Lowland tropical forests
Ecotourists in dipterocarp forest, Bako National Park, Sarawak. @ C.Michael Hogan
Riparian zone of lowland tropical forest along the Monkey River in southern Belize. Source: C. Michael Hogan Lowland tropical forests occur in the Neotropics, parts of Australasia, Southeast Asia and within Africa's Congo Basin, typically within ten degrees latitude of the Equator, north or south. Characteristic rainfall is high in these forests, typically ranging from 1750 to 2000 millimeters per annum; average temperatures are generally in excess of 18 degrees Celsius in each month of the year. Alternate designations for this forest type are Lowland humid forests. Considerable amounts of the temperate zone ecological literature uses the term lowland forest for certain low elevation temperate forests, but the preponderant use of the term without further specification relates to moist evergreen broadleaf forest plant associations in tropical climates. The upper altitude limit for a lowland forest is variable depending on region and aspect of investigation, but typically an altitude of 500 or 1000 meters is applied for the upper bound; the lower elevation limit of lowland tropical forests is sea level.
Contents
Natural history and Ecology
Plant associations within present day lowland forests were quite different from today in the late Pleistocene and even very early Holocene. For example, in cooler, drier times from approximately 10,000 to 20,000 years before present, the present lowland forests of Panama (Ecoregions of Panama) were dominated by entirely different tree species according to pollen records.
Lowland forests typically have a high canopy, with relatively dense evergreen broadleaf high tier trees, such that relatively little sunlight reaches the forest floor. Biodiversity in undisturbed lowland forests can range from moderate to high. Populations of epiphytes and herbaceous species are usually found in abundance as well as tree and shrub tiers.
Abiotic factors include a prevalence of oxisols, soils which are high in ferric iron and oxides of aluminum, lending a reddish appearance to many of the lowland forest topsoils.
Architecture
A lowland tropical forest can be considered to be comprised of the following vertical tiers: emergent canopy, base canopy, middle tier and forest understory. The emergent trees may attain heights of 35 to 50 meters, and sometimes even reach greater stature; these species appear to poke through an otherwise fairly even canopy top, whose height is typically about 25 to 30 meters. Animal life present in these top tiers include many bird species, monkeys and a number of bats. One can most easily view these emergents from a birding tower or ancient Mayan pyramid top that extends above the canopy. Mid tier plants consist of intermediate height trees, tall shrubs and epiphytes that do not require the light illumination intensity of the canopy trees; the mid-tier may also contain abundant fauna, including birds that are able to navigate among considerable leaf and liana (vine) densities. The forest floor receives relatively little sunlight, and thus is rarely choked with vegetation; nevertheless, one finds here a myriad of low growing shrubs, ground trailing lianas, ferns, lichens and mosses. Moreover, there are a plethora of terrestrial avafauna, arthropoda, amphibians and reptiles.[1]
Examples
Instances of Lowland tropical forests in the Neotropics are: Isthmian-Pacific moist forests of lowland areas in western Panama and eastern Costa Rica. While biologically quite diverse, this ecoregion supports only low levels of endemism. The high species richness stems chiefly from the mixing of North and South American biota on this land bridge. The resident fauna, including butterfly, reptile, amphibian, bird, and mammalian taxa are basically broad distribution representative species of the Mesoamerican wet tropical forest ecoregion.
Another example is represented by the lower elevation components of the Petén-Veracruz moist forests in northern Guatemala. This ecoregion is one of the richest faunal regions in the western hemisphere, and is one of the three regions of highest insect richness and endemism. Birdlife International has included this area in its Endemic Bird Area (EBA) project, due to the rich endemic avifauna. This region is also noted for its high canopy forests, one of which engulfs most of the lofty pyramids at the Mayan archaeological site of Tikal.
In the Congo Basin, Atlantic Equatorial coastal forests extend from the Sanaga River in west-central Cameroon south to the west of the Democratic Republic of Congo, immediately north of the Congo River mouth and inland from the coast. This lowland forest system has exceptional levels of species richness and endemism, and contains large areas of evergreen lowland moist forest; moreover, the central portion of this region has one of the lowest human population densities in Africa. Most of the floral and faunal assemblages are intact, including assemblages of threatened large mammals, such as the Western lowland gorilla, mandrill, and Sun-tailed monkey. Important centers of endemism are found in this ecoregion, particularly in some of the coastal mountain ranges.
