Permaculture Designers Manual
CHAPTER 10 –
THE HUMID TROPICS IN PERMACULTURE
Section 10.2 –
Humid Tropics Climatic Types in Permaculture
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WET TROPICS
These are the river basins and wet coasts from Latitudes 0-25°.
Major localities are the Amazon and Congo basins, Central America, Sri Lanka, Malaya, Borneo coasts and New Guinea.
This climate covers about 10% of the earth’s surface (5% of the human population). It is heavily occupied only where terraced alkaline volcanic soils enable sustained cultivation (Java).
It contains remnant tribes of hunter-gatherers (often pygmoid) in remaining forests and is rapidly being ruined by over-exploitation of forests, mining and extensive cattle rearing, mostly developed by large corporations.
Hence, there is a recent tendency for catastrophic wild fires to develop in logged areas such as Borneo and for soils to be leached to low fertility or eroded to ferri-crete or sil-crete subsoil’s. (Figure 10.1)
The sun is mostly overhead, with temperature fluctuating little at about 21-32°C (70-90°F). Humidity is constantly high, frost unknown.
Rainfall is from 152-328cm (60-129 inches), with rain most days and frequent thunderstorms (75 to 150 days / year), usually towards evening or late afternoon.
The landscape features perennial streams, deeply weathered and rotten rock over bedrock (regolith) and rounded hills. There is rapid water run-off and evaporation, with swamps confined to coasts and lowlands.
Rivers usually have flood-plains and extensive alluvial plains and deltaic deposits. Access inland is often by rivers and tributaries.
The vegetation is luxurious, best developed as broadleaf rainforest with lianas and epiphytes, very much mixed as to species-up to 800 tree species per square kilometer.
The shaded floor of the forest contains little growth and has subdued light. Mangroves are extensive on appropriate coastal and estuarine sites.
Growth is rapid, uninterrupted and continuous. Insects and birds are plentiful and varied. Most fauna is nocturnal and arboreal and there are abundant fish and aquatic species. Ground grazers are rare and large herd species do not occur.
About 85% of nutrients are held in plants or animals, so the soils themselves are infertile especially if clear-cultivated, tending to erode and leach to insoluble oxides of iron and aluminum.
Only terraces, flood-plains and new volcanoes keep some soil fertility replenished or held if land is cultivated.
Staple cultivated foods are plantain and banana, cassava, yams, coconut, corn, taro, paddy rice, ducks, pigs, poultry, and fish.
Trade and plantation crops are spices, copra, palm oil, cacao, rubber, banana, Manila hemp, rare hardwoods, balsa, tropical nuts, chicle and drug plants.
Housing is usually raised, steep roofed, thatched, with permeable walls and screened.
Health problems relate to sewage disposal, insect vectors and skin fungi.
Design essentials are for:
Hygienic feces disposal;
Clean water sources;
Integrated and benign insect control techniques;
Gradual replacement of ground crops by trees;
Preservation of natural stands of trees;
Development of river versus road traffic;
Evolution of natural products;
No dig (mulch) techniques on root crops;
Domestic foragers for snail and insect pests;
Appropriate medicinal plants.
WET-DRY TROPICS
These adjoin the wet tropics but are pole ward of them. They take up about 15% of earth’s surface from latitudes 0°-25°, unbalanced in favor of the southern hemisphere.
The Campos, Llanos, Gran Chaco areas of South America, parts of Central America, encircling the Congo basin and many central Pacific islands (Hawaii) are all wet-dry tropical areas, most now developed to grazing.
Winter, the low-sun period, is the dry time, when clear skies and intense sunlight take day temperatures to 38°C (l00°F) or more.
Humidity is low, and strong desiccating winds may blow. Summer, the high-sun period, is like the wet tropics, but episodic flooding is more common and natural erosion therefore greater.
There are no frosts and temperatures range 21°- 27°C (70°- 80°F) in the wet season, 32°- 38°C (90°- 100°F) in dry season.
Rainfall is 25 -152cm (10 – 60 inches), decreasing towards desert margins.
Windward inland slopes and coastal mountains may receive excessive rain to 1m (400 inches), but rain is erratic and least predictable towards the desert margins. Rain shadows evolve on lee slopes or in the lee of mountains.
