Vegetation, the Global Carbon Cycle, and Global Measures

Chapter 22

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Vegetation, the Global Carbon Cycle, and Global Measures Lloyd G. Simpson and Daniel B. Botkin Environmental Studies Program, Department of Biological Sciences, University of California, Santa Barbara, CA 93106 Qualitatively, terrestrial vegetation is known to be an important part of the global carbon cycle. It affects the carbon cycle directly through photosynthesis, respiration, and decay of organic matter; and indirectly by affecting climate. Quantitatively, however, little is known about how vegetation fits into the global carbon budget. Computer models are the primary tools used to assess the functioning of vegetation in the global carbon cycle, but models use poor data and are too simple to give reliable results. The most commonly used estimates of global vegetation reservoir size range between 420 and 830 gigatons of carbon. However, using the most current estimates of carbon density we have calculated a new estimate of 328 gigatons of carbon. This result demonstrates that a concerted effort must be made to systematically measure the biomass of vegetation on a global scale. Until this is done, basic questions such as whether vegetation is a net source or sink of carbon in the global carbon budget will be left unresolved.

Carbon is the basic atom of all organic compounds. Life on Earth requires large amounts of carbon to produce carbohydrates, lipids, proteins, and nucleic acids. Primarily in the form of carbon dioxide (C0 ), carbon is also an important component of the Earth's atmosphere. While it makes up less that 0.04% of the atmosphere, C 0 is significant because of its heat absorbing properties. Small changes in C 0 concentration can change the energy budget of the atmosphere and thereby affect climate which in turn can affect life on Earth. Because carbon is so important to life and can affect climate, and because human activities are leading to a continuing increase in atmospheric C 0 concentration, it is critical that we understand the global carbon cycle. The global carbon cycle is the continuous movement of carbon between the living and nonliving portions of the biosphere, driven in part by biological processes and resulting in a constant supply of carbon to life (Figure 1). The 2

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0097-6156/92/0483-0413$06.00/0 © 1992 American Chemical Society Dunnette and O'Brien; The Science of Global Change ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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global carbon budget quantifies the carbon cycle by describing the location of carbon reservoirs, their size, and the fluxes (rates of carbon movement) among the reservoirs. In spite of its importance, we know surprisingly little about the quantitative aspects of the carbon budget. There have been many attempts in recent years to quantity the global carbon cycle (e.g. i , 2, 3, 4), most of which assume that the cycle is static and in steady-state except for anthropogenic influences. Unfortunately, most of these descriptions of the carbon budget use estimates of reservoir size and flux rates that are based on little if any data appropriate for extrapolation to a global scale. The lack of good quantitative data is a fundamental problem common to all studies of biogeochemical processes at the global level. An important issue that remains unresolved, because of the lack of adequate quantitative data on reservoirs andfluxes,is the location of the so called "missing" carbon. Missing carbon is the carbon added to the atmosphere from the burning of fossil fuel that cannot be accounted for by the measured increase in atmospheric concentration or by diffusion into the ocean (5). There has been a long standing controversy about whether vegetation is a net sink or net source of excess carbon (6). This controversy may not be resolved until there are better assessments of the sizes of the vegetation carbon pools and fluxes associated with it. A proper assessment of the actual and potential role of vegetation in the global carbon cycle requires accurate information about carbon storage and change over time (6 7). f

Vegetation and the Global Carbon Cycle Land vegetation has important effects on the biogeochemistry and energy budget of the Earth (8); most directly through photosynthesis and respiration. Photosynthesis of terrestrial vegetation removes carbon dioxide from the atmosphere, respiration of all life returns C 0 to the atmosphere. Part of the carbon removed by vegetation is stored in the form of nonwoody and woody plant parts. The most striking evidence of these effects is the measurements of annual fluctuations of C 0 concentration in the atmosphere made at Mauna Loa observatory (9) (Figure 2). Atmospheric concentration of C O is at a minimum during the summer growing season when terrestrial photosynthesis is greatest, and at a maximum during the winter when most land vegetation is dormant and photosynthesis on the land is low. Some have estimated that as much as ten percent of atmospheric C 0 could be taken up and released annually by vegetation (2), illustrating the rapidity of flux rates between the atmosphere and terrestrial ecosystems. Marine algae and photosynthetic bacteria also remove large quantities of C 0 , but release most of this almost immediately in respiration. These organisms lack the longevity, large body size, and large storage organs that lead to storage of carbon for months and years. Carbon dioxide has been measured at more that 30 stations, and an annual fluctuation of atmospheric carbon dioxide concentration was observed at all (10). However, the magnitude and timing of the fluctuations varied with geographic location (11). At Mauna Loa observatory, the concentration of atmospheric C 0 2

