brunei, Indonesia, and Malaysia, the three countries that share the island and are responsible for its future.

Geologic Foundation of Biology

At 287,001 square miles (743,330 square kilometers), Borneo is the largest island of the Malay Archipelago. Its geologic origins remain somewhat of a mystery, although most scientists agree that the original surface rose above sea level some 130 million years ago, long after the split of the supercontinent Pangea into the land masses of Gondwanaland and Laurasia. As Gondwanaland's tectonic plate began to fragment and sections gradually moved into the positions they are in today, at times and in places they also converged on the Laurasian plate, causing layers of rock to crunch and then uplift. Around this rock core, sediment and debris carried on ocean currents and forged from the erosion of the core continue to collect and consolidate, thereby shaping the island as it moves, along with the underlying tectonic plate, ever so slowly to the north.

At the geological moment when Borneo rose above sea level, one of the world's oldest rainforest environments began to appear. The island's high level of biodiversity, however, owes much to the dynamics of Pleistocene glaciation.

About 3 million years ago, glaciers began to form in fits and spurts on the surface of the Earth, stretching down from the poles and alpine regions. Some 18,000 years ago, they covered about a quarter of the Earth's surface before they began to melt and recede.

By this time, Borneo had been in place for 50 million years in an area we now call southeast Asia, all the while engaged in the cycle of erosion with the Eurasian landmass and volcanic islands of the Malay Archipelago that built up the Sunda continental shelf. With the formation of glaciers and concomitant lowering of sea level, the shelf emerged from the sea as a land bridge of tropical savanna vegetation linking Eurasia with insular southeast Asia. Exotic plants, animals, and people were able to colonize Borneo. When the glaciers ebbed, the shelf became the floor of shallow seas, and the colonizers were isolated from the mainland. In isolation, plants and animals on the island evolved uniquely.

Climate and Ecoregions

An important influence on the evolution of Borneo's flora and fauna is the Intertropical Convergence Zone, also known as the Thermal Equator, which is an undulating band of consistently low air pressure, high temperatures, and rainfall that follows the movement of the most intense solar rays hitting Earth. These rays move generally north and south across the surface within the tropical latitudes. The Thermal Equator is never far from Borneo, so convectional rainfall is continuous year-round, save for the barely perceptible less-wet period.

Borneo has five types of rainforest ecoregions spread across Malaysia's states of Sabah and Sarawak, Indonesia's Kalimantan, and the Sultanate of Brunei. They are lowland climax rainforest, southwest Borneo freshwater swamp and peat-swamp forests, the Sundaland heath forests (Kerangas), and the Sunda Shelf mangroves. Together, these ecoregions contain thousands of species of flowering plants and trees, including 267 species of dipterocarps (tropical hardwood family with two-winged fruit), and hundreds of species of terrestrial mammals and birds, amphibians, and reptiles, not to mention a plethora of insects, many of which are endemic (evolved in a local biome and found nowhere else).

Lowland Rainforest

The lowland rainforest once covered most of Borneo. What remains are diminishing patches of canopy that together equal about 165,059 square miles (427,500 square kilometers). Emerging from the rusty-red soil, rich in aluminum and iron oxides, are hardwood evergreen dipterocarps, a family of tropical trees that produce seeds with wings that float down from the highest stories of the canopy. Because the soil is relatively infertile, trees have buttressed root trunks that spread widely along the forest floor, enabling them to absorb organic nutrients at the surface.

The luxuriance of life at midcanopy revolves around myriad species of lianas, orchids, and ferns. Growing in the understory—particularly near rivers where sunlight reaches the forest floor—are shrubs, bamboo, rattan, and ferns. Fungi and moss prefer darker places under the canopy, where they break down organic remains into nutrients for trees.

The most significant natural resource of the lowland rainforest is a particular genus of dipterocarps known as Shorea, of which there are 138 species on the island, 91 of them endemic. Some of these flowering trees can grow more than 200 feet (61 meters) tall. In 1990, 50 percent of the world's tropical-wood exports came from Borneo. Dipterocarps—most of which are now critically endangered—were cut down, made into plywood, and exported to Europe and Japan. Rattan, a kind of palm, is also harvested; its core is used to make furniture, and its outer sheath is stripped to make matting and basketry. Indonesia supplies 70 percent of the world's raw rattan, with much of it coming from Borneo.

Finally, the lowland rainforest is a natural resource that draws tourists from around the world. People come to gaze at some of the most unique wildlife on the planet, such as the Bornean orangutan (Pongo pygmaeus), one of two extant species of great apes in Asia; the endemic proboscis monkey (Nasalis larvatus); and the Bornean pygmy elephant (Elephas maximus borneensis), mostly found in the Betung Kerihun, Gunung Mulu, and Ulu Temburong National Parks.

