Topic Introduction

In this topic, you will learn about two closely related ecosystem processes, nutrient cycling (and nutrition) and productivity. It has long been known that plants demonstrate greater productivity on fertile soils, creating the problem of how to explain the luxurious growth of plants on the apparently infertile soils in tropical rain forests. The answer lies in the concept of fertility. The nutrients are present, but large quantities are stored in the vegetation rather than soil. If the vegetation is removed, the soils, with their relatively low pools of nutrients, appear infertile.

Productivity in tropical forests can also be deceiving. There is a huge amount of biomass, implying very high productivity, but how long does it take to develop? Most tropical trees do not have annual growth rings, so establishing the ages of large trees is difficult. Saplings may exist below the canopy for many years with very little growth, until a gap occurs that provides the light they need for a rapid growth spurt. That they can do this implies that they can build up the necessary reserves of nutrients, or that the gap itself makes more nutrients available to the saplings.

Outcomes

By the end of this topic, you should be able to:

• Describe main stores and flows of nutrients in tropical forest ecosystems.
• Describe the different processes in tropical forests that affect plant productivity.
• Contrast and explain nutrient availability on different soil types in the Tropics.
• Assess the role of seasonal flooding in the nutrient cycles of forests.
• Demonstrate how productivity is measured, and outline the potential areas of continued uncertainty.
• Explain the carbon cycle and contrast carbon sinks and flows in different types of tropical forests.

Topic Readings

Required Readings

• Ghazoul, J. and Sheil, D. 2010. Tropical Rain Forest Ecology, Diversity, and Conservation. Oxford, Oxford University Press, 516 pp. Sections 9.4 and 9.5.

Optional Readings

• Jaramillo, V.J., Martínez-Yrízar, A., Sanford, R.L. Jr. 2011. Primary productivity and biogeochemistry of seasonally dry tropical forests. In: Dirzo, R., Young, H.S., Mooney, H.A., Ceballos, G. (eds.) Seasonally Dry Tropical Forests: Ecology and Conservation. Washington DC, USA: Island Press. pp. 109-128.
• Kricher, J. 2011. Tropical Ecology. Princeton, New Jersey, USA: Princeton University Press, 632 pp. Chapters 9 and 10.
• Lüttge, U. 2008. Section 10.2 of Physiological Ecology of Tropical Plants. Berlin, Germany: Springer Verlag. pp. 335-361.
• Crawley, M.J. (ed.) 1997. Plant Ecology. 2^nd Oxford, UK: Blackwell Scientific. 717 pp. Chapter 3.

Fundamental Concepts

PPT Presentations

2.1.1 Ecosystem Processes - Nutrient cycling.pptx Download 2.1.1 Ecosystem Processes - Nutrient cycling.pptx

2.1.2 Ecosystem Processes - Productivity, biomass and carbon.pptx Download 2.1.2 Ecosystem Processes - Productivity, biomass and carbon.pptx

Self-check Quiz (For certificate learning only)

After viewing topic presentations, take this self-check quiz to check your own progress. This quiz is designed to see how well you remember some of the more important issues presented in the topic. There are five questions, and each has a potential of 4 answers, only one of which is correct. When you are ready, click the link Self-check Quiz 2.1 to start taking the quiz.

Pre-readings and Discussion (For certificate learning only)

Once you have read Sections 9.4 and 9.5 of Ghazoul, J. and Sheil, D. 2010. Tropical Rain Forest Ecology, Diversity, and Conservation, choose to post one thought provoking question related to the material that you have read, and answer/respond to at least one other student’s question on Module 2 discussion board Module 2 Discussion.

Graded Assignments (For certificate learning only)

Six Sentence Answer #2

Please refer to the course pages on graded assessments to see details of the Six-sentence Answers (6SA) exercise. You should not attempt to do this assessment until you have read and fully understood the requirements. 

The Case

You are a member of a major international forest consultancy firm that specializes in doing surveys of forests around the world. You have access to field teams and the latest surveying equipment, include LiDAR. In the accompanying course, your company was working on the development of forest businesses on a very large island in South-East Asia. You have now moved on to another project, where you are looking at the effects of past and present management practices on the forests and savannahs of East Africa. This area is very diverse, including many different forest types. They range from grass and tree savannahs through seasonally dry woodlands and forests to Afromontane rain forest.

You have done some work on selecting particular areas for reserves. This was done using species richness as a primary criterion. When you present your results to the project management team, you are asked to explain why the chosen area has so many species. Using the 6SA format, argue why this particular area of tropical forest has so many species.

Remember to follow the structure of the 6SA outlined in the section on graded assessments.

Formatting:

• Double-space the text.
• 12-point font in New Times Roman or Calibri. 
• Put your name (Family or last name, followed by your first name), student number, the title “Topic 1.1 assignment”, and the date at the top of the first page.
• Name the e-file: your Family or last name_your first name_ Topic 1.1_Assignment.doc (or .docx). Example: Innes_John_Topic 1.1_Assignment (or .docx).
• Standard English usage, clear writing style.
• All assignments should be submitted through Canvas before the deadline.

Topic Summary

In this topic, you have learnt about the ecosystem processes associated with nutrition and productivity. As you will have seen, tropical forests are not infertile, although removing the vegetation and converting to pastures or crops can result in major losses of fertility. Nutrition and productivity however cannot be considered alone. You also need to understand the importance of solar energy in driving the ecosystem, and water, which is essential for most forms of life. You also need to examine the traits that enable plants to maximize their chances of survival and reproduction. We examine these in in Topic 3.

Topic Self-review (For self learning)

To review what you have learned in the topic, you are encouraged to use the following reflection questions as a study guide to do a self-review for the topic:

• What are main stores and flows of nutrients in tropical forest ecosystems?
• What are the different processes in tropical forests that affect plant productivity, and how do they function?
• How much variation is there in nutrient availability across different soil types in the Tropics, and what are the causes of this variation?
• How important is seasonal flooding in the nutrient cycles of forests?
• How is productivity measured, and why does our knowledge of the productivity of tropical forests remain so sketchy?
• Describe the carbon cycle in tropical forests. How do carbon sinks and flows vary across different types of tropical forests?