ESCI 407/507: Forest Ecology
Spring 2024
Last updated: 5/29/2024
Lab #7: Carbon Cycling
Introduction:
In this lab exercise, we will be quantifying the amount of carbon being sequestered by a second-growth and an old-growth forest and compare this to the carbon being released to the atmosphere by a vehicle powered by an internal combustion engine. The exercise will be based upon forest sampling data we collected for the previous two labs.
Background:
The potential for substantial climate change resulting from increasing atmospheric concentrations of radiatively-active trace gases has motivated efforts to identify natural and anthropogenic sources and sinks for these gases (Houghton and Woodwell 1989, Post et al. 1990). Among these radiatively-active trace gases, carbon dioxide has the greatest potential to affect global climate and hence, the global carbon budget has received special attention (IPCC 1990). Recent global-scale estimates (Sundquist 1993, Houghton et al. 1992) suggest that emissions of CO2 to the atmosphere from the combustion of fossils fuels and land use account for 5.4 +- 0.5 and 1.6 +- 1.0 Pg of carbon per year, respectively. Dixon et al. (1994) recently reduced the estimate of land use emissions to 0.9 +- 0.4 Pg C / yr. The oceans are thought to be a sink for 2.0 +- 0.8 Pg C / yr while observed increases in atmospheric CO2 concentrations account for about 3.2 +- 0.1 Pg C / yr. This observed increase and the known sources leaves a "missing sink" in the biosphere of about 1.1 +- 1.0 Pg C / yr. The search for this missing sink is at the core of much of the current work on global carbon budgets.
The certainty in the measurements of the various global carbon pools and fluxes varies widely. There have been economic incentives to carefully track the global consumption of fossil fuels and hence, there is more confidence in the estimates of fossil fuel emissions than for the other sources and sinks. Solomon et al. (1993) point out that the oceans and the atmosphere are reasonably well mixed systems and the measurement and modeling of carbon pools and fluxes between the ocean and atmosphere is a tractable problem in physical chemistry and fluid dynamics. For this reason, the estimates of carbon flux between the ocean-atmosphere systems have remained between a fairly narrow, but perhaps not accurate, range of values over the past 20 years (Keeling 1973, Tans et al. 1990, Orr 1993). Solomon et al. (1993) go on to point out that the terrestrial system is fundamentally different than the ocean and the atmosphere in that it is not at all well mixed with respect to carbon. In the terrestrial system, heterogeneity in carbon pools and fluxes exist at all spatial and temporal scales and results from a complex interaction of natural and anthropogenic factors. This heterogeneity has resulted in a wide range of estimates of terrestrial carbon sources and sinks.
The relative certainty in the other carbon pools and fluxes points towards a terrestrial carbon sink. Several lines of evidence suggest that the missing carbon sink is most likely to be found in northern, mid-latitude forests (Tans et al. 1990, Kauppi et al. 1992, Taylor and Lloyd 1992). Dixon et al. (1994) calculated that, if these forests are to account for the missing carbon in the global budget, their net accumulation rate must be about 1.5 Mg C / yr, or about 2 to 3% of their present standing stock. They point out that, although this is within the range of values observed at selected sites, it is considerably higher than the most recent estimate for the continental USA (0.4 Mg C/ha/yr; Turner et al. 1994) and it is up to four times higher than the observed globally-averaged rate of accumulation for northern temperate forests (reviewed by Dixon et al. 1994).
Efforts to reduce the uncertainty in the global terrestrial carbon budget will require better information about the spatial and temporal heterogeneity of carbon storage and carbon flux in various regions throughout the world. Forest ecosystems are particularly important in any consideration of global carbon budgets because they contain about 60% of the global terrestrial carbon stocks (Waring and Schlesinger 1985).
PNW Forests
The forests of the PNW are among the most productive in the world and contain many tree species that attain great ages and substantial stature (Waring and Franklin 1979). Several tree species achieve ages of 500 to 1000 years in natural forests that may contain individual trees 100 to 200 or more centimeters in diameter at breast height with tree heights of 60 to 80 meters. These forests have the capacity to store very large quantities of carbon. An intensively studied, 450-year-old Douglas-fir (Pseudotsuga menziesii) stand on moderately productive (site class 3) land in the central Oregon Cascade Mountains, contained 611 Mg of carbon per hectare in above- and below-ground living and detrital pools and in the mineral soil (Grier and Logan 1977, Harmon et al. 1986). Sixty-year-old stands on comparable land contain between 259 and 274 Mg of carbon per hectare in these same pools (Harmon et al. 1990).
Harmon et al. (1990) have shown that harvesting these old-growth forests and replacing them with young plantations results in a large net release of carbon to the atmosphere, even when storage of carbon in forest products is considered. Although young plantations have a higher net annual rate of carbon uptake than old forests, the total amount of carbon stored in young plantations is minimal when compared to old-growth stands. Harvesting an old-growth stand results in a large increase in the amount of dead wood on the site in the form of tops, branches and roots. Even if the decay rate remained constant after harvest, the amount of carbon released to the atmosphere from this very large detrital pool is substantial during the first several decades after harvest. Similarly, much of the carbon that is removed from the site by timber harvesting is quickly released to the atmosphere during primary and secondary manufacturing processes and by incineration and decomposition of short-lived forest products. Many decades are required before the net carbon accumulation rate by regenerating trees in the plantation exceeds the net carbon emission rate to the atmosphere by the detrital pools and the forest products sector. It may take 200 years before the total amount of carbon stored by the stand approaches pre-harvest levels.
