SCOPE 13 - The Global Carbon Cycle

ABSTRACT

An estimate is given of the annual man-induced net transfer of carbon between the atmosphere and the land biota, including soil organic matter. Effects that enhance the atmospheric pool of carbon are clearing of forests, mostly involving the use of fire, particularly in developing countries of the subtropics and tropics; industrial use of wood; use of firewood; and decomposition of soil organic matter. Transfers in the opposite direction may be effected by world-wide `fertilizing' of ecosystems with CO₂, fertilizing terrestrial ecosystems with plant nutrients other than carbon, and by intentional or inadvertent creation of ecosystems which accumulate high levels of phytomass. Available data are at best reasoned guesses and their accuracy and reliability accordingly low. However, it seems reasonably certain that the land biota is most probably a net source of carbon for the atmosphere, possibly in the order of 1.54.5 x 10¹⁵ g per year. The possibility cannot be ruled out that it is neither a source nor a sink, but it seems very unlikely that it is a net sink, which is required by a number of current geochemical models of the global carbon cycle.

7.1 INTRODUCTION

Man's interference with the pool of carbon represented by the land biota (including soil organic matter) produces effects which enhance and effects which diminish the atmospheric carbon pool. Impacts of the first type comprise conversion of forests, especially clearing by fire, mostly to cropland, pasturage and plantations, industrial and domestic use of wood, and decomposition of litter and soil organic matter. Transfers in the opposite direction may be caused by worldwide `fertilizing' of ecosystems with CO₂, fertilizing terrestrial ecosystems with plant nutrients other than carbon, and by the intentional or inadvertent creation of ecosystems accumulating high levels of phytomass.

The present contribution attempts to give an estimate of the net transfer of carbon per year, which is compared with figures published in the literature. Its main purpose, however. is to throw some light on the difficulties of estimation and to point out the narrowness of our present knowledge which severely limits our understanding of the carbon cycle as a whole and particularly of the further evolution of the atmospheric CO₂ mixing ratio under the influence of human civilization.

7.2 TRANSFERS OF CARBON FROM THE LAND BIOTA TO THE ATMOSPHERE

7.2.1 Clearing of Tropical Forests by Fire

In contrast to past centuries there is, at present, probably only a very limited transfer of carbon into the atmosphere by this process in developed countries of the temperate latitudes. It is important, however, in developing countries, especially in the tropics. Tropical forest destruction for recovery of timber should also be included here because, due to the extreme variety of tree species, only a small fraction of the total phytomass per area is utilized and thereby preserved from decomposition within a short period of time. The total amount of carbon transferred into the atmosphere is determined by the extent of the cleared areas and by the phytomass burned and mineralized per unit of area.

A. Cleared Areas

Due to unreliable statistics, our knowledge of the areas annually cleared is very limited. Though estimates from indirect evidence, such as from per capita fuelwood consumption (Adams et al., 1977), or from the expansion of cultivated land (Revelle and Munk, 1977), yield valuable additional information, it is urgently required that methods of direct observation, e.g. monitoring by aeroplanes and satellites, be further developed and extensively applied (see Chapter 14, this volume). Figures in the literature about areas annually cleared in the tropics vary from 110 000 km² (WWF, cited in Woodwell et al., 1978) and 120 000 km² (Bolin, 1977) to 300 000 km² (Brünig, 1977). It is also estimated that 12% of the area of tropical moist forests (17 x 10⁶ km² in 1950, according to Whittaker and Likens, 1975) is cleared each year (Woodwell and Houghton, 1977; Woodwell et al., 1978, quoted from Hamilton). This would amount to 170 000 to 340 000 km² per year. It is more, if the percentage is consistently applied also to tropical seasonal forests, as in another estimate of the authors mentioned.

There are several reasons for preferring the higher figures. Data derived from official statistics are for a number of reasons more likely to be near to the lower limit of the range of probable values. Forest fires are set up frequently by people only partly integrated into the economies of the respective nations. In other cases, authorities estimate forest area reduction simply from the volume of clearing concessions given to large enterprises, thereby leaving other types of possibly even more extensive clearings out of account. Statistics may even intentionally underestimate the data in view of the fact that tropical forest destruction is increasingly disapproved of by public opinion in the world. A further complication arises from the disagreement of the definition of forest and forest area between different countries and periods.

