Defining “tropical rainforest”

There are many colloquial names for ‘tropical rain forest’. In Australian colonial times the rain forest was described as the ‘scrub’ or ‘brush’. In the recent past, the most commonly used name around the world has been ‘jungle’. In the last few decades, however, the words ‘rain forest’ have actually replaced the old fashioned ‘jungle’. These words “rain forest” are more of a technical term, and were originally used by a German botanist over a hundred years ago; “tropischer Regenwald".

Tropical Rainforest

Ultimately, the best way to define tropical rainforest is simply to describe it's characteristics. While many species of flora and fauna are shared between rain forests, most are unique to that particular region, therefore we don’t usually recognise tropical rainforest on particular species. However, the different species within these rain forests are under the influence of the same environmental pressures, and thus evolve similar features. These similar characteristics, regardless of geography, can then be used to recognise a tropical rain forest.

General structural characteristics include a multi-layered canopy consisting of an undulating ‘sea’ of tree crowns and every now and then, taller ‘emergents’ poking out. Below the main canopy may be many different levels or strata of trees. The very bottom floor of the undisturbed rain forest is relatively open with little undergrowth such as grasses. In some classification schemes rain forest is known as ‘vine forest’ which acknowledges the predominance of vines. Epiphytes are another obvious component of rain forests It is not uncommon to see a large number of both species and individuals on the one tree.

More specific features include, for example, characteristics of the leaves. They are usually leathery, a nice glossy green, and up to 90% may possess the distinctive ‘drip tip’. Many trees in the rainforest will flower and fruit on the branches and/or the trunk; ‘ramiflory’ and ‘cauliflory’ respectively. The roots of many tropical rain forest trees have many interesting variations, including spreading as much horizontally as they do vertically, and the development of ‘buttress’ or ‘plank roots’.

Basic Requirements

Tropical rainforest requires certain conditions 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, with 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 itself also does not vary much during the year compared to temperate regions. On the constantly shaded rain forest floor, the temperature varies even less.

Sun and heat are strong and constant in the tropics

And then there is the rain. Despite the big clue in the name ‘rainforest’ naive visitors are still often shocked by how much it actually rains in the forest. It is of course an essential requirement for rainforest. 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.

If you are travelling through the tropical rain forest you will notice the combination of all these features. The rain forest has very high humidity, and for many people, it is this high humidity that makes the tropical rain forest so uncomfortable.

High rainfall is an obvious feature of rainforest

In addition, there are also 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. ‘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. In even drier situations, an open woodland or savanna will grow.

Distribution of tropical rainforest

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. There are several distinct tropical rain forest regions in the world. Half of the world’s tropical rain forests are in the Neotropics. 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.

The distribution of tropical rainforest in Australia (see map below) is due to various factors such as topography and climate. The presence of the rugged topography so close to the coast 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 ranges in the north east of the continent, but they cool and then drop their moisture. This results in a high and constant amount of rainfall on the coastal lowlands and ranges of North-east Queensland, which thus has the highest rainfall on the continent. Tropical Rainforest takes up only a very small proportion of the continent. In all, only some 0.1% of the countries' land mass is covered in tropical rain forest. 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. It has the town of Cairns at its rough geographical centre, and includes the Daintree region, the Bartle Frere and Bellenden Ker peaks and the Atherton Tablelands.

The distribution of tropical rainforest in Australia

The structure of the rainforest

The rain forest consists of a multi-layered canopy. The main canopy consists of an undulating ‘sea’ of tree crowns with heights of between 25 and 45 metres in lowland tropical rain forest (aerial view of lowland Daintree rainforest, below) . Every now and then, taller trees poke out above all else. These trees are known as ‘emergents’, Below the main canopy may be many different levels or strata of trees. These catch any light that manages to get through the main top canopy. The result is an overlap of many different levels of trees. Looking up from below, the canopy appears closed. Only small gaps reveal the sky above. This then results in very little of the light reaching the floor of the tropical rainforest.

