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Distribution of Tropical rainforest

Courtesy of Damon Ramsey

Requirements and distribution of tropical rain forest around the world

There are several conditions required for a tropical rain forest to grow. Primarily, there needs to be light, warmth, and moisture. The tropics receive a lot of sunlight. This is due to their location on the larger, bulging equatorial region of the globe. Here, there is little variation in the amount of sunlight during the day throughout the year. The time of the sun setting and rising does not vary much more than about an hour of the same time every day. This constant sunlight results in a high temperature. Further, the temperature does not vary much during the year. On the constantly shaded rain forest floor, the temperature will vary even less. Visitors are often shocked by how much it rains. Of course, this is the rain forest, and it is by implication an essential requirement. And not only does it rain a lot, it needs to rain all the time; that is, the rainfall generally needs to be evenly distributed throughout the year. Finally, if you are travelling through the tropical rain forest you will notice another condition. Its result is copious sweating, even when standing still. The rain forest has very high humidity, a combination of the two factors of heat and water. For many people, it is this high humidity that makes the tropical rain forest so uncomfortable. 

In addition, there are certain ‘limiting’ conditions that cannot occur if tropical rain forest is to develop. The temperature must not drop so low as to allow frost to appear (Winter and Atherton 1987). ‘Moisture stress’ cannot occur for prolonged periods; if a dry season lasts for a period of a few months every year, then rain forest will not survive and a monsoon forest will develop (Whitmore 1999). In even drier situations, an open woodland or savanna will grow.

Of course, there are other secondary conditions may determine whether a tropical rain forest will develop. One of these is soil type. If the soil is deeper and more fertile, rain forest may able to develop in areas with less rain or lower temperatures that would normally limit its growth. In fact, this is the situation in large parts of the eastern and southern Amazon rain forest of South America (Whitmore 1999). Following on from this, contrary to many popular beliefs, rain forest does not limit itself to poor soils. Rather, it can grow in wet and warm conditions despite poor soils. Some of the most diverse and luxuriant rain forests in the world grow on rich soils, such as the rain forests on the volcanic soils in areas of Costa Rica, Indonesia and the Atherton Tablelands in Australia.

Tropical rain forests should thus develop in areas where these conditions are met around the world. These rain forests are, by definition, within the tropics, the region bordered by the tropic of cancer in the Northern Hemisphere and the tropic of Capricorn in the south.

Looking at the map below, we can see that there are several distinct tropical rain forest regions in the world. These rain forest regions correlate with the main biogeographical realms (see any biology text for a map of the biogeographical realms, e.g. Beck et al 1991). There is Neotropical rain forest (Central America and South America), Afrotropical rain forest (mainly central and western Africa), Oriental rain forest (mainly Southeast Asia) and Australasian rain forest (New Guinea and North-east Australia). As you can see, half of the world’s tropical rain forests are in the Neotropics (Whitmore 1999). The smallest major zone comprises the Australasian tropical rain forest. Although this and the Southeast Asian regions are quite close, they are distinct regions of tropical rain forest.

Requirements and distribution of tropical rainforest in Australia

Most of the tropical rainforest in Australia is a found in a small area of north-eastern Queensland known as the 'wet tropics', which sits roughly between 15 and 20 degrees south of the equator, well above the tropic of Capricorn. Being in the tropics means there is much less variation in many environmental factors during the year than in temperate areas. For example, sunset and sunrise varies by only about an hour throughout the year. While many different aboriginal groups around tropical Australia recognized many distinct seasons throughout the year, modern Australians generally recognize two main seasons; the 'wet' and the 'dry'.  These two seasons are based mainly on the amount of rainfall. As part of tropical monsoonal Australia, the wet tropics experiences a distinct 'wet season', which occurs in the southern hemisphere summer. Much of the rain falls in the months of December, January February, March and April. The southern winter is much drier in most of tropical northern Australia, but the Wet Tropics region still experiences sufficient rainfall in the so called 'dry season' to sustain rainforest. This results in north-eastern Australia having the highest rainfall in Australia. 

The presence of the rugged topography so close to the coast also determines much about the climate, and can produce a remarkable local variation in this rainfall. Moisture laden winds coming in off the Pacific Ocean try to rise above the range, but cool and drop their moisture, and thus there is a constant amount of rainfall on these coastal lowlands. In the rainforest mountain ranges southwest of Cairns, annual rainfalls are in excess of 8 metres. But even direct rainfall is not the only type of precipitation, for when mist and clouds pass over the valleys and peaks moisture condenses on the plants; a phenomenon known as 'cloud-stripping'. Recent research at James Cook University and CSIRO has suggested that this effect is much more significant than previously realized; at some times of the year, such cloud stripping may be collecting up to 40% more moisture than is measured by traditional rain gauges.

Temperatures on the coastal plains generally don't vary by more than 15 degrees Celsius throughout the year, with temperatures in the early 30's being the maximum in the summer season and lows of the high teens in the winter season. However, the high humidity of the wet season make the heat of the summer seem much more oppressive to many first time visitors to the tropics.

Presently, rainforest in Australia is a very distinct and unusual vegetation type in contrast to the rest of the continent. Most of Australia is a very dry country; in many areas, the evaporation rate exceeds the rainfall, and many outback rivers dry up before they ever reach a sea. The woodlands and deserts are dominated by two genera of trees, Acacia and Eucalyptus. There are several distinct areas of tropical and subtropical rainforest in Australia.

The most northerly rainforests are those of Cape York. Much of this grades into monsoonal rainforest and more open tropical woodland. Located this far north, there are obviously many affinities with the flora and fauna of New Guinea.

The biggest patch of tropical rainforest in Australia is an area known as the 'Wet Tropics'. This is also the area with the highest recorded rainfall. It has the town of Cairns at it's rough geographical centre, and includes the Daintree, Bartle Frere/Bellenden Ker peaks and the Atherton Tablelands.

 Much of the most southerly subtropical rainforest is collectively known as the 'eastern rainforest'. This includes large tracts on the border of southern Queensland and northern New South Wales, and includes Lamington National Park
 


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Lake Eacham, Atherton Tablelands
Tropical North Queensland, Australia.
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