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LABORATORY WEEK 5

CARBON SEQUESTRATION IN

FORESTED ECOSYSTEMS

ESC 150 – Principles of Environmental Science

INTRODUCTION

With concerns mounting about possible shifts in earth’s climate due to human industrial activity, understanding the global carbon cycle has become vitally important.  The carbon (C) cycle, as with any biogeochemical cycle, consists of different pools where the element may reside; these pools are connected by various fluxes, which are transfers of the element from one pool to another.  For example, the tissues of your body represent a pool in which C resides, and loss of this carbon via cellular respiration would be a flux to the atmosphere, which is yet another C pool.  Important reservoirs or pools for the global C cycle include atmospheric CO₂, dissolved CO₂ in the oceans, ocean sediments, certain rocks (e.g., limestone), soils and peat (undecayed organic matter in bogs, etc.), vegetation and of course, oil or fossil fuels are reservoirs of Carbon sequestered many millions of years ago by photosynthetic organisms (here we will focus on living organisms).  The major fluxes that remove C from the atmosphere include photosynthesis by plants and deposition of C in soils and sediments; conversely, fluxes that add C to the atmosphere include cellular respiration, decay, forest fires, and combustion of fossil fuels.  When atmospheric CO₂ is fixed in photosynthesis and the resulting sugars are then converted into wood, C is stored in the vegetation; this storage phenomenon is known as carbon sequestration.  The focus of this lab is on carbon sequestration in forested ecosystems.

Photosynthesis is the means by which plants use the sun’s energy to convert a simple compound (carbon dioxide) to a more complex sugar (glucose).  Plants then break down the chemical bonds in the sugar during cellular respiration, which yields ATP to fuel all of the metabolic activities of the plant, such as growth, maintenance, uptake of soil nutrients, and reproduction.  The process of photosynthesis may be summarized like so:

6 CO₂ + 6 H₂O + Sun Energy → C₆H₁₂O₆ + 6 O₂

[carbon dioxide + water + Sun Energy → glucose (a sugar) + oxygen]

Terrestrial plants such as trees perform photosynthesis, as do single-celled algae in freshwater and marine systems.  Thus, all ecosystems on earth that harbor photosynthetic organisms have the potential to sequester C; of course, some biomes are inherently more productive (rainforest) than others (desert), so rates of C storage vary from place to place.  Temperate zone forests such as those in the southeastern U.S., where it is warm and humid and the growing season is relatively long, have higher biomass per unit area than almost any ecosystem outside of the tropics (only temperate rainforests such as in the Pacific Northwest have higher biomass; Whittaker 1975, Table 5.2).

Once carbon has been acquired by terrestrial plants, it is allocated in different proportions to various plant parts; furthermore, a considerable amount of organic matter eventually makes its way into the soil via leaf litter, falling trees, and decaying roots.  With careful study, it is possible to quantify how C is allocated within a forest ecosystem.  For example, a study of a 50-year old forest in Tennessee, dominated by tulip poplar, revealed that biomass (the mass of living matter) in the forest was allocated as follows:

Table 6.1

       Forest  

Compartment Mass of Organic Matter      % of total  

Tree

Foliage 3,200 1.0

Branches 27,100 8.1

Trunks 94,400 28.2

Roots 36,000 10.8

Understory 8,800 2.6

Forest Floor 6,000 1.8

Soil Organic 159,000 47.5

System Total 334,500 kg/hectare 100.0%

(=334.5 metric tons/ha)

(Data from Cole and Rapp 1981; Waring and Schlesinger 1985, p. 143)

Note that a surprising 59% (47.5 + 10.8) of the organic matter is in the soil and the roots.  This underground fraction, though quite significant in terms of C storage, is difficult to measure, so most studies of carbon sequestration in terrestrial ecosystems consider only the aboveground plant parts.  The “understory” component refers to small herbaceous plants, tree seedlings, and shrubs.