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Dunnette and O'Brien; The Science of Global Change ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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Vegetation, Carbon Cycle, & Global Measures

SIMPSON & BOTKIN

ATMOSPHERE 748 Λ\

-110

•50

-62

**************

/1\

-5.3

-110

********* *

LAND

VEGETATION -328 NPP = -62 -62

SOIL -1300

******************************

* * * *

-110

*************** OCEANS

*

* * * -38000 * * * * * * * * * ******************************** FOSSIL FUELS -4000

Figure 1. The global carbon cycle. Estimates of reservoir size and annual fluxes are from Post et al. (4). Vegetation carbon reservoir was estimated from latest carbon density estimates. A l l values except the atmospheric reservoir are approximate only. All values are in gigatons. Fluxes are next to the arrows and are in gigatons^ear. C . D . K e e l i n g a n d T. P . W h o r f Principal Investigators

360-1

YEAR

Figure 2. Concentration of atmospheric C 0 measured from the Mauna Loa observatory, Hawaii. Data were from Boden et al. (10). 2


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dropped an average of 5.8 ppmv each year between May and September from 1979 and 1989 (Figure 2). Because of the isolation of this location from large land masses, Mauna Loa represents an average of northern hemisphere fluxes. Further north, at NOAA/CMDL station in Point Barrow, Alaska, atmospheric C 0 concentration dropped an average of 13.9 ppmv between April and August from 1979 to 1988. Point Barrow, Alaska is in a region dominated by vegetation that grows rapidly during a short growing season. In the southern hemisphere, the fluctuation is comparatively small and the decrease occurs between October and February, the southern-hemisphere summer. The small decline reflects the southern hemisphere's smaller land mass (about 30% of the Earth's land surface) and smaller amount of land vegetation. Geographical patterns of annual C 0 fluctuations have important implications for the role of vegetation in the global carbon cycle. Dead vegetation also affects the global carbon cycle. Dead organic matter decomposes, releasing carbon dioxide to the atmosphere. Rates of decomposition vary with material, location, and climate. Non-woody organic matter decomposes rapidly; woody organic matter slowly. Decomposition tends to occur faster at the soil surface than below. Decomposition is relatively fast in warm moist climates. In cold climates and in wetlands, decomposition is so slow that there is a net increase of stored carbon in the soil and organic soils called, "histosols," are formed. Vegetation also affects the global carbon cycle indirectly. First, vegetation contributes raw material from which bacteria produce other trace gases such as methane and carbon monoxide (72). Methane is a byproduct of bacterial decomposition of organic matter in oxygen poor environments such as rice paddy fields, wetlands, and the intestines of ruminants and termites. Carbon monoxide is produced directly by vegetation, by burning of vegetation, and, in the atmosphere, by photochemical conversion of methane and non-methane hydrocarbons produced by vegetation such as ethylene, monoterpene, and isoprene (72, 75). Second, vegetation protects soil from erosion and loss of nutrients and organic matter and decreases rates of decomposition. When vegetation is removed, soil decomposition and erosion often increase, because surface water runoff increases, and soil is exposed to wind and sunlight. Deforestation and clearing of grasslands at first leads to an increase in C 0 flux to the atmosphere. Later, with the loss of soil and its nutrients, net C 0 uptake by vegetation may proceed at a slower pace than before. Third, vegetation affects the global carbon cycle indirectly by affecting climate - through changes in albedo, evaporation of water, and surface roughness. Albedo, and therefore the energy budget, is influenced by the type and structure of vegetation. For example, in the northern boreal forest, stands are more open and in the spring there is significant reflectance of solar radiation from snow. This slows the warming process and delays growth (14), resulting in a shorter growing season and a reduced amount of annual carbon uptake. The structure of vegetation also has an influence on surface roughness and on local wind patterns and rates of mixing in the boundary layer of the atmosphere. Forests have a dampening affect on wind, slowing it and causing increased 2