The results of more detailed analyses of the situation in separate parts of the tropical moist forest zone (Amazonia and Central America, Central and West Africa, South and South East Asia) indicate that the lower figure given for the whole earth may be too low. A frequently quoted figure for the rate of forest destruction in Amazonia alone is 100 000 km² /year (Myers, 1976; Sioli, 1977). Annual clearings in Africa are estimated at about 40 000 km² (Synnott, 1977). In South East Asia 30 000 km² of forest are cleared each year for the purpose of selective exploitation of timber only (Myers, 1976), let alone the additional clearings for subsistence agriculture.

A more reliable estimate of past and current changes of the tropical forest area can be gained if forested areas are compared at longer intervals of time rather than annually. In some countries such as Madagascar, the original vegetation has already been destroyed on 90% of the original area (Anonymous, 1975), a state of affairs most likely to be reached by many other tropical countries before the end of this century (see the literature cited in Breuer, 1977). In the Philippines, 40% of the original forest had been lost by 1976 (Boerboom, 1976, cited in Chapter 5, this volume). In 1950, the area covered with tropical rain forest in the world amounted to 17 x 10⁶ km² (Whittaker and Likens, 1975). In 1977, the area covered by predominantly evergreen humid tropical closed forest and open woodland is estimated at 20 x 10⁶ km², half of which is closed forest (Brünig, 1977). If `tropical rain forest' as classified by Whittaker and likens is roughly identical with `closed forest' in Brünig's terminology, the area of this vegetation type would have drastically decreased, probably in the order of 40%, from 17 to 10 x 10⁶ km² in 27 years, or at an average annual rate of 260 000 km². Of course, this comparison must be regarded with caution but the figures correspond well with those given by FAO/World Wildlife Fund, showing a world reduction of the area of rain forest from an original 15.92 to a current 9.35 x 10⁶ km² (cited in Woodwell et al., 1978). As this reduction continued over several decades it should be noted that the annual rate of reduction was certainly not constant duping the period concerned, but increased exponentially along with population growth and intensified economic development.

Reductions of the areas of tropical deciduous forests are not included in these estimates. This vegetation formation may have been and still is suffering even heavier losses since it is predominantly situated in more densely populated regions such as mainland South East Asia. Also, fires are more common and spread more easily. Ajtay et al. estimate the area of this forest type as 4.5 x 10⁶ km² (Chapter 5, this volume) in 1970, which compares with 7.5 x 10⁶ km² in 1950 estimated by Whittaker and Likens (1975). This indicates a significant change even if part of the difference is due to different methods of assessment. Clearing of savannas for agricultural purposes should not be neglected either. Even if the amount of carbon released per unit area is smaller than in forests, it may add up to quite a large sum because of the size of the areas involved. Finally, forests and woodlands are being replaced by ecosystems of subsistence agriculture not only in the tropics but also in temperate zones, as for example in southern South America.

B. Phytomass Burned by Area

Ajtay et al. (Chapter 5, this volume) summarize different estimates of the average carbon content in living phytomass of tropical rain forests. The discrepancy between various figures is not surprising in view of the paucity and low comparability of available data from which the estimates had to be derived. Some individual measurements show very high values for carbon in the total phytomass per area, e.g. 45 x 10³ g/m² (Fittkau and Klinge, 1973), and 34 x 10³ g/m² (Brünig and Klinge, 1975), assuming a carbon content of 45% in dry matter. The wide range of phytomass stocking in relation to site conditions is demonstrated by figures reported by Brünig (1974) from natural rain forest stands on a variety of sites in Borneo. The total carbon content in 55 research plots ranges from 11 x 10³ g/m² to 64 x 10³ g/m², averaging on mesic sites around 45 to 50 x 10³ g/m² . Other measurements indicate lower average values, mainly around 30 x 10³ g/m² of dry matter, equivalent to 13.5 x 10³ g/m² of carbon in the wood (Golley 1975).

For several reasons relatively low figures should be preferred for the purpose of estimating the amount of biomass eventually mineralized. Part of the forest destruction in the developing countries is taking place outside the equatorial forest and concerns other types of forest and woodland with lower phytomasses. Regarding the equatorial rain forest, it has to be assumed that part of the phytomass destroyed represents secondary forest with rather low phytomass per area. As was already mentioned, part of the phytomass is not burned but removed and used as timber, and even the rest is not converted to CO₂ entirely.

