Tropical Rainforest Ecosystem and Biogeochemistry




Figure 1



Biogeochemistry is the study of how the geological, chemical, and biotic systems on Earth interact and influence each other.  The study of biogeochemistry is often focused on how particularly important elements such as carbon (C) or nitrogen (N) are linked and affected by geological, chemical, and biological systems. This linkage affects the cycling of all the important elements. In addition, the biogeochemical cycles of individual elements are strongly connected and interact with each other. Therefore, alterations in one cycle such as the hydrologic cycle, can forcefully impact the functioning of other element cycles. It is important that we understand Earth’s biogeochemical cycles because humans are altering their balance. Humans currently control and consume more than 40% of the primary production (C and N cycles) generated on land and in the oceans, and since the human population is increasing at an exponential rate this rate of consumption is set to increase over time. Humans have already substantially altered the Earth’s biogeochemical cycles by changing steady-state systems that have been in balance for thousands or millions of years. For example, humans have strongly altered the balance of the C cycle by burning fossil fuels and increasing the concentration of C in the atmosphere. Thus, it is important that we understand how our actions impact these cycles in order to predict the consequences of our actions, and to protect the sustainable balance of the planet for ourselves and for the other organisms that depend on these cycles. In this lab we want to explore the importance of forests to the C cycle. 


How does the global carbon cycle work?

Let’s follow an atom of carbon (C) to better understand how C cycles, or moves from one form of C to another and among the different C pools on Earth. Carbon is found in many forms on Earth; as inorganic-C such as bicarbonate and carbonate in rocks, as organic-C like that in plant and animal tissues, and as a gas, for instance carbon dioxide (CO2), methane (CH4) or carbon monoxides (CO). In addition, there are many paths that a C atom can take as it cycles on Earth; therefore, this example illustrates only one possible pathway.


Assume that an atom of C is in sedimentary rock that has been subducted near a volcano. Tectonic activity builds pressure in the volcano, and localized areas of rock melt and begin to release gases from the crater of the volcano. Our C atom is oxidized and released into the atmosphere as CO2 (Figure 2). This CO2 molecule spends time in the atmosphere floating around and absorbing long wave radiation that the Earth emits to space as part of Earth’s radiative energy balance. Because the CO2 molecule is absorbing long wave radiation and preventing the radiation from escaping into space, this warms the atmosphere. Eventually, the molecule of CO2 floats over a forest and is taken up into a tree leaf. The tree uses light energy and the molecule of CO2 in photosynthesis and makes it into sugar, or organic-C. 


Figure 2

Global C cycle


Recall that photosynthesis and respiration are arguably the most important chemical reactions on Earth because together these reactions convert inorganic-C in the oxidized form of CO2, to the reduced form of organic-C in living tissue. Respiration then converts organic-C back to inorganic-C, which is released from the breakdown of these tissues for energy. This set of biological reactions directly or indirectly provides energy for most life forms on Earth, and links together the C biogeochemical cycle.


Going back to our C atom, the new organic sugar molecule in the leaf is then used to make a fruit. A hungry insect eventually finds the new fruit, which contains our C atom. The insect uses the organic-C for energy and incorporates it into its tissues. When the insect dies it falls onto the forest soil and microbes begin decomposing the tissue. Soil microbes take up the molecule of organic-C and use it for energy. They break down the organic-C, oxidizing it in the process, which releases it back to the atmosphere as CO2. Once again the CO2 molecule floats around the atmosphere absorbing long wave radiation and warming the atmosphere. Eventually this molecule makes its way to the ocean where it is mixes with the surface water. Phytoplankton in the water, take up the molecule of CO2 and use it to again make organic-C, which it incorporates into its tissues. The phytoplankton are then eaten by zooplankton, which use the C to make a carbonate shell. Eventually the zooplankton die and sink to the bottom of the ocean, where over time the carbonate shell becomes part of the sedimentary rock to begin the C cycle again.


Most of C on Earth is buried in sedimentary rock as organic-C and carbonate. This C can be stored in the Earth for long periods of time. However, some of it can be released by rock weathering, from volcanic eruptions, or from the extraction of fossil fuels, which are then burned (or oxidized) and put into the atmosphere. During the carboniferous period in Earth’s history, large deposits of organic-C from plants were stored in the Earth’s crust, where over time they turned into coal, oil, and natural gas reserves. Today, humans are tapping into this large store of energy. However, in the process we are also altering the balance of the C cycle by significantly increasing the amount of CO2 and other greenhouse gases in the atmosphere.