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turbulence. Within the canopy air movement is slower and mixing rates are reduced. The lack of vigorous mixing results in a higher concentration of C 0 in the lower portion of a forest canopy than in the atmosphere directly above the canopy (15). Conversely, the C 0 concentration in the air at the crown level, where all the foliage is concentrated, is lower than the atmosphere above. Some carbon released from soil respiration is reabsorbed quickly by the vegetation and does not become part of the global atmospheric carbon pool. Changes in évapotranspiration rates of vegetation can have indirect effects. About 20% of water vapor supplied to the atmosphere annually comes from évapotranspiration of terrestrial systems (16). Some evidence suggests that in some regions, most notably the tropics, a high percentage of local precipitation comes from local évapotranspiration. A change in évapotranspiration caused by removal of vegetation could affect the amount and distribution of precipitation regionally and globally (17) and therefore affect photosynthesis and growth. 2

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Analysis of the Global Carbon Cycle The most common way in which the global carbon budget is calculated and analyzed is through simple diagrammatical or mathematical models. Diagrammatical models usually indicate sizes of reservoirs and fluxes (Figure 1). Most mathematical models use computers to simulate carbon flux between terrestrial ecosystems and the atmosphere, and between oceans and the atmosphere. Existing carbon cycle models are simple, in part, because few parameters can be estimated reliably. The mathematical model of Bjorkstrom (18) is typical of those used to quantify the global carbon cycle and analyze the possible fate of missing carbon. This model is a series of first-order differential equations representing change in carbon reservoirs over time. There are only three principal reservoirs: ocean, atmosphere, and terrestrial organic carbon. The atmosphere is treated as one well mixed reservoir, while the ocean is divided into 12 interconnected reservoirs representing differences in mixing rates and movement of materials among various ocean depths and latitudes. Terrestrial carbon is divided into two biota and two soil reservoirs. One soil and one biota reservoir represent areas changing due to anthropogenic effects. Each reservoir is assumed to be homogeneous and uniform. Fluxes are linear functions of reservoir contents. Reservoir size and the residence time of the carbon in the reservoir are the parameters used in the functions. Between the ocean and the atmosphere and within the ocean, fluxes rates are calculated theoretically using size of the reservoir, surface area of contact between reservoirs, concentration of C 0 , partial pressures of C 0 , temperature, and solubility as factors. The flux of carbon into the vegetation reservoir is a function of the size of the carbon pool and a fertilization effect of increased C 0 concentration in the atmosphere. Flux from vegetation into the atmosphere is a function of respiration rates estimated by Whittaker and Likens (19) and the decomposition of short-lived organic matter which was assumed to be half of the gross assimilation or equal to the amount transferred to dead organic matter. Carbon in organic matter that decomposes slowly is transferred 2

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to the soil reservoir. Soil organic matter decomposition results in a flux of C 0 from the soil to the atmosphere. Once the model was complete, it was adjusted to a steady state condition and tested using historic carbon isotope data from the atmosphere, oceans and polar ice. Several important parameters were calculated and chosen at this stage. Sensitivity analysis indicated that results - dispersal of the missing carbon ~ were significantly influenced by the size of the vegetation carbon pool, its assimilation rate, the concentration of preindustrial atmospheric carbon used, and the C 0 fertilization factor. The model was also sensitive to several factors related to fluxes between ocean reservoirs. One of the most elaborate models proposed thus far was one constructed by Goudriaan and Ketner (20). This model accounted for atmospheric, ocean (subdivided into several compartments), terrestrial biosphere (subdivided into six ecosystems and six components within each ecosystem), and fossil carbon reservoirs and thefluxesamong them, as well as human impact on the biosphere. This model was used to project the global carbon budget from 1870 to 2030 (20). Again, linearfirst-orderdifferential equations represented fluxes of carbon among reservoirs. This model was sensitive to changes in the diffusion coefficient in the ocean, the life-span of vegetation, the rate of land clearing and reclamation, and the C 0 fertilization effect. The C 0 fertilization effect used in this model was relatively high and may in part account for the conclusion that vegetation is a net sink for atmospheric carbon. Another model, first introduced by Moore, et al. (22), was used to examine the role of terrestrial vegetation and the global carbon cycle, but did not include an ocean component. This model depended on estimates of carbon pool size and rates of C 0 uptake and release. This model has been used to project the effect of forest clearing and land-use change on the global carbon cycle (22, 23, 24). It is interesting to note that scientists who use models with a detailed ocean component suggest that excess or missing carbon must be taken up be vegetation. They also contend that estimates of vegetation reservoirs sizes are highly suspect. Those that use models with vegetation featured prominently, on the other hand, conclude that excess carbon must be diffusing into the oceans and that ocean dynamics are poorly understood. These models are too simple to reflect realistic dynamic properties of the carbon budget. Even so, they depend on data that are poorly measured or lacking. Many potentially important compartments are missing or assumed to be unimportant. For example, no model considers carbon transported from terrestrial systems to the oceans through rivers and streams. While the amount is very small, it is continuous and cumulative (25) The dynamics of these models depend strictly on carbon fluxes, but the fluxes are poorly measured or are calculatedfromcarbon reservoir size and assumptions about the residence time of the carbon in the reservoir. In addition, model fluxes are linear functions while in reality few, if any, probably are linear. The models also assume a steady-state condition which suggests that the carbon cycle is structured, stable, and balanced and will remain so indefinitely. This mechanistic view of biogeochemistry allows for little variation even though it is known that fluctuations and variation occur seasonally. The concentration of 2