C. Carbon Released by Forest Destruction

If, with the present inadequate state of knowledge, an attempt at a preliminary estimate is made at all, it seems reasonable to combine relatively high estimates of the areas annually cleared with relatively low estimates of the phytomass burned per unit area. Including destruction of tropical seasonal forest and woodlands as well as forests outside the tropics, an area of 300 000 km² appears to be a reasonable estimate of the area of annual clearings. The amount of carbon released into the atmosphere per unit area may be assumed to equal 12 x 10³ g/m². The annual transfer of carbon to the atmosphere would then be 3.6 x 10¹⁵ _g.

7.2.2 Industrial Use of Wood for Paper, Timber, Log, etc.

Production of  log, pulpwood, and other wood for industrial use (except firewood) is estimated to be approximately 1350 x 10⁶ m³ in 1973/74 (Bolin 1977, after Persson; Chapter 6, this volume). This roughly corresponds to 0.25 x 10¹⁵ g of carbon. A considerable part of this is used for long-lasting structures and would therefore not cause an immediate input of carbon into the atmosphere. Only a certain fraction is rapidly converted into CO₂, mostly paper.

On the other hand, considerable amounts of phytomass are left as residue and waste after the extraction of timber and industrial wood (branches, bark, leaves, etc.), which causes an additional carbon flow into the atmosphere. This is not only the case in the tropical forest, as mentioned above, but also in the boreal forest, where the portion of the phytomass wasted may amount to 80% of the total aboveground phytomass (Brünig, personal communication).

The roles of the northern coniferous forest and, to a smaller extent, of the mixed forests in thinly populated areas of North America are difficult to assess. Though in northern Eurasia and North America there are, of course, no clearcutting operations resulting from population pressure, such as in the tropics, these forests are not unaffected by the expansion of human civilization. It is reported, that, possibly due to the apparent abundance of forest resources, their exploitation is often carried out in a particularly careless way (Adzhiev, 1976), to the effect that even low population density can result in a stress on natural resources, all the more so if large technical projects are initiated as in parts of Siberia.

The living phytomass per unit of area in the taiga is smaller than in temperate and tropical forests, ranging from 13.5 x 10³ g/m² at its southern border to only 5 x 10³ g/m² of carbon in the northern parts (Rodin et al., 1975; see also Rodin and Bazilevich, 1967). Not least for this reason we estimate the net carbon flow from all clearcutting and wood-producing activities outside the tropics including the decomposition of wasted material as well as the eventual decay of the utilized fraction at less than one-tenth of the amount in the tropics, or at 0.3 x 10¹⁵ g C per year, which may well prove too low.

7.2.3 Firewood

The amount and importance of firewood use is often grossly underestimated by researchers from industrialized countries, not only with respect to its role in the carbon cycle, but also in context of world energy systems and consumption trends in general. At least one-third of the world's population (1.3 x 10⁹) depends on firewood for heating and cooking. The estimated average consumption of these people is about 1 t of wood per person per year (Eckholm, 1976). Somewhat higher figures are reported in the literature, usually derived from case studies in selected countries (Openshaw, 1974; for more data see Revelle and Munk, 1977). We may estimate the rate of annual net release of carbon to the atmosphere as 0.3 x 10¹⁵ g, which is possibly an overestimate in so far as part of the material utilized, such as dead branches, would have decomposed readily if left in the forest. On the other hand, it may well be argued that considerably more than 1.3 x 10⁹ people are using firewood in the world.

7.2.4 Decomposition of Soil Organic Matter

Current estimates of the world's content of soil organic matter, including peat, range from 1 x 10¹⁸ g (Baes et al., 1976), through 1.2 x 10¹⁸ g (Bazilevich, 1974), and 2.1 x 10¹⁸ g (Chapter 5, this volume) to 3 x 10¹⁸ g (Bohn, 1976). As a large part of soil organic matter is excluded from exchange with the atmosphere in a short-time perspective, this part of the terrestrial organic carbon is not particularly relevant to the present problem. It is well established, however, that man can reduce this pool by converting natural soils into agricultural soils, especially by draining formerly wet soils, thus exposing humus and peat to oxygen, or by establishing agriculture in areas formerly covered by forest or steppe; or by clearcutting large areas of forest for timber. Soils of this kind may lose half of their carbon content by cultivation (Paul, 1976). However, extrapolations from individual measurements, which in each case refer to particular soil types and to particular agricultural systems, should only be done with extreme caution.

