Introduction

In this module, we examine some of the ecosystem processes that occur in tropical forests. As pointed out in the Introduction to the course, there is potential overlap between this Module and Modules 3 and 4. They have been separated primarily to make it easier for us to present the material and for you to learn about the ecology. In reality, everything is connected in the forest, and any attempt to reduce the ecosystems to more easily studied units risks making it more difficult to understand the extent of this connectedness.

In Modules 2 and 3, we have restricted the material to plants. We considered including material about animals, but this would have made the course too unwieldy. The nutrition of animals is a major subject in itself, as are topics such as reproduction and life strategies. We do however cover the extent to which animals are involved in some of the processes affecting plants, including pollination, seed dispersal, and disturbance. In Module 4, we do include animals, as many of the most important interactions involve animals: herbivore – plant interactions cannot be examined without consideration of the animals involved, whereas predator – prey relations exclusively involve animals. Indeed, any attempt to construct a food web for a tropical forest has to consider animals.

Module 2 addresses some of the fundamental ecological processes that occur in tropical forests. We start off in Topic 1 by looking at nutrient cycling. In the past, scientists have though tropical forests to be very poor in nutrients. This is because when they were cleared, the soils that were left behind quickly became infertile. We now know that this is wrong: tropical forests are rich in nutrients, but the cycling is extremely tight, with very low quantities of nutrients stored in the soil. This contrasts markedly with temperate forests.

We then look at the productivity of tropical forests, and the resulting biomass. When tropical forests were seen primarily as a timber resource, this was important information for foresters, and quite a lot is known about the volumes of timber in many different tropical forests. Today, there are other priorities, with the amount of carbon stored in forests (which as a direct relationship to the amount of biomass) emerging as particularly important.

Topic 2 starts with a look at energy production. This is important in determining productivity, and one of the presentations deals with this. We can find C₃, C₄ and CAM plants in tropical forests, with the proportions varying quite markedly depending on the type of forest. Understanding the different processes that these plants use to capture energy is important.

Water is absolutely critical to plants, and you will learn in some of the case studies (especially the one dealing with savannahs) who water availability can determine whether trees are even present at a site. In areas with abundant water, which includes most moist tropical forests (at least for most of the time), there are still many questions. For example, how does a 70 m tall tree draw water from the soil up into its canopy? The pressures needed to do this are substantial.

While most people assume that tropical forests are evergreen, this is because of there has been such a focus on rain forests. In reality, many tropical forests are deciduous, losing their leaves during the dry season. We examine what ‘evergreen’ actually means, when trees lose their leaves and when new ones are developed. We also look at deciduous forests, examining why when most species in these forests are deciduous, some are evergreen.

The final presentation in Topic 2 looks at the functional traits of plants. We examine how functional traits determine the survival rates of plants in tropical forests, and analyze the trade-offs that occur between different traits.

Topic 3 is devoted to an examination of natural disturbances in forests. While humans also disturb forests, and have done so for thousands of years, the impacts of humans is so important that we have devoted the entire Topic 4 to this issue. A variety of natural disturbances affect forests, ranging from events that occur at a microscale, affecting for example the epiphytes growing on a branch, to large-scale disruptions caused by cyclones, volcanic eruptions and earthquakes.

The impact of humans in tropical forests (Topic 4) has often been discounted in the past. However, some scientists argue that it is much more significant and pervasive than previously acknowledged. This is the subject of some extremely contentious debates amongst scientists, with some arguing vehemently that the impacts of humans have been over-estimated, and others maintaining that they are even more significant than has been proposed.

In the final Topic (no. 5) in this module, we examine the patterns that are found in the vegetation of tropical forests, looking at patterns both in space and in time. This acknowledges that tropical forests are dynamic and in a constant state of flux. This again is a fairly recent realization: they are not climax forests, with a stable species composition. Instead, they are mosaic of patches, some large, some small, each at a different stage of development, and each influenced by a wide range of processes, some of which occur in a near random fashion.