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carbon in the atmosphere is also known to have varied over thousands of years without significant anthropomorphic influence (26). Furthermore, in existing global carbon cycle models, carbon flux is independent of any other factor, including climate, even though these models are used to help predict climate change. These models also fail to consider interactions of the carbon cycle with other biogeochemical cycles. In reality, living organisms require large amounts of nitrogen, phosphorus, and sulfur as well as carbon, and these affect one another. For example, phosphorus is often limiting in many ecosystems therefore an increase in phosphorus availability could result in an increase in production thus affecting the carbon cycle. Fossil fuel burning releases large amounts of sulfur to the atmosphere along with the carbon. Sulfur and carbon react together and form several compounds found in the atmosphere. Importance of Accurate Global Measures Land vegetation is known to play an important role in the global carbon cycle, but our knowledge about reservoir size and flux rates is very limited. In a recent review of the global carbon cycle, Post et al. (4) leave the question of the exact role vegetation performs in the global carbon cycle unresolved. We are not yet able to balance the global carbon budget, and different models give conflicting results. The disagreement on the role of vegetation in the global carbon cycle directly reflects uncertainties and inconsistencies in estimates of carbon pool sizes and flux rates. These estimates are based on carbon density (metric tons per hectare, or kg/m ) and the areal extent to which the density applies. Despite their obvious importance, accurate measures of these factors are lacking for most of the Earth. Several studies, based on models, examined the effects of land-use change on the global carbon cycle and conclude that there is a net release of carbon due to land clearing. However, the results and conclusions of these studies are based on assumed sizes of vegetation carbon pools which are inputs to the models. For example, Melillo et al. (24) concluded that boreal and temperate deciduous forests of the northern hemisphere are net sources of atmospheric carbon. Their analysis used values for carbon density derived by Whittaker and Likens (19) from work by Rodin and Bazilevich (27). Rodin and Bazilevich extrapolated results of small, unrelated studies in Europe and the USSR to estimate total biomass of Eurasian boreal and temperate deciduous forests. Their estimates have since been extrapolated to forests worldwide and are used often today. Commonly used estimates of the global vegetation carbon pool range between 420 to 830 gigatons (4). However, the latest carbon density measurements suggest that in the past, previous estimates grossly overestimated carbon stored in land vegetation. Houghton et al. (22) arrived at a value of 744 gigatons for the carbon content of the global vegetation carbon pool. They computed this value by multiplying the carbon density value derived by Whittaker and Likens (19) for each biome by its estimated areal extent and summing the values for the biomes (Table I). We calculated a new value for the vegetation carbon pool by using the most recent carbon density values. The new value was calculated by multiplying areas of major biomes used by Houghton et al. (22) by the most recent estimates 2


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of carbon density for the closed tropical forest reported by Brown et al. (28) and for the boreal forest reported by Botkin and Simpson (7). In addition, it was assumed that carbon densities for tropical seasonal forest, temperate evergreen forest, and temperate deciduous forest reported by Whittaker and Likens (19) which were used by Houghton et al. (22) -- were overestimated. We reduced the carbon density values of these biomes by 62%, the difference between the value of the tropical forest carbon density reported by Whittaker and Likens (19) and that reported by Brown et al. (28) for tropical forests. This was the most conservative difference between the old and more recent estimates of forest biomass density. Preliminary results from our current work on the biomass of the temperate deciduous forest of eastern North America help substantiate this contention. The reduced values were then multiplied by the corresponding area. The result is a new estimate presented for the first time here, of 328 gigatons for the global vegetation carbon pool (Table I, Figure 1). This value is much lower than any previously suggested.

