SCOPE 13 - The Global Carbon Cycle

ABSTRACT

A general survey is made on various terrestrial ecosystems with respect to carbon flow in and out of the individual system. Emphasis is placed on: (i) tropical, (ii) temperate, and (iii) boreal forests. It can be shown that modern forestry and agriculture techniques significantly reduce the standing crop of biomass in the world's forests; furthermore, oxidation of soil organic matter is accelerated inasmuch as this reduction is not matched by an increase in the rate of photosynthesis in, for example, virgin forests; thus the CO₂ released by man's `cultivating' activities should ultimately end up in the atmosphere or the ocean.

6.1 INTRODUCTION

Due to population explosion and industrialization, man is exploiting the natural resources of earth. There is a risk of man altering the equilibrium in nature due to, for example, the increasing combustion of fossil fuels.

Man's influence on the temperate forest has been far-reaching. These forests still constitute an important resource for human use. Several of the world's most densely populated areas are located within the temperate forest zone, for example large parts of Europe and the U.S.S.R., large parts of the United States and Canada, as well as much of Japan and China. In these densely populated areas, much of the land has now been transformed from forests into various manmade ecosystems. Yet there are vast areas of forest still in existence, for example in Eurasia, in North America, and also in southern Chile, Tasmania, and New Zealand.

Until a century ago, man used the forests of the temperate zone primarily for fuel and building material. Many forests were also grazed. Even though ancient civilization first developed in some subtropical and Mediterranean areas, there is evidence that man already exerted a strong influence on temperate forests several thousand years ago, during the stone age. Many of the forests in Europe were, for instance, subject to shifting cultivation during the late stone age. In addition to direct cultivation, land was cleared in order to increase pastureland. One of the consequences of this land use had been the formation of extensive heathlands in many of the countries around the North Sea, as well as in western France. An analogous development formed the `maquis' and `garrigue' of the Mediterranean countries (Tamm, personal communication).

In many areas, man took advantage of the forests' ability to store large amounts of organic material and nutrients in their biomass and litter layer. Litter was often used as a kind of manure, usually mixed with dung from stables. This was usual in large parts of central Europe. All these activities resulted in an increased liberation of CO₂ from the forests.

During this century, man has begun to exploit other parts of the world, for example the tropical rain forests: These forests are large pools of carbon, and an altered equilibrium here leads to an accelerated flow of carbon into the atmosphere and a decreased assimilation of CO₂ by the rain forests. Most of the nutrients in a tropical rain forest are bound up in the biomass; cutting and utilization of wood from this ecosystem will cause severe impoverishment of the soil. Mires and other peatlands are normally large sinks for carbon. An increased utilization of peat for fuel and soil improvement, as well as draining of bogs, can have severe ecological consequences. New agricultural methods can also alter the pool of carbon in the soil and increase the CO₂ liberation into the atmosphere. Combustion of garbage and sludge will have negative effects on the carbon balance, inasmuch as most of the carbon will be converted to CO₂.

6.2 POOLS OF CARBON IN THE TERRESTRIAL ECOSYSTEM 

6.2.1 Vegetation 

There have been many attempts to estimate the net primary production for the world. Whittaker and Likens (1975) estimate the net primary production for land biota at 105 x 10¹⁵ g dw*/year (Table 6.1). This corresponds to 47.9 x 10¹⁵g C. The estimations for sea production are about 55 x 10¹⁵ g dw/year. To obtain the values for carbon in Table 6.1, the values for dry matter were multiplied by 0.45. The values are net production, i.e. the amount of organic matter remaining after respiration by the photosynthesizing plants.

Gross primary production is about twice the net primary production for the biosphere as a whole. The fraction of the gross productivity respired by plants can be as high as 5075% in many forests, especially in the Tropics. In many other ecosystems, especially in the sea, respiration amounts to 2040% of the gross production (Whittaker and Likens, 1975).

