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’. For many people this probably covers what we will later determine as tropical rain forest. 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”.
Today, though, there are many definitions of ‘tropical rain forest’, even, and most especially, for scientists. These vary around the world, depending on the classification schemes used. One of the most useful of those schemes 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’ (Adam 1997). So even when we scientifically define ‘tropical rain forest’ we run into difficulties. For example, what the public would consider ‘tropical rainforest’ in Australia may be called “closed forests” by Australian scientists, but would be considered mere ‘moist forest’ or ‘wet forest’ by American ecologists. In the end, we must remember that definitions and classifications are all artificial constructs.
Recognizing characteristics of a Tropical Rain Forest
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.
They can be divided for convenience into two types; general structural characteristics of the rainforest, and more specific physiognomic features of the plants themselves.
The rain forest consists of a multi-layered canopy. The main canopy consists of an undulating ‘sea’ of tree crowns and every now and then, taller ‘emergents’ poke 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’ (e.g.; Webb and Tracey 1987), which acknowledges the predominance of vines. Once secure, many will develop large woody stems, generally referred to as ‘lianas’, and may be as long-lived as many of the trees of the forest. 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.
Rainforest leaves are usually leathery, a nice glossy green, and up to 90% may possess the distinctive ‘drip tip’ (Johns 1999). Leaf size in the best-developed topical rain forest is relatively large, being generally described as ‘mesophyll’ (Whitmore 1999) or even larger. Of course, this is a generalisation, and leaf size varies depending on it’s position in the canopy. 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’.
There are also the more subtle ecological features such as spatial complexity and diversity. While such features are important to the definition of what a tropical rain forest is, they are not always so immediately obvious.