Table I. Estimation of the global vegetation carbon pool using the latest estimates of total carbon density for forests.

This Study

Houghton et al. (22)

Biome

Area" 6

(10 ha)

Tropical moist forest Tropical seasonal forest Temperate evergreen forest Temperate deciduous forest Boreal forest Trop, woodland and shrubland Temp, woodland and shrubland Tropical grassland Temperate grassland Tundra and alpine meadow Desert scrub

1352 653 546 612 1179 945 753 425 2051 706 2152

Total

11374

a b c d

e

Carbon Density

Total Carbon

Carbon Density

Total Carbon

(ton/ha)

(gigatons)

(ton/ha)

(gigatons)

200 160 160 135 90 27 27 18 07 03 03

270 104 87 83 106 26 20 26 14 2 6 744

b

76 61 61 51 23 27 27 18 7 3 3

c

c

c

d

e

e

e

e

e

e

103 40 33 31 27 26 20 26 14 2 6 328

From (22). Calculated from (28). Value from (19) reduced by 62%. Value after (7); total biomass density calculated by multiplying above ground estimate by 1.23. Value after (19).


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Obtaining Accurate Global Estimates There are two types of data necessary to obtain accurate global estimates of vegetation carbon pools or biomass. First, it is important to have accurate data on the areal extent of major ecosystems. Matthews (29) found that calculations of global biomass were significantly influenced by the land cover data set used. Second, there must be accurate estimates of biomass density for terrestrial ecosystems. There is a wide range of estimates published for the same ecosystem, each derived by different methods (29), and none having statistical reliability (7). Surprisingly, there is much confusion as to the areal extent of terrestrial vegetation on a global scale. In part, the confusion lies with different ways that vegetation is classified on maps. Various classification schemes are used, including: taxonomic, physiognomic, environmental, and functional. Taxonomic classifications are based on species. Physiognomic classifications are based on shape and form. Environmental classifications separate vegetation by the major determining environmental conditions with which they are associated. Functional classifications separate vegetation by what they do. For example, in terms of the hydrologie cycle, a functional group would be all the vegetation that evaporated water at the same rate, or at the same rate under the same environmental conditions. Because there are different criteria, there is often disagreement on the location and extent of vegetation, especially at a global scale. For example, there is no agreement on the exact boundaries of the boreal forest in North America (30, 31). Three vegetation maps of the boreal forest (32, 33, 34), that we believe to be authoritative and to be determined by independent methods, disagree considerably as to the areal extent of the North American boreal forest. The area that all three agree is boreal forest (the intersection of the three maps) is less than one-half the area represented by the sum of the maps (the union of the maps) (Botkin, D. B.; Simpson, L. G.; Star J. L.; Estes, J. E. In Prep. Vegetation classification and mapping: a brief summary and consideration of the potential of remote sensing) (Figure 3). Most vegetation maps are derived from a variety of sources using different methods and made at different times. This can lead to an overlap between adjacent areas of interest, the exclusion of some areas, and the improper extrapolation of carbon densities, thus resulting in inaccurate estimates of reservoir size. We found that the biomass density of the southern North American boreal forest was over 2.5 times larger than the biomass density of the northern part of the boreal forest (35). Past estimates of boreal forest biomass density extrapolated southern biomass density values to the entire boreal forest, which in part accounts for the large overestimation (7). It is important that a consistent method be developed to map vegetation globally. Most estimates of global vegetation biomass densities are extrapolations from studies never intended to represent large areas (e.g. 19, 36) or they were derived from questionnaires sent to botanists (37). These estimates are still used commonly in the examination and modeling of the global carbon cycle. Some of the earliest estimates were made when almost no quantitative data were available and the data or the estimates were largely speculative. Other estimates are


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Figure 3. Maximum and minimum extent of the boreal forest of Canada from the union and intersection respectively of Anonymous (52), Rowe (55), and Anonymous (34).