Although these stand-level dynamics are well documented, developing a regional carbon budget requires the integration of these results in both the time and space domains. Integration in the spatial domain requires information on the distribution of stand ages, species, site productivity and management techniques. Integration in the time domain requires spatially explicit information on changes in stand age in response to timber harvest, wildfire, succession and changes in management practices and forest product utilization standards. Assembling this information is a challenging interdisciplinary problem in ecology, silviculture, economics, history and social science.
METHODS:
Vegetation Sampling: We will be using the vegetation data you collected at the Second-growth stand and the data you collected at the Old-growth stand. As you will recall, all trees larger than 10 cm DBH were sampled within a 0.1 ha circle (17.8 m radius) and trees larger than 50 cm DBH were sampled within a 0.2 ha circle (25.2 m radius). For the purposes of this lab, we will ignore all trees less than 10 cm DBH and we will ignore understory vegetation. For trees >= 50 cm, we recorded DBH to the nearest 0.1 cm. For all other trees, we will tally trees into 10 cm diameter classes.
For several trees in each plot, I took a short increment core. I did all of the processing of these cores for you. I will provide you with these data in lab.
Vegetation Calculations: Back in the lab, calculate the biomass of each tree using the species-specific equations provided. For trees under 50 cm DBH, use the class mid-points (e.g., use 15 cm for trees in the 10-20 cm DBH class). Calculate the biomass of each tree ten years ago by subtracting the two times the average radial growth over the last ten years. Calculate the current biomass and the biomass for this stand ten years ago (Mg/ha). Assume that about half of this biomass is carbon and calculate the average annual rate of carbon accumulation for this stand in units of Mg C /ha (Mg = 106 grams).
I will provide you with an Excel spreadsheet that
is set up to do most of the calculations for you! All you need to do
is fill in the number of trees in each size class for the trees that were 10-50
cm DBH and list the sizes of all trees that are >50 cm. The spreadsheet
assumes that you have COMPLETE data for four circular plots from the
second-growth stand and four circular plots from the old-growth stand. You will also need to enter data for
the CWD and snags from each plot.
Carbon content of Gasoline: Gasoline is a mixture of several hydrocarbons, mostly heptane (C7H16) and isooctane (C8H18). The Aoctane rating@ is an estimate of the relative amounts of these two compounds. There are some other additives, however, for our purposes, we will ignore these. We will also assume that cars are 100% efficient in the combustion of this gasoline. This is probably not correct (it is probably more like 90 to 95% efficient; note that I am talking about efficiency of combustion, not efficiency of conversion from chemical to mechanical energy; energy conversion efficiency for most internal combustion engines is about 20-25%) but the various emission control devices (like the catalytic converter) probably insure that nearly all the fuel is oxidized.
To calculate the carbon emissions, we need to calculate the carbon content of gasoline. The densities of octane and heptane are 700 and 684 g/l, respectively. Based on molecular weights, the proportion of each of these compounds that is carbon is:
Octane
C = 8*12 = 96
H = 18*1 = 18
----
114 ====> 96/114 =
.84
Heptane
C = 7*12 = 84
H = 16*1 = 16
----
100 ====> 84/100 =
.84
This gives the mass of C in a liter of octane and heptane as 589g and 576g, respectively. For 87 octane gasoline (87% octane and 13% heptane), I get a figure of 587 g C/l. This translates to 4.9 pounds of carbon per gallon of gasoline (587 g C/l * 3.785 l/gallon = 2,222 grams or 2.22 Kg C/gallon; 2.22 Kg C/gallon * 2.2 pounds/Kg = 4.88 pounds C/gallon). These calculations are derived (with some corrections) from Weihe (1997) and various online sources.
Carbon Emissions by the average car: The national average fuel efficiency is (sadly!) about 18 miles to the gallon (actually some recent stats suggest it is actually a bit higher than this)
…….but let’s use the 18MPG value for now…….. and most people drive about 10,000 miles per year. So:
10,000 miles per year / 18 miles per gallon = 556 gallons per year
556 gallons * 3.785 liters per gallon = 2,103 liters per year
2,103 liters per year * 587 g C per liter = 1,234,330 g C per year = 1.23 Mg C per year
Carbon Offset: OK, now how much forest land like this stand is required to balance the carbon emissions by the average car in the U.S.?
Lab Report: If you want to bring your lab grade up a bit, you can prepare
a standard lab report that deal with these calculations and presents the
results. Keep in mind that your lab grade is based on your best 4 lab report
grades. If you have turned in at least four lab reports and you are satisfied
with your grade thus far, you do not need to write a lab report for this lab.