In industrial countries, substantial losses of soil organic matter were caused by cultivation in earlier periods of history. Intensification of cropping on soils already under cultivation the dominant feature in most of these countries today does not in general lead to further losses of humus. In the tropics and subtropics, however, decomposition of soil organic matter is certainly significant, due to the spread of subsistence agriculture to formerly uncultivated areas. It was formerly believed that tropical soils have generally a low organic matter content, but more recently it has been shown that in many cases this assumption is ill-founded (Sanchez and Buol, 1975) and only applies to certain soils and circumstances. Yet even if we assume figures that are too low, a considerable net release of carbon will result from human interference. If the organic matter content of tropical forest soils is assumed to be only 0.5% by dry weight, and it is assumed that one-half of the carbon of the upper 30 cm is oxidized after forest clearing, this would result in a transfer to the atmosphere of almost 0.4 x 10¹⁵ g of carbon each year. In view of the destruction of peat, draining of wetlands, and the losses of humus outside the tropical forest, this figure should be raised to 0.6 x 10¹⁵ g, which still may be a conservative estimate.

7.3 TRANSFERS FROM THE ATMOSPHERE TO THE LAND BIOTA 

7.3.1 `Fertilizing' Ecosystems with CO₂

Since a world-wide stimulation of photosynthesis, resulting from atmospheric CO₂ enhancement, has first been suggested (Machta, 1972; Bacastow and Keeling, 1973), it has become apparent that the reaction of crop plants to variations of the atmospheric CO₂ mixing ratio is a rather complex matter. There are a number of different responses, relative to the photosynthetic mechanisms operating in crop plants and to the different ways in which they control their stomatal activities (Chapter 8, this volume). This is presumably true also for wild species. At present it is impossible, therefore, to establish whether the terrestrial biota of the earth responds to a higher CO₂ content of the air or not. What is possible, though, is to trace some lines of evidence suggesting how plants might possibly react.

Serious doubts have been expressed by plant physiologists with regard to a net transfer of carbon from the atmosphere to the land biota consequential upon an increased CO₂ mixing ratio (Lemon, 1977; Chapter 8, this volume). A net transfer of this kind could be expected only if the current atmospheric carbon content would effectively limit plant growth on earth as a whole. However, most large plant formations are subject to shortages of growth factors other than carbon; examples are nitrogen, other minerals, water, and temperature. Especially, water stress may prevent plants from taking advantage of an enhanced CO₂ supply since it usually happens when temperature and light are in optimal supply. Water stress is much more a common feature in ecosystems than is often assumed; it occurs not only under semi-arid conditions but also in the humid temperate zones during summer (Chapter 8, this volume), and even the canopy of the equatorial evergreen forest is affected for several hours per day (Walter, 1973).

However, in view of the complex interactions of growth factors during phytomass synthesis, atmospheric carbon may have some influence even if water, nutrients, and temperature are likewise in short supply. Carbon may be limiting during certain hours of the day and its higher availability may indeed result in some growth acceleration. Yet a net transfer of carbon also requires that an increased phytomass production will not be balanced by an equal increase in decomposition via respiration. In Table 7.1 the current concept of energy and matter accounting in ecosystems is briefly depicted (Woodwell and Whittaker, 1968; see also Reichle et al., 1975). Gross primary production (GPP) is the total organic matter production of an ecosystem through photosynthesis within a given period of time. Subtracting respiration of the plants for self-maintenance (R_a) one gets net primary production (NPP), which is available for respiration of heterotrophic organisms (R_h). If R_h is smaller than NPP, the remainder is stored in the phytomass (or in soil organic matter), representing a net increase of the standing crop (net ecosystem production (NEP). It depends on the type of an ecosystem whether heterotrophic respiration is carried out mainly by animals (grazing food chain) or by microorganisms (detritus food chain). In forests the latter are far more important, therefore annual R_h is of the same order of magnitude as annual litter production.

NEP of forests and other permanent vegetation is positive as long as the phytomass is increasing, whereas it becomes zero in the so-called climax state. Whether climax forests can be stimulated to resume net growth once there is more CO₂ available is not known. Certainly there are structural limitations beyond which  phytomass in mature forests cannot be increased even under optimal growth conditions. These limits are determined by the availability of light in the interior of the canopy, the maximal height of trees with regard to static stability and water conductance, etc. Since climax forests are always heterogeneous with regard to size and age of the trees, it is conceivable that they respond temporarily to enhancements of the CO₂ content, until after some decades a new equilibrium is reached and NEP drops back to zero.