Lieth (1975) has estimated net primary production in terrestrial ecosystems at 100.2 x 10¹⁵g dw. In Baes et al. (1976) a lower value is given for terrestrial net primary production,about56 x 10¹⁵g/year. Bazilevich et al. (1971) have estimated the total potential phytomass of land at 2.4 x 10¹⁸ g dw. The bulk of this organic mass is in the tropical zone (56%), followed by the boreal (18%), subtropical (14%), sub-boreal (12%), and polar (1%) zones. The majority of phytomass is concentrated in forests (82%). For example, in moist tropical forests, according to Rodin and Bazilevich (1964), the accumulation is approximately 5.0 x 10¹² g/m² or more. The values for subtropical forests and broadleaved forests of the temperate zone are about 0.370.41 x 10¹² g/m². The biomass in northern spruceforests in the taiga regions amounts to about 1.0 x 10¹² g/m². In tropical forests the assimilating parts of the plants account for a large amount, about 0.4 x 10⁸ g/ha or 8% of the biomass. In subtropical forests, the figure is 1.2 x 10⁷ g/ha or 3% of the biomass. The broadleaved forests of the temperate zone have about 0.40.5 x 10⁷ g/ha, about 1% in the assimilating parts, while the amounts for the temperate coniferous forests are about 0.81.7 x 10⁷ g/ha or 58% of the biomass (Rodin and Bazilevich, 1964).

Table 6.1 Primary production and biomass estimates for the biosphere (after Whittaker and Likens, 1975)

The total potential primary production of land is estimated to be 172 x 10¹⁵ g/year. The tropical belt produces 60% of this total, subtropical 20%, sub-boreal 10%, boreal 9%, and polar 0.8%. Forests produce 49% of primary production (Bazilevich et al., 1971). Total phytomass in the world's oceans amounts to 0.17 x 10⁵ g, which is about 15 000 times lower than that of the land, although the productivity is much higher. The total primary production of the oceans is estimated at 4772 x 10¹⁵g/year (Steeman Nielsen et al., 1957; Koblentz-Mishke et al., 1968; Bogorov, 1969). Liebig (1862) made an estimation of what world production would be if the world's surface were covered by a moderately productive meadow (500 g dry matter/m² per year). The data on which the other early estimations were based was too limited for them to be relevant (Whittaker and Likens, 1975). For more detailed discussion on global primary productivity see Ajtay et al. (Chapter 5, this volume).

6.2.2 Fauna

Only very small fractions of all the terrestrial carbon pools are represented by animals. Whittaker and Likens (1975) estimate the total biomass of animals in the world at 906 x 10¹² g C. About 457 x 10¹² g C is bound in continental animals. The annual animal production is about 372 x 10¹² g C for continental fauna and about 138 x 10¹² g C for marine animals. The total herbivore consumption in the world is about 124 x 10¹² g C/year. The continental animals consume about 32.6 x 10¹² g C/year (Whittaker and Likens, 1975). Data on efficiencies of food-use by invertebrate animals is limited and much data on vertebrates is of doubtful applicability to populations under field conditions. Estimations give the somewhat surprising result that the secondary production is less than 1% of the net primary production on land, and 56% in the sea. In most terrestrial ecosystems, animal biomass is concentrated in invertebrates rather than in vertebrates. Vertebrate biomass may, however, exceed invertebrate biomass in grasslands, due to populations of large grazing mammals (Whittaker and Likens, 1975).

6.2.3 Microorganisms

In order to estimate the amount of carbon bound in microorganisms, a mean value of 100 g/m² (dry weight) for microbial biomass was used in most soils (Rosswall, 1976). Microorganisms have a carbon content of about 50% (Rosswall, personal communication). If we approximate the total microbiologically active area of the world at 135 x 10¹² m², we can then estimate the total amount of carbon bound in microbial biomass at 6.8 x 10¹⁵ g C.