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statistically biased, extrapolating from small, restricted studies to large areas. This leads to overestimation because these studies were typically conducted in undisturbed, mature (i.e. high biomass) vegetation. Our recent work in the North American boreal forest (7, 35) represents the first attempt to obtain accurate large-area measures. Conclusion It is clear that qualitatively, vegetation plays an important role in the global carbon cycle. Quantitatively, however, the effect of vegetation is not well known. Models of the carbon budget suffer from grossly inadequate data and improper assumptions including the assumption that fluxes are only linear functions of carbon storage and that the entire budget is in steady-state except for human influences, assumptions clearly contradicted by reconstructions of the history of atmospheric C 0 concentration and understanding of the causal of carbon flux. To determine the quantitative role of vegetation in the global carbon cycle, basic information such as carbon density and distribution must be measured accurately for the entire Earth, and realistic, non-linear models must be developed. 2

Acknowledgments This research was supported by the Andrew W. Mellon Foundation. We would like to thank R. A. Nisbet and an anonymous reviewer for their helpful suggestions and comments on the manuscript; and Κ. E. Simpson for drafting Figure 3. Literature Cited

1 Carbon and the Biosphere Woodwell, G. M.; Pecan, Ε. V. Eds; Nat. Tech. Infor. Center: Springfield, Va., 1973. 2 Bolin, B.; Degens, E. T.; Duvigneaud, P.; Kempe, S. In The Global Carbon Cycle; Bolin, B.; Degens, Ε. T.; Kempe, S.; Ketner, P., Eds.; SCOPE 13; J Wiley & Sons, New York, 1979, (pp 1-56). 3 Moore, Β. III; Bolin, B. Oceanus 1987, 29, 9. 4 Post, W. M . ; Peng, T.; Emanuel, W. R.; King, A. W.; Dale, V. H.; DeAngelis, D. L. Amer.Sci.1990, 78, 310. 5 Broecker, W. S.; Takahashi, T.; Simpson, H . J.; Peng, T. H . 1979. Fate of fossil fuel carbon dioxide and the global carbon budget. Science 1979, 206, 409. 6 Detwiler, R. P.; Hall, C. A. S. 1988. Tropical forests and the global carbon cycle. Science 1988, 239, 42. 7 Botkin, D. B.; Simpson, L. G. Biogeochemistry 1990, 9, 161. 8 Botkin, D. B.; Running, S. W. Machine Processing of Remotely Sensed Data; Symposium; Purdue University: West Lafayette, IN, 1984, pp. 326-332. 9 Keeling, C. D.; Bacastow, R. B.; Bainbridge, A. E.; Ekdahl, Jr., C. Α.; Guenther, P. R.; Waterman, L. S.; Chin, J. F. S. Tellus 1976, 28, 538. 10 Trends '90: a compendium of data on global change; Boden, Τ. Α.; Kanciruk P Farrell, M . P. Eds.; ORNL/CDIAC-36; Oak Ridge Nat. Lab., Oak Ridge, TN, 1990.


