Integrate > Workshops > Systems, Society, Sustainability and the Geosciences > Activity Collection > Determining Carbon Storage in Garcelon Bog

Determining Carbon Storage in Garcelon Bog

Holly A. Ewing, Bates College
Bates College, Environmental Studies
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Summary

This is a three-week lab investigating the mass of carbon stored in trees, shrubs, and peat in a local bog. The collaborative lab includes field collections of material from trees, shrubs, and peat and then analysis of organic content and calculation of the mass of the carbon pool. Laboratory work includes getting dried weights and loss-on-ignition for organic content. The groups of students who collect material in the field undertake calculations in Excel, examination of scientific literature on carbon content and root-to-shoot ratios, and area delineation in ArcGIS. Initial groups focused on one pool (i.e., shrub, tree, or peat) are disassembled and then reconfigured to bring expertise from a student's original group to a synthesis group which is responsible for the final calculation of the total amount of carbon in the bog and the write-up for the lab. The lab is designed to teach skills in collaboration and group problem solving in addition to environmental science content.

Learning Goals

Learning objectives of this activity:
- Apply previously encountered concepts and calculations in concentration, mass, volume, scale, and unit conversions to a larger and more complex problem.
- Work effectively in groups to solve a large and difficult problem by first becoming an expert in a part of the problem and then bringing that expertise to a new group that undertakes a synthetic analysis.
- Effectively organize field and laboratory data in Excel spreadsheets such that moderately sophisticated (for the introductory student) calculations can be undertaken, understood, and shared with others.
- Understand some of the difficulty behind providing estimates of "error" when dealing with a large and complex problem involving many imperfect measurements and extrapolation.
- Reflect on the larger discussion of carbon sequestration in which this analysis of the carbon content of a single bog resides.

This lab sequence aims at resolving misconceptions such as
- "I don't really need to understand this calculation (or how to use this software or what we are doing or...) since my lab partner(s) does(do)."
- Scientific research is produced by brilliant individuals working alone.
- The things we can see are the things that matter the most.
- There is one perfect answer to how much carbon (or whatever) there is in a particular ecosystem pool.

Prerequisite concepts and skills students should already know or have:
- sampling, chemical composition, pools, fluxes, mass, volume, scale, unit analysis, unit conversions
- skills in tree identification and some way of getting tree biomass per unit area (I use the point-center-quarter method, but there are other methods). Note that the tree group biomass also depends on allometric equations from Jenkins et al. (2004), so some familiarity with "best fit lines" or regression is helpful.
- facility entering data in Excel and using it for basic calculations
- ability to make a polygon and calculate its area in ArcGIS (not required since this could be done just by estimation on a map or likely with the help of something like Google Maps)
- ability to work collaboratively in small groups with changing composition
- ability to find papers within the primary literature using an index such as Scopus, JSTOR, Science Direct, or Web of Science is useful but can be taught in the lab if needed

Context for Use

The lab was written for an introductory environmental science class taken by majors and non-majors in Environmental Studies (a required introductory core course for the Environmental Studies major). The course assumes no science or math background at the college level and is designed to teach skills in collaboration and group problem solving in addition to environmental science content. This three-lab sequence occurs a little past half-way through the course after students have had other labs that introduce them to the use of Excel and ArcGIS software and concepts related to sampling, chemical composition, pools, fluxes, tree identification, mass, volume, and scale.

The course has a lecture (with 40-60 people, nearly all first- and second-year students) and a lab period (15-20 students each). Labs are only three hours long; a four hour lab would be much better, and this lab would not be possible with less than three hours. The exchange of group members happens only within a lab section. I do a summary of what all groups from all lab sections found in the lecture period (see below).

While dependent upon the skills and concepts listed above, the lab could be adapted for other environmental science, biology, or geology courses that do not cover all of these topics or skills.

I teach this course in the winter in Maine. This makes finding shrubs under the snow and getting through the frost to the peat somewhat difficult in some years, but it can be done. I actually prefer teaching it in winter to fall since we do less damage to the bog (and we are less likely to lose a student to the muck) in the winter.

However, practice working in collaborative lab groups is essential. I have students change lab partners every week so that they get to know others in the class, practice working with people with different styles and abilities, and are less inclined to form cliques that exclude other students.

This is the biggest and most difficult academic problem any of them has ever faced, but it often ends up being their favorite field lab and the data lab that gives them the confidence to try big collaborative projects later in their career as undergraduates.

Description and Teaching Materials

The handouts that students receive contain the details of how the activity works. I give them considerable latitude in how they actually sample in the field to estimate the mass of carbon held in each of the shrub, tree, and peat pools. Anyone adapting this will likely want to take their own approach given the site and equipment available. For reference, though, here is an overview of what my students usually do:

SHRUBS: This group uses clippers or loppers to harvest shrubs from a known area. Then they get a field wet weight for the whole mass (using something like a Pesola spring scale and the shrubs in a plastic bag). In the lab, they get a dry weight and organic content for a subsample. They go to the literature for information about carbon content of organic matter and root-to-shoot ratios.

