Are Forests Helping in the Fight Against Climate Change?

This assignment explores the role of forests in carbon storage and their impact on climate change, focusing on the research conducted by Bill Munger at Harvard Forest.

Group 1

Research Background: In the 1990s, scientists began to wonder what role forests were having in this exchange of carbon in and out of the atmosphere. Were forests overall storing carbon (carbon sink), or releasing it (carbon source)? Bill is one of the scientists who decided to explore this question. Bill works at the Harvard Forest in central Massachusetts, a Long-Term Ecological Research site that specializes in setting up big experiments to learn how the environment works. Bill and his team of scientists realized they could measure the CO2 coming into and out of an entire forest. They built large metal towers that stand taller than the forest trees around them and use sensors to measure the speed, direction, and CO2 concentration of each puff of air that passes by. Bill compares the CO2 in the air coming from the forest to the ones moving down into the forest from the atmosphere. With the CO2 data from both directions, Bill calculates the Net Ecosystem Exchange (or NEE for short). When more carbon is moving into the forest than out, NEE is a negative number because CO2 is being taken out of the air. This often happens during the summer when trees are getting a lot of light and are therefore photosynthesizing. When more CO2 is leaving the forest, it means that decomposition and respiration are greater than photosynthesis and the NEE is a positive number. This typically happens at night and in the winter, when trees aren’t photosynthesizing but respiration and decomposition still occur. By adding up the NEE of each hour over a whole year, Bill finds the total amount of CO2 the forest is adding or removing from the atmosphere that year.

Bill and his team were very interested in understanding NEE because of how important it is to the global carbon cycle, and therefore to climate change. They wanted to know which factors might cause the NEE of a forest to vary. Bill and other scientists collected data on carbon entering and leaving Harvard Forest for many years to see if they could find any patterns in NEE over time. By looking at how the NEE changes over time, predictions can be made about the future: are forests taking up more CO2 than they release? Will they continue to do so under future climate change?

Source 1.1

As humans drive cars and use electricity, we release carbon in the form of carbon dioxide (CO2) into the air. Because CO2 helps to trap heat near the surface of the earth, it is known as a greenhouse gas and contributes to climate change. However, carbon is also an important piece of natural ecosystems, because all living organisms contain carbon. For example, when plants photosynthesize, they take CO2 from the air and turn it into other forms of carbon: sugars for food and structural compounds to build their stems, roots, and leaves. When the carbon in a living tree’s trunk, roots, leaves, and branches stays there for a long time, the carbon is kept out of the air. This carbon storage helps reduce the amount of CO2 in the atmosphere. However, not all of the CO2 that trees take from the air during photosynthesis remains as part of the tree. Some of that carbon returns to the air during a process called respiration.

Another important part of the forest carbon cycle happens when trees drop their leaves and branches or die. The carbon that the tree has stored breaks down in a process called decomposition. Some of the stored carbon returns to the air as CO2, but the rest of the carbon in those dead leaves and branches builds up on the forest floor, slowly becoming soil. Once carbon is stored in soil, it stays there for a long time. We can think of forests as a balancing act between carbon building up in trees and soil, and carbon released to the air by decomposition and respiration. When a forest is building up more carbon than it is releasing, we call that area a carbon sink, because overall more CO2 is “sinking” into the forest and staying there. On the other hand, when more carbon is being released by the forest through decomposition and respiration, that area is a carbon source, because the forest is adding more carbon back into the atmosphere than it is taking in through photosynthesis.

Source 1.2

Question 1a

Short answer

First, is the forest a carbon source or sink when more carbon is given off by respiration and decomposition than is taken in by photosynthesis? Second, when is the forest a carbon sink?

When is the NEE positive and when is it negative?

Source 1a.1

Group 2

Scientific Data: Use the data below to answer the scientific question.

Source 2.1

Question 2a

Short answer

What data will you graph to answer the question?

Independent variable:

Dependent variable:

Question 2b

Short answer

Below is a graph of the data: Identify any changes, trends, or differences you see in your graph. Write the story of the graph.

Source 2b.1

Group 3

Interpret the data: Make a claim that answers the scientific question.

Question 3a

Short answer

Make a claim that answers the scientific question: Is the Harvard Forest a carbon source or a carbon sink, and how has the net ecosystem exchange (NEE) changed over time?

Question 3b

Short answer

What data or evidence gathered from text or graphics lead you to your claim?

Question 3c

Essay

Explain your reasoning and why the evidence supports your claim. Connect the data back to what you learned about how the processes of photosynthesis, respiration, and decomposition influence the carbon cycle in forests. Make sure you also explain related scientific principles from the cytology unit: photosynthesis, cell theory, cellular respiration, cell organelles, the 7 characteristics of life, and macromolecules like enzymes.

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