If you choose NOT to write a report for this lab, I still expect you to prepare a one page write up that provides the answer and also considers what has been left out of these calculations (have we accounted for ALL of the forest carbon budget?) and considers various sources of uncertainty in our calculations
Excel Spreadsheets:
carbon2024_og.xlsx
carbon2024_2ndgrowth.xlsx
Note that, in these for the 2ndgrowth file, I’ve entered data for you for one interior plot for 2013 to combine with the data from your three interior 2015 plots.
These spreadsheets will walk
you through the calculations. As in
previous labs, fill in the data in the areas shaded blue. Links in the spreadsheets will carry out the
calculations for you.
Snags: You will note
that the spreadsheets do not include snags.
How do you think including these would change your calculations? OK, now modify these spreadsheets to include
snags. Here are some spreadsheets that
will enable you to incorporate snags.
Note that the pages for the living trees are identical to those in the
spreadsheets above. The page for
detrital calculations includes a new section for including snags. You will need to use the height estimates
we made for each snag. In the
spreadsheet, I made some assumptions in order to calculate the top diameter of
each snag. In order to do this, you will
need to enter the canopy height measurement that we made for each plot.
carbon2024_2ndgrowth_snag.xlsx
Your
lab report should include estimates of the C flux for the two stands with
and without the snag calculations.
Literature Cited
Dixon, R.K., Brown, S., Houghton, R.A., Solomon, A.M., Trexler, M.C. and Wisniewski, J. 1994. Carbon pools and flux of global forest ecosystems. Science 263:185-190.
Grier, C.C. and Logan, R.S. 1977. Old-growth Pseudotsuga menziesii communities of a western Oregon watershed: biomass distribution and production budgets. Ecological Monographs 47:373-400
Harmon, M.E., Franklin, J.F., Swanson, F.J., Sollins, P., Gregory, S.V., Lattin, J.D., Anderson, N.H., Cline, S.P., Aumen, N.G., Sedell, J.R., Lienkaemper, G.W., Cromack, Jr., K. and Cummins, K.W. 1986. Ecology of coarse woody debris in temperate ecosystems. Recent Advances in Ecological Research 15:133-302.
Harmon, M.E., Baker, G.A., Spycher, G. and Greene, S.E. 1990a. Leaf-litter decomposition in the Picea / Tsuga forests of Olympic National Park, Washington, USA. Forest Ecology and Management 31:55-66.
Harmon, M.E., Ferrell, W.K. and Franklin, J.F. 1990b. Effects on carbon storage of conversion of old-growth forests to young forests. Science 247:699-702.
Harmon, M.R., S.L. Garman, W.K. Ferrell. 1996. Modeling historical patterns
of tree utilization in the Pacific Northwest: Carbon sequestration implications.
Ecological Applications 6(2):641-652 Harmon-ModelingHistoricalPatterns-1996.pdf
Houghton, R.A. and Woodwell, G.M. 1989. Global climate change. Scientific American 260:36-44.
Houghton, R.A., Callander, B.A. and Varney, S.K. (eds) 1992. Climate Change 1992. Cambridge University Press, Cambridge.
IPCC 1990. Climate Change: The Intergovernmental Panel on Climate Change Scientific Assessment. Houghton, J.T., Jenkins, G.J. and Ephraums, J.J. (eds), Cambridge University Press, Cambridge.
Kauppi, P.E., Mielikainen, K., Kuusela, K. 1992. Biomass and carbon budget of European forests, 1971 to 1990. Science 256:70-74.
Keeling, C.D. 1973. Industrial production of carbon dioxide from fossil fuels and limestone. Tellus 25:174-198.
Orr, J.C. 1993. Accord between ocean models predicting uptake of anthropogenic CO2. Water, Air and Soil Pollution 70:465-482.
Post, W.M., Peng, T.-H., Emanuel, W.R., King, A.W., Dale, V.H. and DeAngelis, D.L. 1990. The global carbon cycle. American Scientist 78:310-326.
Solomon, A.M., Prentice, I.C., Leemans, R. and Cramer, W.P. 1993. The interaction of climate and land use in future terrestrial carbon storage and release. Water, Air and Soil Pollution 70:595-614.
Sundquist, E.T. 1993. The global carbon dioxide budget. Science 259:934-941.
Tans, P.P., Fung, I.Y. and Takahashi, T. 1990. Observational constraints on the global atmospheric CO2 budget. Science 247:1431-1438.
Taylor, J.A. and Lloyd, J. 1992. Sources and sinks of atmospheric CO2 Australian J. Botany 40:407-418.
Waring, R.H. and Franklin, J.F. 1979. Evergreen coniferous forests of the Pacific Northwest. Science 204:1380-1386.
Waring R.H. and Schlesinger, W.H. 1985. Forest Ecosystems: Concepts and Management. Academic Press, New York.
Weihe, P. 1997. Tree measurement and carbon cycling: a laboratory exercise. Bulletin of the Ecological Society of America. 78(2):142-143.
Return to ESCI 407 Lab Index Page
Return to ESCI 407 Syllabus
Return to David Wallin's Home Page