In addition to the burning of fossil fuel, which is altering the composition of the atmosphere, humans are impacting the C cycle through land use change. By converting forests to agricultural land, clear-cutting and burning tropical forests, and by converting rural lands to urban areas, humans are decreasing the sequestration capacity of the Earth and increasing the amount of C in the atmosphere. Scientists have determined that in the last 650,000 years or more, the concentration of CO2 in the atmosphere has never risen above 300ppm. However, since the industrial revolution, the concentration of CO2­ in the atmosphere has been steadily increasing and today is ~388ppm. In the next 50-100 years we expect this to continue to increase up to more than 600ppm, which will have important consequences for the radiative balance of the Earth and global climate.



Before Coming To Class

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 some context to determine how important forests are to the global C cycle.


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. In all of these problems you should assume that the internal change in the atmosphere is zero.


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.



Tools you will need

Biogeochemists use several different “tools” to understand changes in biogeochemical systems. They calculate flux rates, or the movement of a material through a given area over a specific amount of time. In addition to flux measurements, we need to be able to measure the amount of material that is in the reservoir or pool that holds the material that we are interested in measuring. There are four main reservoirs or pools that we need to understand to measure Earth’s biogeochemical cycles: the atmosphere, land, ocean, and rocks. The “cycling” is simply the movement of elements between these pools – for example, primary production moves C from the atmosphere pool to the land pool, and respiration of organic matter moves the C back to the atmosphere pool. Cycles involving these four pools interact with one another and feedback within and between the reservoirs. To understand cycles, we must also be able to calculate the rate of movement between pools, and determine what factors drive or control the cycling between pools. To do this, it is useful to use the principles of mass balance.


Mass Balance

Mass balance equations are often used to describe the state of a system because they can be used to measure both the flux and the change in pool size in a given system. These mass balance equations operate in the same way regardless of the scale or size of the system of interest. In general, how much a system changes, or the net change in a system due to a perturbation, is dependent upon the amount of input plus the output, plus the internal change.


Net Change = Input + Output + Internal Change


Mass balance equations are useful because they allow one to make predictions about the impact of changes on resource flow and changes in the amounts of materials or elements we have on Earth. This mass balance equation (Net change = Inputs + Outputs + Internal Change) 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

To understand element cycles, biogeochemists also calculate residence times of materials in the pool, which is the average amount of time that the element or material spends in the pool before being removed.  If there are 100 trees in a forest, and 5 are removed each year, then the residence time is equal to the pool size divided by the flux rate (in or out, it doesn’t matter), or (100 trees) / (5 trees per year) = 20 years. If the residence time is very short that often means that the component is converted to something else quickly or that it is very reactive.


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?” 




Let’s examine a simple system using a mass balance equation. Say you have $500 in your savings account. This is considered your pool or reservoir. Your bank gives you 10% interest on the money in your account each payday (month). Therefore your internal change is 10% of $500 or $50. On payday, you deposit $100 into your account; this is your input. However, your monthly rent is due the next day, so you take out $200 to pay it; this is your output. Therefore to calculate your net change, you simply add up all the components: Net Change = Input ($100) + Output (-$200) + Internal Change ($50) = $ -100 + 50 = $-50. Therefore your net change is minus $50 (Figure 3), which means that your initial pool size of $500 will decrease and now you will only have $500 – 50 = $450 in your account. Notice that in this case there is a relatively small pool size ($500) and a relatively large net change or flux rate (-$50 per month), so the system will fall out of its initial balance rapidly. In fact, your reservoir of money will be gone in a matter of months! Large pool sizes on the other hand are often difficult to disturb; for example, if you have $500,000 in the bank, and still only had an internal change of $50 and the same rent and income, your net change would be small relative to the pool size and your reservoir would last a long time. 


Figure 3

Mass balance example




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 1

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


Question 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 3

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


Question 4

What factors are responsible for the increase in global deforestation?


Question 5

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


Question 6

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


Question 7

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


Question 8

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


Question 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

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