How would one go about acquiring the data in table 6.1?  Soils are analyzed by digging soil pits, collecting samples, and then determining soil bulk density (g/cm³) and soil organic matter (SOM).  Loss-on-ignition, a test for determining the fraction of organic matter in soil, involves burning a sample of soil in a muffle furnace; the difference in mass of the soil before and after combustion indicates the amount of organic matter, which burns off at very high temperatures, leaving behind the inorganic fraction such as sand and clay.  Armed with measures of bulk density and SOM, it is possible to determine the C stored in soils on a per hectare basis.  The fraction of total organic matter tied up in foliage may be measured using “traps” to catch leaf litterfall, most of which rains down in autumn.  “Litter” includes both “fine litter” (leaves, small twigs, etc.) and coarse woody debris (CWD), i.e., fallen trees that add a huge amount of organic matter to the soil when they die or are blown down in storms.  The living above-ground plant biomass may be determined by destructively sampling trees, i.e., cutting them down, oven-drying the various components (trunk, branch, leaf), and then weighing everything.  Rather than cutting down any trees, we will employ allometric equations to estimate the organic matter stored in aboveground plant parts.

Allometric equations are used to quantify relationships between aspects of size and shape.  In our case, we will use an allometric equation to help predict total plant mass based on just one simple measure: tree dbh (diameter at breast height, 4.5 feet above the ground).  Then, with our estimates of tree biomass, we can determine the amount of carbon stored in a plot of a given size, or we can estimate the carbon stored per hectare (1 hectare = 2.47 acres).  Martin et al. (1998) studied trees of the southern Appalachians and derived an equation relating the tree diameter (D, in cm) with the total above-ground dry biomass (in kg) of the tree:  

Biomass (kg) = 10^{(-1.25 + 2.66 log10(D))}

A carat (“^”) is used in Excel spreadsheets to indicate that what follows is an exponent. As an example, a tree with 20 cm diameter (where log₁₀ of 20 equals 1.301) would have a total predicted biomass as follows:

Biomass = 10^{(-1.25 + 2.66 (1.301))}

  Biomass = 10^{(-1.25 + 3.461)}

  Biomass = 10^2.211

Biomass = 162.6 kilograms (approximately 358 pounds)

Therefore, using the allometric equation of Martin et al. (1998), a 20 cm diameter tree has an approximate dry above-ground mass of 163 kilograms.  A 40 cm tree would have a mass of about 1027 kg (2,264 lbs), which is far more than just twice that of a 20 cm tree.  It is often true that allometric relationships are non-linear, due to biophysical constraints on, for example, the mass that may be supported by a stem of a given diameter.   

  The Martin et al. study from Tennessee measured the total organic matter in trees, not just carbon.  Organic matter consists of mostly C, H, O, and N.  So how does one determine the mass of carbon alone in organic matter?  This is a key point if we are to quantify carbon sequestration precisely.  As an example, the percent C in glucose (C₆H₁₂O₆) may be determined using some simple math.  The atomic masses of C, H, and O, are 12, 1, and 16, respectively.   Therefore, the total mass of a molecule of glucose is (6 x 12) + (12 x 1) + (6 x 16) = 180.  The percentage of the mass of glucose that is due to carbon is thus 72/180, or 40%.  Trees are not giant lollipops made solely of simple sugars, however!  Much of the glucose produced by a tree is converted to cellulose, a major component of wood.  Trees are also made up of other substances (lipids, proteins, nucleic acids) that have differing percentages of C.  Studies have determined that the dry weight of trees (i.e., excluding the water content) is on average about 45% carbon (Likens and Bormann 1995, p. 113).  Using the data from Table 6.1 (Cole and Rapp 1981), we find that the total aboveground biomass (AGBM) for trees in Tennessee is 124.7 metric tons/ha; therefore, the total carbon stored in trees is 56.1 metric tons C/ha (i.e., 0.45 x 124.7).  Table 6.2 below has data on average biomass and carbon storage for major forest ecosystems of the world.

Table 6.2.  Aboveground biomass and carbon (C) storage in forest types of the world.