Table 7.1 Simple concept of energy and matter accounting in ecosystems (imports and exports are disregarded)

	GPP		Gross primary production
	Ra		Respiration of autotrophs
			(dark respiration)
	NPP		Net primary production
	Rh		Respiration of heterotrophs
			(consumers and decomposers)
	NEP		Net ecosystem production
			(net change in living and dead
			phytomass, litter, zoomass and
			soil organic matter)

Net ecosystem production is likewise zero in ecosystems where, for various reasons such as irregular water supply, periodic disturbance by flooding, or, most importantly, human influence, permanent vegetation does not develop and the plants remain short-lived. These ecosystems cannot cause a significant net withdrawal of carbon from the atmosphere no matter how the plants respond to an enhanced atmospheric carbon content, since the phytomass produced additionally is soon decomposed. This clearly applies to agriculture. Yet it appears that these plants above all could benefit from a CO₂ fertilization, as proven by numerous experiments (reported in Wittwer, 1974). In intensive agriculture, plants enjoy absence from heavy shortages of other growth factors. Also stomatal action in non-perennial plants, especially if adapted to sunny environments, seems to favour the CO₂ uptake into the substomatal cavity even at the expense of considerable water losses, in contrast to the behaviour of many trees (Larcher, 1973).

One group of ecosystems would certainly be able to cause net transfers of carbon from the atmosphere if they responded to a higher atmospheric CO₂ content in an accelerated growth rate: immature forests, as well as other early and intermediate successional stages following commercial exploitation or other disturbances of vegetation and/or site by man. This is all the more the case if these systems grow on productive substrates, thereby being exempt from nutrient shortage. Not only closed forests or initial stages of forests are relevant here, but also the innumerable trees, bushes, hedgerows, etc., in parks, towns, and along roadsides, forming part of the cultivated landscape all over the world. It is a general feature of the expansion of human civilization on earth that not only ecosystems in their earliest successional stage are favoured as in the case of agriculture but that also half-grown permanent vegetation is maintained everywhere at the expense of climax vegetation. We shall see later that these plant formations are presumably responsible for quite large withdrawals of carbon from the atmosphere anyway. Whether this is further reinforced by an increase of the carbon content of the air is not clear.

The preliminary conclusion drawn here is that the two preconditions for a net withdrawal of carbon through photosynthesis stimulation by atmospheric CO₂ increase are seldom met at the same time: either plants are sensitive to CO₂ enhancement, but, because of their short lifetime cannot store the assimilates added; or they could store them but seem to react insufficiently to CO₂ enhancement. There appears no way to confirm any of these hypotheses at present. All we know is that in the northern hemisphere the seasonal oscillations of atmospheric CO₂ have not significantly changed in amplitude during the past 15 years (Hall et al., 1975), which means that the ratio of semiannual net photosynthesis to semiannual net respiration remained unchanged. Analyses of tree-ring width should theoretically be capable of yielding information on changes in NEP but the intricacies connected with that technique (for a detailed description see Fritts, 1976) must be kept in mind. It is doubtful that any statistically significant data could be achieved at present.

Several attempts have been developed to date aiming at modelling the terrestrial biota's response to variations in atmospheric CO₂. They either assume CO₂ excess and growth acceleration to be proportional (Machta, 1972); or increase in growth to be logarithmically proportional to the CO₂ increase (Bacastow and Keeling, 1973; Revelle and Munk, 1977); or an increase that follows the MichaelisMenten kinetics well known from enzyme biochemistry (Kohlmaier et al., 1978). Other parameters such as the photosynthetically active phytomass (Revelle and Munk, 1977) and the area covered by forests (Siegenthaler et al., 1978) are also taken into account (for problems of modelling the biota see also Keeling, 1973).

Approaches such as these are indispensable for long-term projections of the future carbon cycle and will be very valuable once the parameters upon which the calculations are to be based are better known than they are today. For a short-term analysis the specific type of a biota growth function is of less importance, since for relatively small variations in atmospheric CO₂ every such function can be approximated reasonably by a linear relationship. The `biota growth factor' is assumed in the literature to be between 0 and 0.4, expressing the fact that for a 1% increase in atmospheric carbon dioxide; NPP will increase by 0 to 0.4% (Keeling, 1973; Bacastow and Keeling, 1973; Revelle and Munk, 1977; Siegenthaler and Oeschger, 1978). If, in view of the above explanations, we assume a `biota growth factor' of only 0.05 (the minimum estimate by Revelle and Munk, 1977), a current CO₂ excess of about 14% (Chapter 3, this volume) and a global annual NPP of 53 x 10¹⁵ g C on the land, photosynthesis today would be larger by 0.05 x 0.14 x 53 x 10¹⁵ = 0.37 x 10¹⁵ g C, relative to preindustrial conditions.