The landscape is of intermittent streams, some wadis and flood plains, karst (limestone) areas with sinkholes, cenotes and absence of surface water (Yucatan, Mexico).
Hills are rounded, but gully erosion can develop rapidly on slopes. Extensive inland swamps may develop in flooded areas and lakes in rifts are common (Africa).
Rivers often have dangerous bar ways of silt and sand due to active erosion sequences.
These regions contain the vast savannah grasslands of the tropics, with thorn-bush and flat-topped Acacia trees (Africa), evolving to steppe grassland on plateaus, with baobabs and dry-deciduous trees.
Grasses reach 1-6m (4-20 feet) in the wet season, and are often burnt off. African areas contain enormous numbers of herd species: zebra, gnu, antelope and therefore large carnivores.
Arboreal species occur only within tree islands and the gallery forests of valleys. There are termites, ostrich, rhea, locust and large numbers of reptile species and insects.
Soils are generally more fertile and alkaline than the wet tropics, especially where they are less leached by rain towards the deserts.
Cultivated land is still at risk from erratic rain and erosion, leaching and wind effects.
Due to overgrazing and fire, erosion may extend the desert into these areas, or into dry-summer subtropics. Serious soil erosion results from short term shifting cultivation (less than a 15 year fallow period).
Staples are corn, millet, wheal, beans, potatoes, cucurbits, peanuts, cattle and goats, sheep, and game products.
Herding of low-yield large herds is a major erosion hazard.
Plantation crops are sugar, cotton, peanuts, and pineapple. Sisal Exports are cattle and sheep products, and hardwoods from gallery forests.
Houses and granaries are generally mud or pise with thatch.
Design essentials are for:
Small domestic water storage and reticulation;
Hedgerow against winds;
Inter-crop tree legumes such as Acacia albida;
Improved stock varieties and stock management;
Natural herding system of local herd species;
Mulch use of grasses;
Increased tree crop of high forage value;
Decreased fire frequency;
Tree stands for fuel and structural timber;
No-tillage (cut and mulch) grain techniques;
Low bunds for water retention.
Chisel plow and sod seeding techniques;
Greater reliance on in-village tree crop near wells and ponds;
Reclamation of eroded lands using pioneer species;
Keyline techniques of flood control;
Soakage pits and impoundment of run off by low bunds or swales across slopes;
Tree forage and tall grass hand fed to domestic stock;
The use of manures in gardens;
The development of domestic fuel wood systems near villages;
MONSOON TROPICS
These are really a sub-type of wet-dry tropics, but influenced by nearby continental land masses and oceanic winds onshore.
They are confined to the lndo-Thailand region, northern Australia, East Java, Timor, Southern New Guinea and extend Latitude 0° to 35° north in India.
Despite only about 8-10% of the world’s land surface, monsoon areas contain large human populations.
Late summer heating of the continents causes onshore sea winds and with luck, heavy rains. The dry (winter) season reverses winds from cool interiors to coasts, giving a cool period not experienced in the wet dry tropics (temperature: 13° – 21°C (55° – 70°F).
Temperatures rise and dry hot winds develop (to over 38°C – 100°F) in spring, with heat increasing until the onset of the monsoon.
About 60% of the rain falls in summer, but rain is erratic and varies from 102cm-1000cm (40-400 inches), depending on topography and distance from the coast.
Floods and droughts are equally unpredictable, but common. Most activities are determined by the monsoon rain (transport, fishing and farming).
Tropical forests once clothed the hill slopes and river plains, and grasslands extended towards deserts as savannah.
Population pressure, deforestation and marginal agriculture have devastated this ecology in India. Dry-deciduous broadleaves are common, teak and bamboo once extensive.
Tree canopy is less dense than in wet tropics, so that dense understory is also developed.
Mangroves occupy river mouths and low coasts.
Large native animals are now rare in the Indian sub-continent, but reptile life is abundant, as are feral or native deer, buffalo and primates.
Monsoon Australia is better vegetated, with scattered eucalypt and Acacia trees, riverside forests and very low human populations to date. Large marsupials, feral buffalo, and marsh waterfowl are abundant.
Soils are lateritic, often very hard in the dry season and of low nutrient status.
Some are cracking clays.
Housing is often mud-pole structures, thatched, steep-roofed, with wide eaves and good drainage for wet period.
Design essentials are similar to those of the wet dry tropics.
