Topic Introduction

In this topic, we look at two subjects that are only marginally related: vegetative reproduction and the growth and survival of seedlings. Vegetative reproduction represents an important survival strategy for a plant, and in a number of species, it is the most important form of reproduction. It is particularly important in areas experiencing frequent disturbances, such as tropical forests in areas affected by tropical cyclones. Examples of these are given in Module 5 (the forests of the Caribbean are a particularly good example). There are various mechanisms involved, including sprouting from stem buds and underground roots, adventitious shoots, stem suckers and replacement shoots. Some species can grow adventitious roots from twigs, small branches and even leaves, a useful adaptation in areas with cyclone activity.

When combined with the losses to predation, the chances of a flower leading all the way to a mature individual is very small. The highest losses occur at the flowering and seed stages, but very few seedlings survive to adulthood, implying significant losses at this stage in the life cycle of trees. Many elements of seedling survival in natural situations remain poorly understood, such as the role of mycorrhizae. In the Dipterocarpaceae and Fagaceae, ectomycorrhizal associations may enable seedlings to tap into existing networks, potentially giving them access to nutrients and photosynthates. Other families are linked to arbuscular mycorrhizal associations, and do not have this early advantage.

Seedling mortality can be very high in the first one to two years. If shade-intolerant seedlings germinate in shade, mortality can be 100%. Seedlings germinating in their optimal environmental conditions may still have mortality rates exceeding 20%. There are many different causes of mortality, as described in the second presentation in this topic.

An important distinction, which is examined in detail in this topic, is the different strategies adopted by pioneer and shade-tolerant species. This is a convenient but misleading classification, as many species fall somewhere in between these two groups, and a continuum exists with many species showing mixed traits. These are also examined in this topic.

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

• Describe the different forms of vegetative reproduction.
• Using Piper as an example, compare the different strategies of vegetative reproduction.
• Describe the different life phases of forest trees.
• Outline the principal factors affecting the growth of trees.
• Compare the factors leading to the mortality of trees at different stages in their development.
• Contrast the life strategies of shade-tolerant and pioneer tree species.

Topic Readings

Required Readings

• Ghazoul, J. and Sheil, D. 2010. Tropical Rain Forest Ecology, Diversity, and Conservation. Oxford, Oxford University Press, 516 pp. Sections 10.4, 10.5 and 10.6.

Optional Readings

There is no supplementary materials for the topic.

Fundamental Concepts

PPT Presentations

This topic contains two presentations:

3.3.1 Plant reproduction - Vegetative regeneration.pptx

3.3.2 Plant reproduction - Growth and survival.pptx Download 3.3.2 Plant reproduction - Growth and survival.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 3.3 to start taking the quiz.

Pre-readings and Discussion (For certificate learning only)

Once you have read Sections 10.4, 10.5 and 10.6 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 3 discussion board Module 3 Discussion. 

Graded Assignments (For certificate learning only)

Six Sentence Answer #7

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. Your company is working on a contract 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 been working on the design of forest corridors that will connect different patches of currently isolated forest, and have established nurseries to grow the seedlings that you will use in the plantings. Using the 6SA format, argue why it would be important to encourage wildlife to introduce seeds from the forest patches into the corridors.

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, we have looked to aspects of the life cycles of trees. The first is vegetative reproduction, which is an important process in areas experiencing cyclones and other disturbances. Many trees can regenerate from suckers, sprouts or from leaves, twigs and small branches lost from the parent tree. This enables rapid post-disturbance recovery.

We also looked at the survival and growth of seedlings, saplings and trees. Very few germinants grow to reach maturity, with a wide range of factors causing mortality at all stages of growth. Some of these are density-dependent, whereas others are independent of density. In the topic, we have examined these factors, and related them to the survival rates of trees in tropical forests.

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 different forms of vegetative reproduction, and how do they work?
• Using Piper as an example, can you compare the different strategies of vegetative reproduction?
• What are the different life phases of forest trees?
• What are the principal factors affecting the growth of trees and how do these change over the lifetime of a tree?
• What leads to the mortality of trees at different stages in their development?
• How do the life strategies of shade-tolerant and pioneer tree species differ?