Building a Carbon Budget:
Are Bennington College Forests a Source or Sink for Carbon?
First installment for an ongoing lab project, Biodiversity, Fall 2004
I. Background
We know that atmospheric carbon (carbon dioxide) is increasing. We know that this is due in large part to burning of fossil fuels. We know that climate will change in response to this, so it is important. However, there are many loose ends – potentially important variables that we have to guess at, with varying degrees of confidence.
One of the biggest of these loose ends is the role of vegetation in the global carbon cycle. Plants remove carbon dioxide from the atmosphere through photosynthesis (primary production); in fact, this is one of the two main pathways by which carbon leaves the atmosphere (the other is absorption in the ocean). But some of that ‘photosynthetically fixed’ carbon is released again to the atmosphere through decay and burning of biomass and through the respiration of plants and the animals that eat them. If carbon dioxide is being consumed by photosynthesis faster than it is released by these processes, vegetation serves as a carbon ‘sink’, potentially compensating, in part, for our injections of carbon dioxide. However, if total organic pools of carbon (‘biomass’) are decreasing (if respiration and decay are greater than photosynthesis), then ecosystems become sources of atmospheric carbon.
(Some important terminology: Total photosynthesis – production of sugars by plants from sunlight, water, and carbon dioxide – is referred to as gross primary production or GPP. Plant respiration is the burning of some of these sugars to run the plant. The remaining photosynthate accumulates as new biomass. This excess of GPP over respiration is net primary production or NPP – effectively, growth. Production of biomass of consumers – all animals, fungi, etc. – is referred to as secondary production.)
Unfortunately, it is very difficult to determine the ‘carbon status’ of ecosystems. Carbon dynamics have been well-documented for only a very few ecosystems. At regional (let alone global) scales, we do not know whether natural ecosystems are net producers (sources) or consumers (sinks) of carbon dioxide; results and claims are conflicting.
This fall, we will initiate a study to assess the carbon status of forest stands on the Bennington College campus. To determine whether these forests are sources or sinks, we will need to assess the amounts of carbon held in different components of the ecosystem and the rates at which carbon moves into and through the ecosystem. In other words, we will construct a ‘carbon budget’. Or rather, we will begin the task; it will take a number of years and the efforts of many classes to gain a very full picture of what’s happening.
Here is a schematic diagram of a generalized (could be global)
carbon budget:
In forested ecosystems, the bulk of photosynthesis happens in canopy trees, and most biomass is accumulated in the form of wood and soil organic matter (OM). Here are examples of forest carbon budgets from a study in the Amazon and another from North Carolina (in the latter, the numbers are in g C m-2 yr-1 (grams of Carbon per square meter per year); the percentages are for changes in response to an experiment).
II. Our job: the general agenda
We will begin making the measurements required to develop a carbon budget specifically for two or three sites on campus. To do this we must decide the best, most feasible way to put numbers of the boxes and arrows on a diagram like those above. What are the most important things to measure? What are appropriate techniques for measuring them? What is a good ‘sampling design’ (i.e., how many measurements do we need, distributed in what manner, so as to give us an accurate and unbiased estimate of the variable of interest)?
We will begin with some of the important things that are easiest to measure:
A. Living tree aboveground biomass: Living trees constitute the largest single ‘reservoir’ of carbon in our forests, and it’s a good starting point to establish total tree biomass. It is not practical or desirable to remove all of the trees for drying (why would we want to dry them?) and weighing, so we need some other way to estimate their mass. How would you estimate tree biomass? It’s also not practical to measure every tree in the entire forest, so we must sample (choose a subset of the entire population of trees for measurement). How would you design such a sampling scheme?
B. “Litter” production: Some of the biomass produced each year remains in the pool of ‘living’ vegetation mass (mostly the wood in trees), but a large portion dies and is dropped as dead leaves, twigs, etc. – ‘litter’. This eventually decays and becomes either soil OM or returns to the atmosphere. The first thing to do, though, is measure the rate of litter production. Think about how to do this.
Part of your job will also be to think about how additional components of the carbon cycle for our forests might be measured/estimated. We might work on some of these this semester; others will wait for other classes in other semesters, but I will ask you to propose some research designs. Here are some examples of things that we’ll need to learn (it’s not a complete list; you might think of other things that would be important to know):
C. Changes in tree biomass: Accumulation of woody biomass is the primary way in which forests can become carbon ‘sinks’. Of course we can simiply repeat part ‘A’ above in subsequent years, but can we estimate ‘standing biomass’ for the past?
D. Large woody debris production and decay: In addition to litter, living biomass is converted to dead biomass as large branches and whole trees die and fall. This ‘large woody debris’ (LWD) then decays, its carbon entering either the atmospheric pool or the soil OM pool. How do we determine 1) the rate of production of LWD, 2) its rate of decay, and 3) what happens to its constituent carbon?
E. Soil organic matter: Soil OM is the other large pool of carbon in most terrestrial ecosystems. How would you measure it? How would you assess whether the pool of carbon in soil OM is changing? (Where does it come from? Where does it go?)
F. Understory vegetation: Shrubs and herbs don’t contribute a lot to NPP in most forest ecosystems, but they can still be significant. How can we measure biomass, and production in the understory.
G. Roots: This is one of the big challenges for terrestrial ecosystem researchers. How do you determine root biomass? How do you measure the rate at which roots grow and die and decay?
III. Specific tasks and assignments
This will be the next installment.
IV. Some links and references
There are many, many websites that bear on this project, and many books and papers in the primary literature. Here are just a couple of websites. You can find more. We’ll also be doing some relevant readings, to be assigned later.
– KDW, 1 September 2004