Unit 10c. Tropical Rainforest Ecosystem and Biogeochemistry



In this lab we want to explore the importance of forests to the carbon cycle. Before coming to class please read Sustaining the World's Forests in State of the World 1998. If you prefer, you can skip the section, "The Impact of National Policies." Also read a short summary entitled Forest Loss Unchecked p. 104-105 from Vital Signs 2002, from the WorldWatch Institute. The articles will give us an overview of the state of the Earth's forests and give us a context to determine how important forests are to the global carbon cycle.


Figure 10.c.1



In Class Activity

In class, number off by two’s to create two groups. Once that class has be broken into two large groups, break up into small groups of 3 and examine the following statements:


  1. Tropical forests produce large quantities of oxygen through photosynthesis. If we clear-cut all of the world’s tropical forests, we would lose that oxygen production mechanism and oxygen consuming organisms, like ourselves, would be in danger.


  1. If we burn all the world’s tropical forests, we not only lose the oxygen that they produce each year, but we would consume huge amounts of oxygen from the atmosphere in the burning process, and oxygen consuming organisms would be in jeopardy of extinction.


Group 1 should explore the first statement and group 2 should examine the second statement using knowledge of ecosystems and biogeochemistry. The goal is to develop some conclusions about the validity of these statements.


After sufficient time has passed to calculate the answers, groups 1 and 2 will explain their conclusions to the other group and discuss questions related to the readings.


You may want to use the following concepts and relationships to help you:


Mass Balance


Net change = Inputs + Outputs + Internal Change


This mass balance equation can be used to ask several questions. For example, if you were interested in the effect of photosynthesis on the net change of oxygen in the atmosphere, then the equation would be written as follows:


Net change = Inputs (the input of O2 to the atmosphere from photosynthesis) + Outputs (in this case none) + Internal change (in this case none)


If you were interested in the effects of burning on the net change of oxygen in the atmosphere, then you would include the amount of burning into the equation as an output, because burning consumes and thus removes oxygen from the atmosphere. Again, in all of these problems you should assume that the internal change in the atmosphere is zero.


Residence Time


RT = (total amount in the pool) / (input or output rate)


Note that to calculate the residence time the system must be at "steady state". Steady state is a stable condition that does not change over time or in which change in one direction is continually balanced by change in another. In addition, either the input or output of materials can be used in the denominator to calculate residence time. For our current atmosphere, consider that we are at steady state with respect to oxygen. Remember to interpret the residence time in light of the question “what would it take to disturb or change this system?” 


Photosynthesis and Respiration


CO2 + H2O <--> CH2O + O2


The process that causes the forward reaction (left to right), which produces oxygen, is photosynthesis. The processes that cause the reverse reaction, which consumes oxygen, are respiration and biomass burning.


All chemical reactions can be written in terms of the "number of atoms" that participate in a reaction. The term often used by chemists is a "number of moles" of one compound react with a "number of moles" of another compound. A "mole" is equal to 6.022 x 1023 atoms. In the above reaction we see that in burning or respiration, one mole of O2 (or two moles of O) reacts with a CH2O compound to produce one mole of CO2 (or one mole of C).


Because the assigned problem deals with the "weight" of oxygen and carbon in grams, we need to convert from moles to grams. This is done using the atomic weight of the substance, where a mole of C weighs 12 grams, and a mole of O weighs 16 grams (12 and 16 are the atomic weights of C and O, respectively – thus O2 would weigh 32 grams per mole). Because C and O do not weigh the same amount, and you must find out given the above reaction how much O is used in burning a certain amount of C, you must be able to convert between the two using the following relationships:


(12 g/mol of C) / (32 g/mol of O2) = 0.375 g C / g O2 (read “g of Carbon per g of Oxygen”)

- or -

(32 g/mol of O2) / (12 g/mol of C) = 2.667 g O2 / g C


Numbers you will need to make the calculations to answer the question:



Be sure to check that the "units" in your equations cancel properly, or your answer will almost certainly be wrong.



After discussing your conclusions about tropical deforestation and the global carbon balance, discuss the following questions as a class or in small groups.


Question 10.c.1

What types of ecosystem services do forests provide? What types of pressures do forest ecosystems face?


Question 10.c.2

What is the difference between an old growth, secondary growth, and plantation forest? Create a matrix that explores: a. similarities, b. differences and c. quantities of these three forest types.


Question 10.c.3

What are some of the costs and benefits associated with plantation forests?


Question 10.c.4

What factors are responsible for the increase in global deforestation?


Question 10.c.5

How are the causes and consequences of deforestation different between developing and developed countries?


Question 10.c.6

What affects do roads and road building have on forest ecosystems?


Question 10.c.7

What types of ecological impacts result from large-scale deforestation? Is there a more sustainable way to manage forests?


Question 10.c.8

What role do governments play in managing forest resources? Should governments or communities be responsible for managing forest resources?


Question 10.c.9

Explore table 2-1. Principles and Criteria for Forest Stewardship. Do you think that these principles are realistic? Can they be implemented?


Question 10.c.10

What policies and practices should be implemented to sustainably manage our forest ecosystems?





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