The canopy of trpoical rainforest

Measurements in some rainforests have shown that only some 5% of the sunlight may reach the forest floor. This means that the very bottom floor of the rain forest has little undergrowth, such as grasses, and undisturbed rainforest thus has a relatively open under story. The plants that do grow down here in the relative darkness often have relatively huge leaves to catch what is left of the sunlight. In the lowland forests they belong to the giant herbs such as native bananas and gingers. Seedlings have to be dropped away from the parent tree to avoid competition with the parent tree. If animals do not disperse the fruit, or wind dispersed seeds do not fall far from the parent tree, a blanket of seedlings may grow near the tree (see image below). These may grow to a few inches and then just halt development, a permanent carpet of miniature plants frozen in growth. These seedlings may actually stay like this for many years; they are biding their time, just waiting for a gap in the canopy, at which time they will enjoy a sudden burst of growth.

Seedlings on the rainforest floor

Epiphytes

Visitors often see these growths in the canopy and may assume they are or a natural growth of the tree, parasites on the trees or bird’s nests. However, epiphytes are simply plants that grow atop other plants. They, like pretty much all other plants, use sunlight for photosynthesis. In the thick vegetation of the jungle, little light reaches the lower levels, so epiphytes are ‘getting a leg up’ by growing up and on top of other trees. Many epiphytes include plants that have small 'seeds'. This enables easy dispersal through the air and up into the canopy to find a place to grow from. Orchids have hard capsules that may open and release hundreds of tiny seeds. Ferns are even lighter; the sporophytes seen underneath the fronds release thousands of tiny dust like spore that, with just a whiff of air, may waft up into the canopy to settle on a tree.

A fern growing as an epiphyte

Vines

Vines are some of the most obvious aspects of this ecosystem. It is not uncommon to see a large number of both species and individuals crowded on the one tree. In some areas, there may be almost as many climber species or individuals as there are trees. Once secure, the lianas may develop large woody stems'. The 'rattan palms' of Asia and Australia develop the longest stems of any plant in the world, with records of stems over 500 feet. Barring collapse, some of these woody climbers may be as long-lived as many of the trees of the forest. While vines are found all over the world, over 90% of species are found in the tropics. Climbers generally do what their name suggests; they climb other vegetation to gain access to sunlight. There are many more climber species in the tropics than there are in the temperate areas. Despite popular belief, most species of climbers have an anti-clockwise bias around their host trunk. Climbers in the rainforest create some of the most interesting and aesthetically pleasing patterns and formations. Many of the lianas seem to hang in the air like an Indian rope trick, naturally creating the question of how they climbed up through empty space. The answer is that of course they didn’t; when younger, these climbers grew up trees or through a canopy which has since disappeared through death and/or collapse, leaving the climber hanging. Similarly, some of these climbers hanging in the air create a pleasing spiral pattern hanging freely in the air. When the climber was younger it grew around a branch or tree that has since disappeared; but the climber retains its spiral growth pattern (as pictured below).

Rainforest Vine

Cauliflory

The flowering and fruiting on the branches and/or the trunk of a tree is called ‘ramiflory’ and ‘cauliflory’ respectively. Early explorers, and even visitors today, have mistaken them for a parasite on the plant. They can make for a spectacular show in the tropical rainforests. The example pictured is a Daintree Satin-ash in the Cape Tribulation lowlands

A rainforest tree fruiting from its' trunk

Roots of rainforest trees

The roots of many tropical rain forest trees have many interesting variations. In many trees, the roots will spread as much horizontally as they do vertically. Erosion by running water may eventually reveal this complex network just underneath the ground. Many trees also develop large flanges above the ground that flank the tree on either side. These are called ‘buttress’ or ‘plank roots’ and give the tree the appearance of a rocket (as in the image below). Although some species are capable of developing them, and some are not, the development of buttress roots is a reflection of environmental pressures and not an indicator of species. Reasons suggested for such development include support and stability, allowance of air exchange in water-logged soils, and the capture of leaf litter. In the latter, the roots act as a nutrient ‘sink’, collecting fallen debris for breakdown. These areas are then used by sheltering animals, and thus the tree then collects further nutrients via the animal’s waste products.

Buttress root of a rainforest tree