Forests Have Had a Good Week
What is Their Real Impact on Carbon in the Atmosphere?
A long-term ecological research site in central Massachusetts gained additional relevance this week. The Harvard Forest in Petersham, MA, has added value to the management and environmental impact of forests since its establishment in 1907. For the last 35 years, it has been part of the National Science Foundations Long-term Ecological Research Program, and a major site for research on the carbon balance of forests. The research is firmly embedded in the historical patterns of land use, including the deforestation and reforestation of the New England landscape, over the last 400+ years.
That history is captured in a series of exquisite and detailed dioramas housed in the Fisher Museum at the Harvard Forest. Sadly, the museum is closed in the current situation, but the dioramas can be viewed on line (see link under Information Sources below). It is a history worth knowing, as it pertains to the role of forest ecosystems globally in terms of carbon balances and climate change impacts.
The extra relevance of research at the Harvard Forest derives from the first agreement reached at the on-going COP26 climate summit in Glasgow, which was to limit and reverse deforestation. A certain reverence for the role of forests in the climate system was suggested by Boris Johnson’s description of forests as “great teeming ecosystems,” “cathedrals of nature” and “the lungs of our planet.” President Ali Bongo of Gabon offered one of the more ecologically aware statements by extending the analogy for the Congo basin to serving as “the heart and lungs of the African continent” yielding moisture to the atmosphere that brings rain to the Sahel and the Ethiopian highlands. After the speeches, about 100 countries, containing about 85% of the world’s forested lands, pledged to end deforestation by 2030.
All good, but reports from Glasgow included a couple of troubling countervailing ideas. The first was that previous agreements in 2004 and 2007 to end deforestation had little impact, with rates doubling since 2000 and increasing by about one-third since 2014. It was noted that there is still more financial gain in clearing forests than protecting them.
More relevant to the goals of this site and this essay, and to the research at the Harvard Forest, was a pair of conflicting comments about the actual role of forests in altering the carbon dioxide content of the atmosphere. One source said that 16% of human-driven emissions of carbon dioxide to the atmosphere comes from land use, including deforestation, but also stated that forests, through tree growth, absorb 20% of emissions from the burning of fossil fuels.
Can these both be true? What determines the carbon balance of forests, and the impact of global forests on carbon dioxide? And the question we always ask in these essays – How do we know?
Forests are perhaps the most complex biological systems on Earth. A previous essay touched on the wide array of strategies trees use just in constructing, displaying and losing foliage. Leaves are central to the carbon balance of forests, as the site of photosynthesis, but twigs branches, stems, woody roots and fine roots all play important and tightly integrated roles.
In terms of carbon flows, leaves gain and lose carbon continuously, so measuring net photosynthesis requires a very short attention span. When excess carbon is exported from leaves to the rest of a tree, it can reside in twigs that might last 4-5 years, branches that can be held for decades, and trunks, boles or stems that can be active for centuries. Root systems have the same wide array of residence times.
And then there is soil, where dead tree materials fall, and where carbon can reside for a season, a decade or a century. Some soils contain humus up to 1000 years old.
Just to add to the problem, trees and soils are not fluid like water or air, so unlike the oceans or the atmosphere, which are relatively well-mixed, the major components of forest ecosystems are immobile. Understanding forests in detail can mean measuring individual trees and small plots of soil.
Given all this, how can we determine whether a forest stand of any size is gaining or losing carbon, and then how the balances of the individual forests of the world contribute to an annual global number?
This complex topic will require two essays! This first one will deal with the carbon balance of individual stands, how that can be measured, and some examples of gains and losses. The next will take the methods used in those stand up to the global level and compare different approaches to measuring the carbon balance of all the forests of the world.
So for this essay, we return to the Harvard Forest, to long-term and detailed ecological research, and then to the introduction of a relatively new technology that allows direct measurement of carbon balances. Full disclosure, I was involved in the first proposal for long-term work at the Harvard Forest in 1988, and was active there until the early 2000s.
The traditional approach to measuring the carbon balance of a forest is to inventory all of the components of the soils and vegetation and sum them up to a total. This is detailed work requiring hours of precise and often repetitive measurement of both pools (organic stuff in soils and trees) and fluxes or flows (carbon fixation by leaves through photosynthesis, carbon dioxide release through respiration and decomposition). Applying this approach to a number of experimental conditions, like warming of soils, application of nitrogen fertilizer, or simulated hurricanes, allows understanding of the processes that control those whole-stand carbon balances.