Freshwater and Peat-Swamp Forests

As elevation increases on the island, the ecological characteristics of the lowland rainforest give ground to altitudinal zonation, through which species of flora and fauna gradually change mainly because of decreased temperatures and changes in rainfall patterns. The result is a changing mix of tropical, deciduous, needle-leaf trees to grasslands, alpine meadows, stunted trees, mosses, and lichen. Kayan Mentarang National Park in Indonesia and Mount Kinabalu in Malaysia (the tallest mountain on the island) are remarkable examples of this geophysical process.

The biological communities of lowland rainforests are similar to the swamplands and Sundaland heath forests of Borneo; differences in species stem mainly from the prevalence of standing water and certain other abiotic conditions. In the peat-swamp forests, dead leaves and wood never fully decompose in the soggy ground, thereby creating a spongy layer of rank, acidic matting called peat, which can be 66 feet (20 meters) thick and 5,000 to 10,000 years old. The forests now maintain an unusually high number of species of flowering plants and ferns. Trees can take root in peat, but the tallest trees grow around the margin. Maludam National Park in Sarawak is the largest peat forest of its kind on the island.

Sometimes, draining peat swamps for agriculture goes hand in hand with setting forest fires that ignite the peat in which fires can simmer for long periods. These fires can create harmful smoke and haze, which can spread throughout southeast Asia. One particularly notorious episode during the dry years of 1997–98 was exacerbated by the tropical atmospheric effects of El Niño.

The destruction of peat swamps amounts to a global warming double-whammy. Peat serves as a carbon sink, a reservoir for greenhouse gases that otherwise would be accumulating in Earth's atmosphere and adding to climate change. Research by the United Nations Collaborative Initiative on Reducing Emissions from Deforestation and Forest Degradation (UN–REDD) shows that peat is more effective than forests at sequestering carbon. Eliminating swaths of peat swamp reduces this carbon sequestration service, whether it is the result of burning the swampland or draining it, thus immediately degrading this function of the peat. By the same token, when masses of peat are consumed by fire, they release stored carbon to the atmosphere at a very high rate. Borneo, with one of the world's most massive peat areas, has thus become a UN–REDD priority area.

Sundaland Heath Forests

Acidic, sandy soils constitute the waterlogged terrain of the Sundaland heath forests, where a lack of nitrogen hinders a more complex variety of plant growth. Locally, these forests are known as kerangas, which is an Iban word that means "infertile soil in which rice cannot grow." Yet certain plants such as sundews, nepenthes, and bladderwort have adapted to the environment by becoming carnivores, seducing insects and even small frogs as entrees. Crocodiles, gibbons, and orangutans (the “men of the forest”) inhabit this thoroughly watery region. Birds are less abundant in this region of the forest, particularly frugivorous birds. A few endangered insectivorous birds live in the region including the crestless fireback, grey-breasted babbler and hook-billed bulbul.

Sunda Shelf Mangroves

Along the coastal lowlands of Borneo—particularly where freshwater rivers, the longest being the Rajang of Sarawak, open into the seas—a remarkable ecoregion began to develop some 50 million years ago, centering on a family of trees called mangroves. The Sunda forest of mangrove trees and shrubs, or mangal, consists of an almost-unmatched assortment of endemic marine and terrestrial plants that have adjusted over time to differing degrees of the brackish water that washes the coastal plain.

The most prominent types of mangroves are Avicennia, Rhizophoras, and Nypa palms, which can be found in sizable stands in Bako National Park in Malaysia and Tanjung Puting and Gunung Palung National Parks in Indonesia. Most of the mangrove forests of Borneo, however, have been cleared for aquacultural, agricultural, or urban development.

Environmental Pressures

Extensive habitat loss and poaching, offset by new discoveries and evolutionary processes, continually alter Borneo's level of biodiversity. However, there has been a rapid decrease in the populations of large animals such as the Bornean clouded leopard (Neofelis diardi) and the island's quite rare Borneo Sumatran rhinoceros (Dicerorhinus sumatrensis harrissoni)—a subspecies of the Sumatran rhinoceros and considered a “living fossil” related to the extinct, ice-age woolly rhinoceros—boasting the unique distinction of bearing patches of reddish-brown hair. These decreases are a clear indication that human culture impairs the inimitable species-generating power of the Cool Equatorial Rainforest biome.

Even though an international consortium of green-culture enthusiasts agreed in 2007 to establish a Heart of Borneo, where about a third of the island's less accessible forested region would be preserved from industrial-scale logging, enforcement is typically not up to the task of facing down corporate interests. Plans for increases in oil, gas, and coal production; the further building of dams; and expansion of palm oil plantations may finally put to rest the image of Borneo as an idyllic wilderness on the fringes of the known world. The World Wildlife Fund (WWF) noted in a 2021 report that 43 million hectares were lost to deforestation from 2004 to 2017. The WWF further noted that deforestation and forest degradation in the region continued at an alarming rate. Fires also threaten the region. Many of the fires in the region originate in the plantation sector. Fires in 2015 and 2019 reportedly burned nearly 4.4 million hecates of forest.

Bibliography

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