TREES: The tree group usually uses the point-center-quarter method (see refs) for estimating tree density and size. In the field, they also take tree cores from a couple of trees of each species (paper birch, white pine, and spruce make up most of the trees), and this is used in the lab to determine the percentage of biomass that is organic (nearly all). They then use Jenkins et al. (2004, see refs) to determine the mass associated with a tree of a given diameter. They also need to use the literature to get a value for carbon content of organic matter and root-to-shoot ratios.

PEAT: This group uses a soil auger with many extensions or a Russian peat corer to collect peat from different depths below the surface. This particular bog has more than 5 m of peat in most places, so having lots of extensions is a helpful thing. They take known volumes of peat and get wet weights, and then they dry and burn subsamples of this in the lab to get measures of water and organic content. They also need the literature to determine the carbon content of organic matter. This is always the biggest carbon pool, and this is also the group with the smallest number of sampling locations since it takes a while to dig through the snow and ice and figure out how deep the peat is in a particular place.

All groups have to delineate the area they are calling the "bog" and then extrapolate their measures to the bog as a whole. If you have access to GPS units, having them mark their points and then see where they were on an aerial photo is usually quite instructive. Since there is considerable heterogeneity in vegetation in this bog, I make sure that all groups get around to all the kinds of areas, but they don't usually walk as far as they think they do.

Make sure to see the Teaching Notes below since they contain details on sequence and execution that will increase the probability of success in this complex lab.

student handouts:

- introduction to the three-lab sequence
- color aerial photo of the site
- overview Excel template for data entry
- instructions for what they need to have done in preparation for the last lab as well as the requirements for the write-up

Links:
Introduction for Students to the Garcelon Bog Lab (Microsoft Word 47kB Jun21 12)
Color Aerial Photo of Garcelon Bog
Excel Template for Data Entry (Excel 2007 (.xlsx) 19kB Jun21 12)
Instructions for Student Syntheses (Microsoft Word 45kB Jun21 12)

Teaching Notes and Tips

A few notes on preparation, timing, and interaction that greatly impact the ability of students to complete this lab:

1. Students do need time before they head to the field the first week in which to plan their approach. It is a little hard since they have never seen the site, but they have been in forests and open areas and have seen aerial photos for places they have worked.

2. Because there are only three main pools (peat, shrubs, and trees) at the outset, there will be subgroups within each of these groups (i.e., though there might be six students assigned to "shrubs," when they are in the field, and then when they are back in the lab (weighing, burning, and dealing with data), they need to be split into two groups of 3, each of which needs the data from the other group but each of which does its own version of the calculation of how much carbon there is in shrubs). Groups larger than three (or max four) students do not work effectively in the field or lab.

3. Make sure that students have completed all the lab work before they come to the second week of this lab; they need all the time they can get in their first groups (pool-based: tree, shrub, peat) to pull together the estimates for that group.

4. I have found that this lab works best if I give the students the Excel template since this is the biggest data entry, analysis, and synthesis task any of them has ever encountered. If they can get through the data analysis within their individual groups (peat, shrub, or tree) and really understand it in the second of the three lab periods, the last lab period goes much better. If one or more of the groups has not done what they should have and are still working to find errors and complete their calculations when they enter the last lab period, the time for synthesis in the new group is cut short and the final group never really comes together.

5. It can be helpful if you identify ahead of time some papers where students might find numbers like "carbon content of peat" since it can take a while in lab for them to find those papers and then figure out where in the paper the needed number is.

6. Since all the synthesis groups (those with one person each from shrub, tree, and peat groups) come up with a different "answer" about how much carbon there is in Garcelon Bog at the end of the third lab period, I take the time in lecture after all the labs to put all this information on the board so everyone from all groups can see this before they finalize their lab report. Inevitably, the "answers" vary by several orders of magnitude (in terms of kg of C in the bog as a whole), though they do tend to clump around something reasonable (to one or maybe two sig figs). This then allows a discussion of why the numbers vary and what they think a reasonable level of precision is for reporting purposes.

Assessment

I grade the group lab report for clarity, completeness, appropriate reporting of precision, writing (technical and style), and presentation of data (and clarity, completeness, and precision there too). I do not grade on whether they got the "answer" right since the "answer" is highly dependent upon methodology and assumptions about the size and depth of the bog. I do want them to realize this dependency, however, and that is part of what I hope to see in their discussion.

Students receive both a group grade for the report and an individual grade based on their contribution to the report. The latter is assessed through my observations in lab and the evaluations I ask each of them to write about their group members and themselves (see end of synthesis handout for students).

References and Resources

Cottam, G. and J.T. Curtis. 1956. The use of distance measures in phytosociological sampling. Ecology 37(3): 451-460. Describes the point-center-quarter-based calculations for basal area that can be used to determine tree "abundance."

IPCC (Intergovernmental Panel on Climate Change, many useful reports for background on global climatic change and calculation of carbon pools and fluxes): [ http://www.ipcc.ch/]

Jenkins, J.C., D.C. Chojnacky, L.S. Heath, R.A. Brdsey. 2004. Comprehensive database of diameter-based biomass regressions for North American tree species. United States Department of Agriculture, Northeastern Research Station General Technical Report NE-319. Includes the allometric equations needed by the tree groups for turning DBH (diameter at breast height) measures into biomass.

Author Notes

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