           Aboveground Vegetation

        Biomass         Carbon

Ecosystem         (kg/ha)                      (kg/ha)

Boreal coniferous forest 51,300 23,100

Temperate forests:

Coniferous forest 307,300 138,300

Deciduous forest 151,900 68,400

Tennessee forest (table 6.1) 133,500 60,075

Tropical Forest 292,000 131,400 

(Waring and Schlesinger 1985, p. 142, Table 6.5)

Note that the data in Table 6.2 are in terms of AGBM for all of the forest plants, not just the trees.  Be careful with numbers for biomass and carbon storage: the numbers may or may not include soil or belowground parts (e.g., roots), and the units may be in terms of either biomass or carbon.

During this lab, we will measure trees and calculate aboveground tree biomass in plots of standard size.  From these data, we will estimate the storage of carbon (kg C/ha) in the woodland or forest that you sampled.

SPECIFIC OBJECTIVES

1. Become familiar with the concept of carbon sequestration, especially as it relates to anthropogenic changes in the global carbon cycle;

2. Practice field techniques commonly used by forest ecologists, including tree

identification, measurement of dbh, and laying out sampling plots;

3. Perform calculations in order to quantify carbon storage;

4. Enhance quantitative reasoning skills by manipulating numbers and reaching

conclusions based on numerical data.

MATERIALS

Flexible measuring tape (cloth or plastic) to measure trees

20 m long string

4 stake flags- you can construct these out of household materials

Roll of orange plastic ribbon to place around the perimeter of the 5 x 5 m plot.

Data sheet – see below

Clipboard

Pencil or pen

Appropriate field clothing (long pants and long sleeves and close-toed shoes)

Insect repellant and sunscreen (if seasonally appropriate; optional)

PROCEDURE

There are three parts to this exercise:

1) set up a 5 x 5 meter plot in a natural or semi-natural area in which you will measure trees

2) measure dbh, (in centimeters) and identify as best you can all the trees in your plot (refer back to the previous lab on tree identification to help), you can also simply note that there are ___many trees of of type A, B, C etc.

3) calculate biomass for each tree, then perform all the calculations and answer the questions in the lab worksheet.

Part I. Laying out a 5 x 5 Meter Study Plot

The first step is to find a place where you can lay out a 5 x 5 meter plot.  A nearby forest would work just fine, or a wooded area in your own backyard.  The key is that the area must not be just a barren lawn with a single tree in it, for example – that would hardly be representative of forests in Georgia.  So try your best to find a forest or semi-natural area.    Then lay out a square plot that is 5 meters on each side. (Note that 5 meters = 16.4 feet, or 16 feet and 5 inches.)  Use the 20 meters of orange string provided in your lab kit, and mark each corner with a stake flag.  Make sure that the corners are right angles (90 degrees) so that the plot is truly square.  I recommend practicing in an open area before setting up your plot in the forest.  If the plot is not square, its area will not be equal to 25 m² (that is going to be important for your calculations later).  Use the roll of string or ribbon to lay down on the ground along the perimeter of your plot; this ribbon plus the stake flags will allow you to readily see the boundaries of the plot.  Here is what your study plot ought to look like:

 

Figure 6.1. Layout of 5 x 5 m carbon sequestration plots.  “N” indicates magnetic north.

(Your plot does not have to be aligned North-South if you do not have a compass.)

Part II.  Measuring Trees in Plots

Use the data sheet provided (see below) to record the diameters of all of the trees in your 5 x 5 meter plot.  Tree diameter, or dbh (diameter at breast height), is measured using a flexible tape measure at a height of 4.5 ft (1.37 m) above the ground.  For our purposes in this lab, a tree is defined as any woody plant with a diameter at breast height of 2.5 cm (1 inch) or greater.  You can just ignore any saplings that are smaller than this.

