What is a tropical rainforest?

There are many colloquial names for tropical rainforest.

In the recent past the most commonly used name around the world has been ‘jungle’, and for many people this probably still covers what we think of as tropical rainforest.

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”.

In contrast to most of the rest of the world, in Australia we often combine the two words “rain forest” into the one word ‘rainforest’, and on this site I will swap spellings regularly.

scientific definitions of tropical rainforest...

There are many definitions of ‘tropical rainforest’, even (and most especially) for scientists. These vary around the world, depending on the classification schemes used.

One of the most useful of those systems is the Holdridge pyramid. This is particularly popular in the Americas where it originated, and it’s often used to distinguish between different types of ‘rain forest’ in parts of the Neotropics, especially Central America. Rain forest in this scheme is actually divided into ‘moist forest’, ‘wet forest’, and ‘rain forest’. It uses the factors of water and temperature, and their variations, to determine the type of vegetation.

Within Australia, a system designed by Specht is commonly used. It is based on canopy cover and the height and form of the dominant vegetation. In this classification, tropical rain forest comes under the general vegetation type ‘closed forest’.

Thus, even when we try to scientifically define ‘tropical rainforest’ we run into difficulties; what a member of the general public in Australia would consider ‘tropical rainforest’ may be called ‘closed forest’ by Australian scientists, but would be considered mere ‘moist forest’ or ‘wet forest’ by American ecologists.

In the end, we must remember that these are all just human categories for dealing with the natural world.

Ultimately, the best way to define tropical rainforest is simply to describe its characteristics.

describing the tropical rainforest...

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 rain forest 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.

We can use two main types of characteristics to recognize a tropical rainforest; the general structural features of the overall rainforest, and the more specific features of the plants themselves.

Structural features

There are various obvious structural features of the tropical rain forest. For example, it has a multi-layered canopy, which consists of an effectively complete top layer, while below there may be many different levels or strata of plants.

The very bottom floor of undisturbed rainforest is relatively open with little undergrowth such as grasses.

Another distinctive feature is the amount and variety of plants growing on other plants, such as vines and epiphytes growing in the canopy.

Specific features

There are also more specific features of the organisms that live within the rainforest. For example, tropical rain forest leaves often look very similar despite growing on unrelated plants; they are usually leathery, a nice glossy green, and many possess the distinctive ‘drip tip’.

Many tropical rain forest trees develop features that are rare outside the rainforest; they may flower and fruit on the branches and/or the trunk (‘ramiflory’ and ‘cauliflory), and the roots of many trees spread out as much horizontally as they do vertically, developing into ‘buttress’ or ‘plank roots’.