Topic Introduction

In this topic, we examine some more of the ecological processes that occur in tropical forests. The mineral nutrition of plants is an important determinant of productivity, but two other fundamental resources are equally important, if not more so. These are solar energy and water. Both are fundamental to plant life.

Solar energy is an essential driver of the system. Without solar energy, there would be very little life (there are a few organisms that are completely independent of solar energy, deriving their energy form, for example, submarine volcanic vents). Plants are able to convert solar energy into chemical energy, specifically carbohydrates. To do this, there must be adequate light, and there must also be water and carbon dioxide. Photosynthesis is examined in the first presentation.

Carbon dioxide is not a limiting factor for plants: it is ubiquitous, and atmospheric concentrations are rising due to human activities. In contrast, the availability of water can be highly limiting, and determines the type of forest that will be present at a site, or even if there will be any vegetation at all. Water availability can be a challenge even in moist climates, and the case study looking at Danum Valley (Case 5.2) looks at this in detail.

Photosynthesis in plants occurs in organelles known as chloroplast. These are concentrated in the leaves, although in a variety of species, it may also occur on branches and on the main stem (mainly indicated by the presence of a green surface to the stems and branches). In temperate forests, foliage is dropped in autumn as leaves tend to be susceptible to freezing injury and the low light levels of winter make it inefficient to retain leaves. In the moist Tropics, seasonal variations in light and temperature are less of an issue, and many non-annual patterns of leaf fall occur, with many plants being evergreen, or at least semi-evergreen. In the seasonally dry Tropics, moisture rather than temperature is the critical factor, and many plants are dry season deciduous. Leaves are subject to damage from herbivores and other agents, providing another reason for replacement. The third presentation in this topic looks at these issues.

Plants have developed a number of traits that enable them to utilize resources and function more efficiently, thereby enhancing their competitive ability. Functional traits in plants are examined in the final presentation in this topic.

Outcomes

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

• Discriminate between C3, C4 and CAM photosynthetic processes.
• Compare how different forms of leaf placement affect energy capture.
• Analyze the importance of sunflecks for understorey plants.
• Describe how water availability affects the growth of plants through their life cycle.
• Contrast the responses of plants in tropical moist and tropical seasonally dry forests to water stress.
• Combine water stress with other factors to assess how forest plants may respond to changing water availability.
• Describe patterns of seasonality in tropical forests.
• Compare the phenology of plants in tropical moist and tropical seasonally dry forests.
• Outline the hydraulic properties of roots, stems and shoots and explain how these differ among different plant groups.
• Describe how plant functional traits affect the composition of tropical forests.
• Analyze the tradeoffs between different functional traits and how these contribute to species diversity.

Topic Readings

Required Readings

• Ghazoul, J. and Sheil, D. 2010. Tropical Rain Forest Ecology, Diversity, and Conservation. Oxford, Oxford University Press, 516 pp. Chapter 10.

Optional Readings

• Crawley, M.J. (ed.) 1997. Plant Ecology. 2^nd Oxford, UK: Blackwell Scientific. 717 pp. Chapters 1 and 2.
• Giraldo, J.P., Holbrook, N.M. 2011. Physiological mechanisms underlying the seasonality of leaf senescence and renewal in seasonally dry tropical forest trees. In: Dirzo, R., Young, H.S., Mooney, H.A., Ceballos, G. (eds.) 2011. Seasonally Dry Tropical Forests: Ecology and Conservation. Washington, DC, USA: Island Press. pp. 129- 140. (Especially for 3.3.4)
• Lüttge, U. 2008. Physiological Ecology of Tropical Plants. 2^nd Berlin, Germany: Springer. 458 pp. Chapters 4 and 5.

Fundamentail Concepts

PPT Presentations

This topic contains four presentations:

2.2.1 Plant form and function - Energy capture.pptx Download 2.2.1 Plant form and function - Energy capture.pptx

2.2.2 Plant form and function - Water.pptx Download 2.2.2 Plant form and function - Water.pptx

2.2.3 Plant form and function - Leaf senescence and renewal.pptx Download 2.2.3 Plant form and function - Leaf senescence and renewal.pptx

2.2.4 Plant form and function - Functional traits.pptx Download 2.2.4 Plant form and function - Functional traits.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 2.2 to start taking the quiz.

Pre-readings and Discussion (For certificate learning only)

Once you have read Chapter 10 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 #3

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.

This job that you are working on involves assessing where future reserves should be located. You have already identified a number of rain forest areas that have very high species richness. When you presented these to the company management team, one member suggested that these areas are obviously very high productivity and should be converted to agriculture to support the local population. Using the 6SA format, take what you now know about nutrition and productivity to build an argument for maintaining these areas as forest.

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 some of the important ecological processes that govern plants in tropical forests. Plants require sunlight, water and nutrition, and if any of these are limiting, then productivity will be inhibited. Remember that this can work both ways: it is frequently a shortage that is a problem, but it is also possible to have too much. For example, many plants of the understorey cannot survive in full tropical sunlight. Water can also be too abundant, resulting in waterlogged, anaerobic soils that severely limit the species that can survive in such conditions.

Over time, plants have evolved a number of traits that have enabled them to adapt to particular conditions. These often involve trade-offs between different properties: fast growth, for example, is generally associated with lighter, wood with less strength that the denser wood associated with slow-growing species. This makes the trees more susceptible to mechanical damage. As we will see in future modules, many traits have arisen because of the relationships between plants and animals – either to encourage (e.g., pollination and seed dispersal) or discourage (e.g., herbivory) interactions.

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 the differences between the C3, C4 and CAM photosynthetic processes?
• How do different forms of leaf placement affect energy capture?
• What is the importance of sunflecks for understorey plants?
• How does water availability affect the growth of plants through their life cycle?
• What are the differences in the responses of plants in tropical moist and tropical seasonally dry forests to water stress?
• How does water stress combine with other factors to affect the response of forest plants changing water availability?
• What are the different patterns of seasonality in tropical forests?
• What are the differences in the phenology of plants in tropical moist and tropical seasonally dry forests?
• How do the hydraulic properties of roots, stems and shoots vary among different plant groups?
• How do plant functional traits affect the composition of tropical forests?
• What are some of the tradeoffs between different functional traits, and how do these contribute to species diversity?