It is not possible in this format to even outline the extensive and intensive effort required to construct a carbon budget in this manner, but a recent scientific article captures the scope and nature of the work. Adrien Finzi is the first of 25 authors on a paper which runs to 37 pages and represents an amazing compilation of research at the Harvard Forest across all of the disciplines that relate to forests and carbon. The abstract describes this paper as combining the results from “hundreds of thousands” of individual measurements!
This is a characteristic of large-scale environmental research, or Earth System Science that I’ve described before, where an immense amount of effort on dozens of collaborative research projects by large, integrated teams of scientists and technicians are often needed to get to a single or a few crucial numbers. The Harvard Forest report exemplifies this, as those hundred of thousands of observations are boiled down to a few net carbon balance numbers for individual stands and the research site as a whole, while also capturing the breadth and depth of understanding of forest carbon dynamics that underlie those summary numbers.
That wealth of information and understanding is also crucial to the validation and extension of the newer technology that allows the measurement of net carbon balances over larger areas and at sites across the globe.
To approach this newer technology, let’s look at the question of forest carbon balances at a different scale. If the goal is to measure just the overall carbon balance, without knowing the details, you could, for example, put a big plastic bag around an entire stand of trees, draw air through the bag and measure the change in carbon dioxide in the air as it passes through the bag. This approach is often used for individual leaves or small soil plots, but an entire forest? Crazy idea, yes?
And of course it has been tried – but maybe just once. In the 1950s the U.S. Atomic Energy Commission funded work by Howard Odum and others to construct that giant bag, in a tropical forest no less. It worked to some extent. Changes in the carbon dioxide content of the air drawn through the bag provided an estimate of the real-time carbon balance of the forest, but you can visualize all the unrealistic conditions of temperature, humidity and wind speed inside that big bag.
Still, it makes a nice visual demonstration of the concept!
So is there a way to measure that net carbon balance without the artificial construct of the big bag? There is. The method is called Eddy Covariance, and this again returns us to the Harvard Forest.
Steve Wofsy and colleagues established the first eddy covariance site at the Harvard Forest in the late 1980s. It is now the longest running forest site in the world.
What is eddy covariance and how does it work? It is a great example of a simple concept that is very challenging to implement!
Instead of a bag, visualize an imaginary boundary above the forest, say 10-30 feet above the tallest tree. All you have to do is measure how much air passes across this boundary in either direction, and how much carbon dioxide is in that air. Multiply the two together and you have your net carbon balance.
The complication arises from those curved arrows in this figure that depict the eddies in the movement of the air. Forests project a rough surface to the atmosphere so that even a slight breeze causes the swirling pattern of air movement seen in the image. These eddies can change direction, relative to the position of our red line boundary, many times in a second! So, actually what you have to do is measure the rate of air movement several times a second AND measure the concentration of carbon dioxide in that air at PRECISELY the same time.
The concept of eddy covariance was developed in the 1950s, but it was not until the 1980s that the technologies for making these two measurements simultaneously was sufficiently developed. Three-dimensional sonic anemometer is the name of the instrument for measuring rates of air flow, and infrared gas analyzer is the term for the instrument measuring carbon dioxide. Put these two on a tower reaching above the canopy, and you have your measurements. The challenge is making those measurements simultaneously 6 to 10 times per second!
So what is the carbon balance at the Harvard Forest? A comparison of results from three different tower locations addresses the apparent contradiction cited at the opening of this essay – how parts of a forested landscape can both gain and lose carbon simultaneously.
The first tower established at the Harvard Forest is called the EMS tower in this diagram. NEP stands for net ecosystem production or net carbon balance. This tower has measured a positive net carbon balance over an intact mixed-species forest in nearly every year since the first measurements in 1992 (note the annual pattern of carbon gain during the summer period of active growth, and loss during the winter, when photosynthesis is negligible).
This forest continues to accumulate carbon, so is an example of a forest “absorbing emissions of carbon dioxide.” Because of all the detailed measurements made inside those stands at the Harvard Forest, there are explanations for the changes in net balance from one year to the next, and validation of the scale of carbon gained as estimated from the tower data.
The overriding pattern of increased storage over time results largely from the history captured in those dioramas shown above. Forests recovering from disturbance can continue to gain carbon, primarily in the wood of the dominant trees, for a very long period of time.
Indefinitely? No. At some point, disturbance of some kind will reverse the long-term accumulation of carbon. At the Harvard Forest, researchers had the foresight and the opportunity to place a second tower in a location dominated by hemlock trees, and another one in a planned clearcut.
The hemlock stand also gained carbon for many years, about as rapidly as the mixed forest around the EMS tower, but notice the downturn in the most recent years. Hemlock is subject to an invasive pest called hemlock wooly adelgid. This new arrival began to impact the carbon balance of the hemlock stand around 2013 through decreased growth and increased mortality. As trees decline and die, respiration and decomposition overbalance photosynthesis, and the carbon balance turns negative.