In the Australasia the Borneo lowland rainforests are a good example of biodiverse lowland forests of the region. This ecoregion is made up of lowland dipterocarp forests. All of Borneo, Java, Sumatra, and mainland Malaysia and Indochina were once part of the same landmass during the Pleistocene Glacial period. Land bridges connected all of these islands, fostering waves of migrations of animals, plants, and humans. Today Borneo is separate but shares similarities in flora and fauna with these other landmasses. The geology of these Borneo rainforests consists of limestones, volcanic rocks, schist-gneiss complexes, and sedimentary rocks. Soils are primarily ultisols and inceptisols, generally older, infertile soils. Based on the Köppen Climate Classification System, this ecoregion falls in the tropical wet climate zone. Monthly rainfall exceeds 200 millimeter year-around. The region harbors one of the world's richest assemblages of flowering plants. Forests contain a high diversity of trees, with as many as 240 different tree species present within one hectare.
Archaeology
Lowland forest at Tikal, Guatemala viewed from above canopy top. Note the tops of ruined temple appexes protruding above the canopy. Source: C. Michael Hogan Even though lowland tropical forests are inherently low in nutrients and relatively unproductive in regard to agriculture,[2] the presence of early man is often associated with lowland forests compared to montane forests, since land cultivation and transport were more approachable in these gently sloping topographic regimes. Notable associations of very early settlement in the Americas is documented; for example human habitation in present day central Panama has been documented to have occurred as early as 11,000 years before present.[3]
Associations of Lowland tropical forests with the Mayan civilization are evidenced by many extant sites including the extensive Tikal archaelogical site in Guatemala and the Nim Li Punit archaeological site in southern Belize.
Conservation
Sizable portions of the world's lowland tropical forests have been converted to agricultural uses, even though the inherent productivity is low, due to the thinness of soil and lack of nutrients. Coffee ,cocoa, banana, mango, papaya, macadamia, avocado and sugarcane are some of the chief crops that are produced in lands that were originally lowland tropical forest. For example, by the end of the 20th century 40 million tons of bananas were produced annually, the total annual mango crop amounting to over 13 million tons. Lowland tropical forests yield over 200 varieties of fruit, compared to only 20 for temperate forests. The tropical forests of New Guinea, for example, contain 251 tree species having edible fruit, of which merely 43 had been established as cultivated crops by the year 1985.[4] Coffee production in Mesoamerica amounted to more than US$ 3 billion in 1970. Much of the genetic diversity needed to avoid pest damage to tropical crops is derived from resistant wild stock, illustrating the importance of conserving large intact tracts of these lowland tropical forests. Too often the tropical forest conversions are simply to monocultural cropping, thus eliminating the inherent biodiversity of this ecosystem. There are some examples of polycultural cropping in the coffee and cocoa industries; for example, it has been shown that coffee can be compatibly grown adjacent to intact primary forest in certain parts of western Panama. Cocoa farming has been shown to be viable in a polyculture environment in the Bocas del Toro region of northwest Panama. Replications of prehistoric farming are being studied in the Chaa Creek area of Belize.
Palm oil plantations are the greatest threat to Lowland forests in Malaysian Borneo, both in the states of Sabah and Sarawak. The palm oil tree is not only an alien species to the Southeast Asia region, but supports virtually none of the native understory or wildlife; in fact, the ground surface below the palm oil canopy resembles an ecological dead zone. Indonesian Borneo is also threatened by ongoing widespread slash-and-burn practises, which not only create massive amounts of habitat destruction, but also cause widespread air pollution as far as Malaysia and Singapore.
Besides the widespread losses of lowland tropical forest due to agricultural land conversion, deforestation due to logging continues at a rapid pace; for example, native tropical rainforests in Indonesia will be effectively logged out by the year 2018 and Papua New Guinea within three to six years thereafter, based upon present timber harvest rates.[5]
References
^ John R. Packham, David J. Harding, G.M. Hilton and R.A. Stuttard. 1992. Functional ecology of woodlands and forests. 407 pages
^ Patrick S. Bourgeron. Spatial Aspects of Vegetation Structure. in Frank B. Golley. Tropical Rain Forest Ecosystems. Structure and Function. Ecosystems of the World (14A ed.). Elsevier Scientific. pp.29–47. ISBN 0444419861
^ R.C.Bailey, G. Head, M. Jenike, B. Owen, R. Rechtman, and E. Zechente. 1989.Hunting and gathering in tropical rainforest: is it possible American Anthropologist, 91:1 59-82
^ Peter W. Stahl. 2006. Archaeology in the Lowland American Tropics. 332 pages
^ N. Myers. 1985. The primary source W. W. Norton and Co., New York, N.Y.
^ China is black hole of Asia's deforestation, Asia News, 24 March, 2008