7.3.2 Fertilizing Ecosystems with Plant Nutrients other than Carbon

Carbon accumulation in world biomass may increase if the supply of other limiting factors is increased by human interference. Agriculture, however, where this is mainly practised, cannot contribute significantly to eliminating carbon from the atmosphere because most of the crops are quickly decomposed by respiration in animals and man. Mineral fertilizers should not be considered therefore. As the supply of growth factors in non-agricultural ecosystems is, so far, rarely subject to intentional interference by man, we have to look for unintentional interference, such as global increase in temperature, rainfall, or nutrient supply. Only the latter is considered here. It has been suggested that terrestrial ecosystems might be inadvertently fertilized by NO_x  pollution of the atmosphere (Broecker, 1977; Revelle and Munk, 1977). Since plant growth on the land is often limited by the availability of nitrogen even in ecosystems where growth may be considered to be primarily restricted by water shortage (Chapter 8, this volume) an additional input of nitrogen from the air may well have a certain effect. It should be borne in mind, however, that in many cases decomposition of organic matter in the soil is equally stimulated by enhanced nitrogen supply. It has been shown, furthermore, that separating stimulatory impacts of air pollution on ecosystems from growthsuppressing effects is very difficult (Smith, 1974), and that the latter might have become important in temperate forests (Likens et al., 1977).

Annual nitrogen fixation by combustion of fossil fuel is estimated to be 19 x 10¹² g per year (Söderlund and Svensson, 1976). If 20 to 25% of this amount is incorporated into additional terrestrial phytomass with an average C/N ratio of 50, roughly 0.2 x 10¹⁵ g of carbon will be withdrawn from the atmosphere annually. There is at present no possibility to prove estimates of this kind empirically, though it seems, rather, to indicate the upper limit of the possible effect.

7.3.3 Creation of Ecosystems where Phytomass Can Accumulate

Several human activities are likely to cause large removals of carbon from the atmosphere by photosynthesis on the land, even if there is no response to an enhanced CO₂ mixing ratio. One such activity is reforestation, planting trees on sites where they would not otherwise spread within a reasonable period of time. The amount of carbon withdrawn from the atmosphere by reforestation is estimated by Bolin (1977) to be about 0.3 x 10¹⁵ g per year. This figure covers only the areas where forest successions are intentionally initiated by man. Man also creates biotopes where forests and woodlands develop spontaneously, e.g. on land where agriculture is abandoned and on forest clearings in the tropics not successfully converted to agricultural land or pasturage.

In order to get an idea of the magnitude of this effect it may be assumed that one-third of the annual forest clearings in the tropics, or 100 000 km² per year, are reinvaded by secondary forest with an average lifetime of 10 years before being cut again. If NEP is estimated to be 2.2 x 10³ g/m² of dry matter, equivalent to 990 g/m² of carbon, there results an annual net withdrawal of carbon from the atmosphere of 1 x 10¹⁵ g. Evidently, even assuming a very high regrowth rate, as done in this example, the carbon budget of tropical forests cannot be balanced at present, since the annual losses, as estimated above, amount to 3.6 x 10¹⁵ g.

A third kind of human impact to be discussed here is maintenance of half-grown permanent vegetation, as mentioned above. Immature forests in the temperate regions may be particularly important. Even if they do not always have a higher NPP than natural forests would have in the same site, they are often characterized by higher NEP than near-climax stages since on the average they are not full-grown. Forests in many parts of the temperate zone have increased in phytomass during the last century and this trend may well persist at present. Numerous analyses of broadleaved and mixed temperate forests have shown that NEP may amount to 3060% of NPP (Möller et al., 1954; Whittaker and Woodwell, 1968; Duvigneaud and Denaeyer-De Smet, 1970; for more literature see Duvigneaud, 1971). This fraction of the NPP is stored in the biota as wood. Since there are certainly many forests in the temperate zone not equally distinguished by such a high NEP/NPP ratio for instance, the one analysed by Harris et al. (1975) it may be assumed here that only 20% of the temperate forests in the world are fast-growing. Given an area of 2.4 x 10⁶ km² for this fraction of all temperate forests, an NPP of 540 g C/m² per year, and a NEP/NPP ratio of 0.4, there results an annual net storage in wood of 0.5 x 10¹⁵ g of carbon. The same figure is reported by Woodwell et al. (1978), derived from a different set of assumptions.