The clearcut tower adds another page to the story. After cutting, regrowth is limited in the first couple of years before a full canopy is re-established, while warmer soils and the decay of “slash” or downed material left behind by the loggers increase carbon loss. This “stand” then shows a net carbon loss for 3-4 years, before photosynthesis and regrowth gain the upper hand and the line turns upward. This loss is in addition to whatever happens to the wood removed from the site.
This one diagram, and this set of studies using this standardized approach, captures the dichotomy of carbon gains and losses by forests. Regrowing stands can accumulate carbon for decades and appear to offset human emissions. Disturbance, either by introduced pests or cutting, as in the diagram, or climate events like blowdowns during major storms, can reverse the gains.
Deforestation, by removing the trees and changing the use of the land, for example conversion to agriculture, makes the losses of carbon permanent. Reforestation can initialize additional increases in carbon storage if the new stands remain undisturbed. Fires can cause catastrophic losses of carbon, redressing decades of accumulation in an instant, and are an increasing part of the forest carbon story.
We will put all of these landscape dynamics together in the next essay, as we work up to an estimate of forest carbon balances at the global scale, but thanks to decades of careful, detailed research at the Harvard Forest, and other sites, we have a firm foundation for measuring and understanding the carbon dynamics of forests and making those global projections.
To wrap up this first essay, here are a few major points about carbon balances at the site or stand level:
- Forests can help in the effort to reduce carbon dioxide in the atmosphere. If they are allowed to grow in the absence of disturbance they can be a carbon sink for decades to centuries.
- Reforestation can also reduce carbon in the atmosphere if, again, the new forests are allowed to grow without disturbance.
- In the absence of human disturbance, natural disturbances such as wind damage and natural fires or tree mortality will eventually limit carbon accumulation in a forest.
- Harvesting converts the land into a carbon source, which can be increased depending on the fate of the wood removed from the site.
- Introduced pests and diseases can also turn a forest into a net carbon source.
- Deforestation turns formerly forested landscapes into long-term sources of carbon to the atmosphere.
Information Sources Used for This Essay
Current sites in the National Science Foundation Long-Term Ecological Research Program are described here:
Sources for the Harvard Forest, the Fisher Museum and the dioramas are here:
https://harvardforest.fas.harvard.edu/
https://harvardforest.fas.harvard.edu/fisher-museum
https://harvardforest.photoshelter.com/galleries/C0000I60YVnjGj0w/G0000OR2a2UhZafY/Diorama
A book summarizing long-term historical and ecological research at the Harvard Forest is:
Foster, D. and J. Aber. 2004. Forests in Time: The Environmental Consequences of 1,000 years of change in New England. Yale University Press. 496pp.
Munger, J.W., C. Barford and S. Wofsy. “Exchanges between the Forest and the Atmosphere” - Chapter 10 in this volume, describes eddy covariance and early results from the EMS tower
Four articles reporting on statements made at COP26 summit and commitments about deforestation are here:
https://www.nytimes.com/live/2021/11/02/world/cop26-glasgow-climate-summit
The full article on the carbon budget at the Harvard Forest is:
Finzi, A. et al. 2020. Carbon budget of the Harvard Forest Long-Term Ecological Research site: pattern, process, and response to global change. Ecological Monographs 90: https://doi.org/10.1002/ecm.1423
A reference for the tropical forest bag experiment is:
Odum, H.T. and C. Jordan. 1972. Carbon balance of the large cylinder. In: H.T. Odum and R.F. Pigeon (eds.). A Tropical Rain Forest. Atomic Energy Commission, Washington, DC
Sources of information on Eddy Covariance include:
Skinner, R.H and C. Wagner-Riddle. 2012. Meteorological Methods for Assessing Greenhouse Gas Flux. Chapter 21 in Managing Agricultural Greenhouse Gases. Elsevier. https://doi.org/10.1016/B978-0-12-386897-8.00021-8
Wofsy, S.C., M. L. Goulden, J. W. Munger, S.-M. Fan, P. S. Bakwin, B. C. Daube, S. L. Bassow, and F. A. Bazzaz. 1993. Net Exchange of CO2 in a Mid-Latitude Forest. Science 260: 1314-1317 https://doi.org/10.1126/science.260.5112.1314
Wikipedia: Eddy Covariance https://en.wikipedia.org/wiki/Eddy_covariance
The image of the eddy covariance tower is from:
Torn, M.S. and C. Buechner. Ameriflux Management Project. https://eesa.lbl.gov/projects/ameriflux-management-project/