Are forests helping the fight against climate change?
*This is a modified version of the original Data Nugget that has been designed to be used on the DataClassroom web-app. The original pencil and paper activity can be found here on the Data Nuggets website
Featured scientist: Bill Munger from Harvard University
Written by: Fiona Jevon
Background
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.
Bill setting up a large metal tower in Harvard Forest in 1989, used to measure long term carbon-dioxide exchange.
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)? 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?
Scientific Question: Is the Harvard Forest a carbon source or a carbon sink, and how has the net ecosystem exchange (NEE) changed over time?
Scientific Data:
1. Look at your scientific question, and decide which variables might be most important to answer the scientific question. Which did you choose?
Independent variable:
Dependent variable:
2. Make a graph based on your two variables. Add a line of best fit with the Regression line check box. Add it as a screenshot below:
3. Identify any changes, trends, or differences you see in your graph. Include your graph and specifically refer to it when describing those changes, trends, or differences.
Interpret the Data:
4. Make a claim that answers the scientific question.
5. What evidence was used to write your claim? Reference specific parts of the tables or graph.
6. 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.
Your next steps as a scientist:
7. Science is an ongoing process. What new question do you think should be investigated?
8. What future data should be collected to answer your question?
Independent variable(s):
Dependent variable(s):
9. For each variable in the previous question, explain why you included it and how it could be measured.
10. What hypothesis are you testing in your experiment? A hypothesis is a proposed explanation for an observation, which can then be tested with experimentation or other types of studies.
Digital Extension
These questions are a digital extension of the original Data Nuggets activity. The data manipulation and graphing tasks within are best completed here on DataClassroom.
11. What conclusion can you draw about Harvard Forest and its Net Ecosystem Exchange? Make a graph with Net Ecosystem Exchange on the Y-axis and year on the X-axis. Add a line of best fit with the Regression line check box. Refer to your graph as evidence in your answer. You can skip this question if you already made this graph in the Data Nugget activity above.
12. Graphs don’t always have to tell us decisive stories or show clear trends. Sometimes, they help reveal anomalies or outliers. Of all the years shown here as data points, which year do you see as an outlier that you would want to examine more closely? Why?
13. Graphs can often tell different stories depending on how we present the data. Adjust the y-axis by changing the Y-axis range from “Auto” to “Fixed” with a range of -1000 to 1000. Would your claim from #1 change based on this new visualization? Why or why not?
Statistics Extension
14. Make a graph with just NEE on the Y-axis and no other variable on the plot. Add descriptive statistics to your graph to show the mean and 95% confidence interval around the mean.
15. How confident are you that the Harvard Forest is consistently functioning as a carbon sink? Refer to your graph in #4 as evidence for your response here.
16. Does there appear to be a statistically significant trend in the data? Run a graph-driven hypothesis test to see if there is a significant relationship between the variable year and the variable NEE. Another way of thinking about this is that you are testing whether or not the slope of the line of best fit is significantly different from 0 (horizontal; indicating no relationship).
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