In summary, man's influence on the successional state of ecosystems as represented by the three examples mentioned could lead to an annual net storage of carbon in the terrestrial vegetation in the order of 1.8 x 10¹⁵ g.

7.4 RESULTS AND CONCLUSION

The man-induced net transfer of carbon between the land biota and the atmosphere per year, as estimated in this contribution, is indicated in Table 7.2. It has been mentioned already that the figures do not represent well-founded estimates but are just reasoned guesses about the order of magnitude, and some of them may even fail in that. None the less, there is reason to believe that the resulting figure for the net transfer is somewhat biased to the lower side since the items indicating transfers from the biota to the atmosphere may in part be underestimates, which is less likely for most of the figures describing transfers in the opposite direction. The range of possibilities should be assumed to be 1.54.5 x 10¹⁵ g C/year, the most probable interval being 2.53.0 x 10¹⁵ g. In Table 7.3 this estimate is compared with the estimates of other authors.

Table 7.2 Man-made transfer of carbon to the atmosphere from the land biota

Activity	1015 g C/year
Forest clearing in the Third World	+3.6
Industrial use of wood	+0.3
Firewood	+0.3
Soil organic matter decomposition	+0.6
Reforestation	0.3
Regrowth in the tropics	1.0
Regrowth in temperate regions	0.5
Growth stimulation by CO2	0.3
Growth stimulation by NOx	0.2
Sum	+2.5

The gross transfer to the atmosphere amounts to 4.8 x 10¹⁵ g C/year, or about as much as the annual input from fuel in 1976 (Chapter 3, this volume). But 2.3 x 10¹⁵ g or almost 50% are recycled to the land biota. The dominant cause for this withdrawal appears to be man's activity in favouring the initial and intermediate stages of permanent vegetation, whereas the increase in atmospheric CO₂ or the higher availability of other nutrients, both potentially resulting in an accelerated growth rate, might be of secondary importance as yet. In other words, there need not be any increase in NPP due to fertilizing effects whatsoever; there need only be an increase in NEP at the expense of heterotrophic respiration in order to explain a permanent flux of carbon to the land biota.

Table 7.3Estimates for transfers of carbon to the atmosphere from non-fossil sources

Authors	1015 g C/year	Remarks
Woodwell et al. (1978)	218	Net transfer
	48	(most probable)
Brünig (1977)	6	Tropical forest clearing
		only, calculated from the
		author's figures of areas
		and phytomasses
Woodwell and Houghton (1977)	2.520	Net transfer
	5	(most probable)
Given estimate	1.54.5	Net transfer
	2.53	(most probable)
Adams et al. (1977)	0.44	Estimated from wood
		consumption per
		capita
Wong (1978)	1.9	Net transfer
Bolin (1977)	0.41.6	Net transfer

The largest portion of the net transfer is caused by direct and indirect human influence on the living phytomass in forests. Yet soil organic matter may account for 2025% of the net transfer, which might be even underestimated. The roles of wetlands and freshwater ecosystems have not been considered here. Semiterrestrial ecosystems such as marshes and peat-bogs may be a noticeable source of carbon once they are drained, whereas lakes, streams, estuaries, and coastal areas of the sea are sometimes considered to be quite effective sinks. Significant amounts of carbon may be transported to the sea sediments as dissolved humus in blackwater streams such as the Rio Negro in the Amazon system.

Regarding the three major forest biomes in the world, it is clear that the tropical forest though not exclusively the equatorial rain forest is the most important. Even allowing for extensive regrowth in parts of the clearcut areas, the fluxes to and from the atmosphere cannot be balanced and a net transfer to the atmosphere of at least 2 x 10¹⁵ g, more probably of 3 x 10¹⁵ g of C per year is almost certain, judging from geobotanical observation. In contrast, the temperate forests are an effective sink whose importance may be even greater than estimated here. In spite of intensive logging, the annual gains in phytomass may well outweigh the losses due to harvesting and industrial processing of wood. For the northern coniferous forest (taiga) the picture is unclear. Since plant growth in the boreal zone is slow, its impact as a sink is limited in the short run. It cannot be excluded, however, that the taiga is a more important source to the atmosphere than assumed here.