To measure dbh, wrap the tape measure around each tree trunk at a height of 1.37 m, then read the circumference directly from the tape measure.  From circumference you will have to calculate diameter, and if you measured in inches you will have to convert to centimeters.  For smaller trees, use the Vernier calipers (they are faster, plus they measure diameter directly rather than circumference).  See the handouts from earlier in the course for a reminder about how to make these conversions, but briefly, diameter (D) = circumference (C) / 3.1416, and 1 inch = 2.54 cm.  Record dbh in centimeters (cm), to the nearest millimeter (mm), for example “51.6 cm.”

Part III. Calculations

Remember, we are going to use the allometric equation of Martin et al. (1998) to use our measurements of tree diameter (D, in cm) to estimate the total above-ground dry biomass (M, in kg) of each tree.  “Log” in the formula refers to log base-10 (i.e., log₁₀):  

Biomass (kg) = 10^{(-1.25 + 2.66 log(D))}

We can re-write this in a format that Excel spreadsheets can understand:  

Biomass   = 10^(-1.25+ 2.66 * (log(D)))

Note that in Excel the “*” indicates multiplication, and the “^” (carat) symbol indicates raising 10 to the power of all that follows in parentheses.  Excel will also find logs for you, using the formula “log(C11)”, where C11 is the cell address where the dbh data is found.  Your cell address will be some other value.  Remember to follow the correct order of operations!

You may enter your raw tree data into an Excel spreadsheet (not required) or you may use a calculator.  The lab worksheet that you will submit for a grade (a separate document) indicates the calculations you need to perform, plus it gives you the formulas.  Briefly, though, you will calculate the total aboveground tree biomass (kg) in your 5 x 5 m plot, and from this you will find the total carbon stored in trees in your plot (kg) (assuming that 45% of the mass of organic matter consists of the element C).  You will also solve for biomass and carbon storage on a per hectare basis.  Remember, a hectare is 10,000 m², and our plots are only 25 m², so to convert from a “per plot” basis to a “per hectare” basis, you multiply by 400 (since 25 goes into 10,000 four hundred times).  

 REFERENCES

Cole, D.W. and M. Rapp. 1981. Elemental cycling in forest ecosystems.  In Dynamic

Principles of Forest Ecosystems, (D.E. Reichle, ed.), pp 341-409. Cambridge

 University Press, London and New York.

Likens, G.E. and Bormann, F.H. 1995. Biogeochemistry of a forested ecosystem, 2^nd ed.,

Springer-Verlag, NY.

Martin, J.G., B.D. Kloeppel, T.L. Schaefer, D.L. Kimbler, and S.G. McNulty. 1998.  

Aboveground biomass and nitrogen allocation of ten deciduous southern

Appalachian tree species.  Canadian Journal of Forest Research 28: 1648-1659.

Waring, R.H. and W.H. Schlesinger. 1985. Forest Ecosystems: Concepts and

Management. Academic Press, Inc. Orlando, FL. 340 pp. (see p. 143)

Whittaker, R.H. 1975. Communities and Ecosystems, 2^nd ed.  Macmillan Publishing Co.,

NY. (see p. 224)

Data Sheet: Lab Week 5 - Carbon Sequestration

Plot Dimensions: 5 x 5 m,  Plot Size:25 m²

Plot Location (brief description):   

Student Name:  _______________________________________ Date Sampled: ____________       

_____________________________________________________________________________________

NOTES ON METHODS:

Measure and record the dbh (in cm, to nearest mm) of all trees, defined as woody plants with dbh ³ 2.5 cm.

Identify trees to species if possible, but “maple” or “oak” or “pine” is also OK.  You may also write “deciduous species” or “evergreen broadleaved species”

Trees must be rooted in a plot in order to be counted.  

For trees with multiple stems at breast height, put all dbh measurements of those stems within a set of brackets, like so: (12.4 cm, 13.9 cm).  Later you’ll calculate carbon storage separately for each stem.

_____________________________________________________________________________________

 TREE DATA (³ 2.5 cm dbh, that is, ³ 1 inch)

Tree # Tree Species dbh (centimeters)

Example Pine or Type A 43.9

1

2

3

4

5

6

7

8

9

10

11

12