The activities are as follows:
- Teacher Guide
- Student activity, Graph Type A, Level 3
- Student activity, Graph Type B, Level 3
- Student activity, Graph Type C, Level 3
- PowerPoint of images and diagrams
- Grading Rubric
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.
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?
Featured scientist: Bill Munger from Harvard University
Written by: Fiona Jevon
Additional teacher resource related to this Data Nugget:
- There are several publications based on the data from the Harvard Forest LTER. PDFs for all papers can be found online here. Citations below:
Wofsy, S.C., Goulden, M.L., Munger, J.W., Fan, S.M., Bakwin, P.S., Daube, B.C., Bassow, S.L. and Bazzaz, F.A., 1993. Net exchange of CO2 in a mid-latitude forest. Science, 260(5112), pp.1314-1317.
Goulden, M.L., Munger, J.W., Fan, S.M., Daube, B.C. and Wofsy, S.C., 1996. Exchange of carbon dioxide by a deciduous forest: response to interannual climate variability. Science, 271(5255), pp.1576-1578.
- Barford, C.C., Wofsy, S.C., Goulden, M.L., Munger, J.W., Pyle, E.H., Urbanski, S.P., Hutyra, L., Saleska, S.R., Fitzjarrald, D. and Moore, K., 2001. Factors controlling long-and short-term sequestration of atmospheric CO2 in a mid-latitude forest. Science, 294(5547), pp.1688-1691.
Urbanski, S., Barford, C., Wofsy, S., Kucharik, C., Pyle, E., Budney, J., McKain, K., Fitzjarrald, D., Czikowsky, M. and Munger, J.W., 2007. Factors controlling CO2 exchange on timescales from hourly to decadal at Harvard Forest. Journal of Geophysical Research: Biogeosciences, 112(G2).
Wehr, R., Munger, J.W., McManus, J.B., Nelson, D.D., Zahniser, M.S., Davidson, E.A., Wofsy, S.C. and Saleska, S.R., 2016. Seasonality of temperate forest photosynthesis and daytime respiration. Nature, 534(7609), p.680.
- Our Changing Forests Schoolyard Ecology project – Do your students want to get involved with research monitoring carbon cycles in forests? Check out this hands-on field investigation, led by a team of Ecologists at Harvard Forest. Students can contribute to this study by monitoring a 20 meter by 20 meter plot in a wooded area near their schools.
- Additional images from Harvard Forest, diagrams of NEE, and a vocabulary list can be found in this PowerPoint.