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11 Gammon, R. H.; Komhyr, W. D.; Peterson, J. T. In The changing carbon cycle a global analysis; Trabalka, J. R.; Reichle, D. E., Eds.; Springer-Verlag: New York, 1986. 12 Crutzen, P. J. In The Major Biogeochemical Cycles and Their Interactions Bolin, B.; Cook, R. B., Eds.; SCOPE 21; John Wiley & Sons: New York, NY, 1983; pp 67-111. 13 Freyer, H.-D. In The Global Carbon Cycle; Bolin, B.; Degens, E. T.; Kempe, S.; Ketner, P., Eds.; SCOPE 13; J Wiley & Sons, New York, NY, 1979; pp 101128. 14 Hare, F. K.; Ritchie, J. C. The Geographical Review 1972, 63, 333. 15 Oke, T. R. Boundary Layer Climates; Second Edition; Methuen & Co.: New York, NY, 1987. 16 Westall, J.; Strumm, W. In The Handbook of Environmental Chemistry; Huntzinger, O. Ed.; Springer-Verlag: New York, NY, 1980, Vol. 1 Part A; pp 1749. 17 Waring, R. H.; Schlesinger, W. H. Forest Ecosystems: Concepts and Management; Academic Press: New York, NY, 1985. 18 Bjorkstrom, A. 1979. A model of CO interaction between atmosphere,oceans, and land biota. In The Global Carbon Cycle; Bolin, B.; Degens, E. T.; Kempe, S.; Ketner, P., Eds.; SCOPE 13; J Wiley & Sons: New York, NY, 1979; pp 403-457. 19 Whittaker, R. H.; Likens, G. E. In Carbon and the Biosphere; Woodwell, G. M.; Pecan, Ε. V. Eds.; Nat. Tech. Infor. Center: Springfield, V A , 1973; pp. 281300. 20 Goudriaan, J.; Ketner, P. Climatic Change 1984, 6, 167. 21 Moore, B.; Boone, R. D.; Hobbie, J. E.; Houghton, R. A.; Melillo, J. M . ; Peterson, B. J.; Shaver, G. R.; Vorosmarty, C. J.; Woodwell, G. M . In Carbon Cycle Modelling; Bolin, B., Ed.; SCOPE 16; John Wiley & Sons: New York, NY, 1981; pp 365-386. 22 Houghton, R. A.; Hobbie, J. E.; Melillo, J. M . ; Moore, B.; Peterson, B. J.; Shaver, G. R.; Woodwell, G. M . Ecol. Mono. 1983, 53, 235. 23 Houghton, R. A.; Boone, R. D.; Fruci, J. R.; Hobbie, J. E.; Melillo, J. M . ; Palm, C. A.; Peterson, B. J.; Shaver, G. R.; Woodwell, G. M . ; Moore, B.; Skole, D. L.; Myers, N. Tellus 1987, 39B, 122. 24 Melillo, J. M.; Fruci, J. R.; Houghton, R. A.; Moore III, B.; Skole, D. L. Tellus 1988, 40B, 116. 25 Lugo, A. E.; Brown, S. Vegetatio 1986, 68, 83. 26 Botkin, D. B. Discordant Harmonies: a new ecology of the 21st century; Oxford University Press: New York, NY, 1990. 27 Rodin, L. E.; Bazilevich, Ν. I. Production and Mineral Cycling in Terrestrial Vegetation; Oliver and Boyd: London, GB, 1967. 28 Brown, S.; Gillespie, A. J. R.; Lugo, A. E. For. Sci. 1989, 35, 881. 29 Matthews, E. Progress in Biometeorology 1984, 3, 237. 30 Larsen, J. A. The Boreal Ecosystem; Academic Press: New York, NY, 1980. 31 Larsen, J. A. The Northern Forest Border in Canada and Alaska.; SpringerVerlag: New York, NY, 1989. 2


22. SIMPSON & BOTKIN Vegetation, Carbon Cycle, & Global Measures 425 32 Anonymous Physical Geographic Atlas of the World; Acad. of Sciences U.S.S.R. and Board of Geodesy and Cartography GGK, U.S.S.R, 1964. 33 Rowe, J. S. Forest Regions of Canada.; Can. For. Serv., Dept. Fisheries and the Environment: Ottawa, Ont. Canada, 1972. 34 Anonymous National Atlas of Canada; Dept. of Energy, Mines, and Resources: Ottawa, Ont., Canada, 1974. 35 Botkin, D. B.; Simpson, L. G. In Global Natural Resource Monitoring and Assessments: preparing for the 21st century; Cini, F. G. Ed.;. Amer. Soc. of Photo.

and Remot. Sen.: Bethesda, MD, 1990, Vol. 3; pp 1036-1045. 36 Ajtay, G. L.; Ketner, P.; Duvigneaud, P. In The Global Carbon Cycle; Bolin, B.; Degens, E. T.; Kempe, S.; Ketner, P., Eds.; SCOPE 13; J Wiley & Sons: New York, NY, 1979; pp 129-182. 37 Olson, J. S.; Pfuderer, H. A.; Chan, Y. H. Changes in the Global Carbon Cycle and the Biosphere; ORNL/EIS-109, Oak Ridge National Laboratory: Oak Ridge, TN, 1978. RECEIVED September 4, 1991


Vegetation, the Global Carbon Cycle, and Global Measures

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