Though much fruitful work can be done in the future, it is doubtful that direct accounting of carbon transfers, as attempted here, will alone be sufficient for yielding convincing results, however carefully it is carried out. The problem has to be tackled by various approaches which, from different points of departure, may lead to converging results. One such approach, independent from geobotanical observation, is isotope analysis of tree rings, where changes in the ¹³C/¹²C ratio of the air can be traced over many decades (additionally to the references quoted below, see Freyer and Wiesberg, 1974; Galimov, 1976; Rebello and Wagener, 1976; Tans, 1978; Freyer, 1978b; Chapter 3, this volume). Clearly, the limited number of investigations carried out hitherto do not permit final conclusions. Yet some of the analyses available to date support the idea of a net flux of carbon from the land biota to the atmosphere during the last century. Wagener (1978), Stuiver (1978), and Freyer (1978a) calculated cumulative net fluxes of carbon from the land biota of 208 x 10¹⁵ g, 120 x 10¹⁵ g, and 70 x 10¹⁵ g respectively, since the middle of the last century. Still less data are available on the current annual net transfer, but it is noteworthy that Wagener (1978) and Freyer (1978) preliminarily calculated an annual net flux from the biota of 3 ± 2 x 10¹⁵ g C, well within the range of the estimate given here. Though this coincidence is encouraging, it still requires corroboration by further studies.

The idea that the land biota continues to be a sizeable net source of carbon for the atmosphere is inconsistent with the current concepts of oceanic CO₂ uptake and mixing (Broecker et al., 1971; Bacastow and Keeling, 1973; Oeschger et al., 1975; Oeschger and Siegenthaler, 1978; Siegenthaler and Oeschger, 1978; Chapter 15, this volume). Though the models developed to date could account for a net transfer from the biota to the atmosphere in the past (Siegenthaler et al., 1978), in general they require the land biota to be at least neutral, or they even require it to be a net sink of carbon at present, since the ocean is considered to absorb only a relatively small fraction of the annual input of fossil CO₂. The annual CO₂ increase in the atmosphere is 2.9 x 10¹⁵ g C (58% of the annual fossil fuel input; see Chapter 3, this volume). If there is a net transfer of about 2.5 x 10¹⁵ g C/year from the biota, additionally to the 5 x 10¹⁵ g/year originating from fossil fuel, and if there is no other significant sink for CO₂ known to date, the ocean has to withdraw 4.6 x 10¹⁵ g C/year from the atmosphere, the airborne fraction being only 39%. This obviously exceeds the ocean's absorption capacity as conceived by most of the current geochemical models.

It is an intriguing question whether the evidence from oceanographic measurements and computations or from direct observation of the earth's vegetation carries more weight. Since the validity of the concepts of oceanic mixing was first questioned by ecologists (Woodwell and Houghton, 1977), oceanographers have been successful in corroborating their ideas (see for instance Siegenthaler and Oeschger, 1978). It is noteworthy that models of quite different design such as box models (Bacastow and Keeling, 1973; Niehaus, 1976), box diffusion models (Oeschger et al., 1975), and also models accounting for advective mixing (Chapter 15, this volume), all come to the conclusion that the biota cannot be a source of carbon at present. In contrast, each of the results presented in this contribution can much more easily be called into doubt with reference to the inadequacy of available data, and more generally, in considering the immense heterogeneity of the terrestrial biota.

Yet we consider the ecologist's arguments in this dispute as not irrelevant. Over thousands of years, man's influence has resulted in a steady decrease in the amount of carbon fixed in wood and in soils. In history, this trend has been countervailed temporarily and locally, examples being the expansion of the forested area during the Thirty Years' War in Central Europe (161848), or the more recent recovery of European forests from their generally poor condition until the nineteenth century, which, as mentioned above, may still be going on at present. Being fully aware of the possible impact of these countervailing effects, it is still hard to believe that the general trend has been reversed completely: that during millenia of human history, distinguished by low population numbers and a low standard of technology, there was a steady net transfer of carbon from the biota to the atmosphere, whereas presently, under the impact of explosive spread of civilization and population pressure unpreceded in history, there should be no such transfer.

The controversy between geobotanists and geochemists as to where all the carbon man is currently releasing goes to and why the atmosphere's carbon budget is not perturbed even more seriously in consequence of the releases from fossil and non-fossil sources (Woodwell et al., 1978), is unsettled. From direct observation of the world's vegetation it has to be concluded that it is most probable that at present the land biota is a net source of carbon of considerable size, that, due to coincident effects poorly understood, it may well be neither a source nor a sink in a short-term perspective, but that it is extremely improbable that it is a net sink at present and in the decades to come.

ACKNOWLEDGEMENT

I am indebted to Prof. Brünig (Hamburg) for valuable suggestions. 

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