Question

Background: Ancient lycophytes were the dominant plants on Earth during the Carboniferous Period, which is named for the coal that formed from those ancient plants that lived 300 million years ago. As humans use fossil fuels such as coal and gas, we release carbon in the form of carbon dioxide (CO₂) into the air. Because CO₂ traps 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. 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 cellular 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.

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)? Dr. Bill Munger is one of the scientists who decided to explore this question. Dr. Munger 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. Dr. Munger 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. Dr. Munger 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, Dr. Munger 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, Dr. Munger finds the total amount of CO₂ the forest is adding or removing from the atmosphere that year.

Dr. Munger and his team were 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. Dr. Munger 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?

Scientific Question: Is the Harvard Forest a carbon source or a carbon sink, and how has the net ecosystem exchange (NEE) changed over time?

Use the data below to answer the scientific question:

Year	NEE (grams carbon/ meters2/year)
1992	-164
1993	-179
1994	-173
1995	-282
1996	-194
1997	-163
1998	-157
1999	-213
2000	-261
2001	-426
2002	-270
2003	-212
2004	-458
2005	-543
2006	-458
2007	-537
2008	-612
2009	-358
2010	-36
2011	-150
2012	-339
2013	-218
2014	-459
2015	-194

Student Questions

1. Forests as carbon sinks or sources: After reading the background information, fill in the table below. (1 pt)

Hypothesized forest processes	Forest is a carbon source or sink?	NEE is positive or negative?
More carbon given off by respiration and decomposition than is taken in by photosynthesis.
More carbon taken in by photosynthesis than is given off by respiration and decomposition.

2. Make a graph of the data over time. Add a trendline to help identify any trends in the data. (1 pt). Write one sentence describing what you see. (1 pt)

3. Make a claim that answers the scientific question. What evidence was used to write your claim? Reference specific parts of the table or graph. (2 pts)

4. 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. (2 pts)

Read the Wehr et al. 2016 Nature paper that is posted to Blackboard.

5. What assumptions were Wehr and colleagues testing in regard to net ecosystem exchange? (2 pts)

6. What did Wehr and colleagues measure to test the assumptions in #5? (2 pts)

7. What were the major findings of Wehr et al. 2016 with regard to NEE? (2 pts)

8. What are the implications of this paper? (2 p

Answer 1

1. Forest is a carbon sink since the NEE values are negative which indicates that more carbon is taken in by photosynthesis than is given off by respiration and decomposition.

2.

NNE (grams carbon/ m2 / year) 07 1992 -100 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 NNE (grams carbon/ m2 / year)

There is an overall decrease in NEE over a long term with certain exceptions where it becomes less negative such as in 2003 or 2010. This reflects that forest acts as a carbon sink

3. Claim : Forests are excellent carbon sequesteration chambers.

The part of the table from 1992 to 2008 can be referred to note that as the forest ages and the trees grow old they become efficient sequesteration chambers of carbon.

4. For the above claim, it is assumed that no new plantation has occurred and the forest is growing naturally. In addition, no envirnomental disasters which lead to the loss of plant life in forest have happened.

The claim that forests act as excellent carbon sequesteration chambers is supported by the consistently increasing negative values of NEE over the years. This reflects that the forest is growing, the plants are maturing, new plants are produced . The resulting effect of population growth of plants is that more photosynthesis takes place and the ratio of photosynthesis to decomposition rate increases over the years. Although there is adequate amount of decomposition of dead organic matter such as dead leaves, stems,etc. and respiration, this is overridden by the high photosynthesis of the forest as a whole