6.2.4 Soil Organic Matter

The decay of soil organic matter is one of the largest CO₂ inputs to the atmosphere. The mass of organic carbon in the world's soils, which is often quoted, is based on the carbon contents of nine U.S.A soils (Twenhofel, 1926). An extrapolation of these values for the whole world's soils give 710 x 10¹⁵ g C (Rubey, 1951). More recent and more extensive data, such as the FAOUNESCO world soil map, should give a more accurate estimation of the organic carbon in the world's soils. These FAOUNESCO values indicate that the amount of soil organic carbon in the world is about 3000 x 10¹⁵ g (Bohn, 1976). Bohn has calculated organic carbon content based on a soil depth of 1 m. This will, in some instances, give an overestimation, but Bohn argues that many soils are much deeper than 1 m, and this is a realistic mean value. The largest quantity of litter and surface humus is found in shrubby tundras (0.84 x 10¹² g/m²) followed by spruce forests in central, southern, and northern taiga (0.45 x 10¹², 0.45 x 10¹² , and 0.30 x 10¹² g/m² respectively). Broadleaved forests have about 0.15 x 10¹² g/ha, closely followed by subtropical forests with 0.10 x 10¹² g/ha. The steppes often have about 0.062 x 10¹² g/m². In the tropical rain forests, the amount of litter and surface humus is very small, only about 0.020 x 10¹² g/m². In savannas it is even smaller, about 0.013 x 10¹² g/m² (Rodin and Brazilevich, 1964). Bohn (1976) has estimated the total organic carbon content in soils in South America at 301 x 10¹⁵ g C, in North America at 665 x 10¹⁵ g C, and in the rest of the world at 1980 x 10¹⁵ g C.

6.3 CHANGES INDUCED BY MAN

6.3.1 Forests

A. The Effects of Forestry on the Carbon Cycle

In most countries in the world, the total wood production has increased due, for example, to the use of fertilizers, draining of swamps, and rationalization. The estimates of the total global area of forests are, however, somewhat uncertain. Only 46% of the world's forests are confirmed by reliable values. The estimates are insufficient for 33% of the world's forest areas (Persson,1974).

The world's total area of forests has been estimated at 2.8 x l0⁷ km². This corresponds to 22% of the global land area. Coniferous forests have been estimated at about 40% of the total forest area (Table 6.2) (Persson, 1974; Skogsstyrelsen, 1976). There are about 300 x 10⁹ m³ of wood in the world's forests with a diameter (1 m above ground) of more than 2030 cm for deciduous trees and 510 cm for coniferous trees. This would correspond to about 37.5 x 10¹⁵ g C. However, it is to be noted that the calculated amount of carbon in the tree-stems only constitutes a minor part of the total carbon pool in the forest ecosystem. The part of the carbon pool which is removed from the ecosystem when cutting the trees, will probably sooner or later be mineralized or burnt.

Table 6.2 The amount of stem wood in the forests of the world and forestry cutting (from Skogsstyrelsen, 1976)

Figure 6.1 gives the changes in volume of cutting between 1950 and 1973 (roundwood). During this period cutting increased by about 80%. The increase varies in different parts of the world and is most marked in Asia at little over 200%, but is only about 2025% in Europe and North America. The above increase in cutting can mainly be explained by the fact that roundwood cutting in several countries in the Far East is now much better reported.

The annual cuttings of coniferous and deciduous forests are shown in Table 6.3. According to Table 6.2 the share of industrial wood in North America was approximately 96%, while in Africa and Asia (except U.S.S.R) it was only 14% and 29%, respectively. Most of the wood in Africa and Asia is used for fuel and will thus rapidly be converted in CO₂.

The cutting of fuel wood has decreased in industrialized countries, while it remains almost unchanged in Africa and increased considerable in Asia (except U.S.S.R.) (Figure 6.1(b)). This is an average which is not relevant for Japan, since it is a highly industrialized country (Skogsstyrelsen, 1976). The increased cutting of fuel wood would result in a global increase in fuel wood consumption from 1025 x 10⁶ m3 in 1961 to about 1120 x 10⁶ m³ in 1970. This means an increase for the type of wood which is rapidly converted to CO₂. At the same time, the amounts of wood used for pulp and paper production have rapidly increased (Figure 6.1(c)). Thus, an increasing amount of the organic matter in the world's forests is converted to CO₂ rather rapidly. However, the timber and pulp production fluctuates over the years due to the state of the market. Thus, the production of pulp and paper in the major producing countries fell by 13% in 1975 (Anon., 1976).

Figure 6.1 The global cutting (a), cutting of fuel wood (b), and cutting of pulp wood (c), all figures in 10⁶ m³ solid volume under bark (after Skogsstyrelsen, 1976. Reproduced by permission of the National Board of Forestry, Sweden.)

Table 6.3 Removals in certain countries with extensive logging in 1973. (Skogsstyrelsen, 1976)

	Coniferous			Broadleaved
	industrial			industrial			Total	Total
	wood	fuel wood	total	wood	fuel wood	total	1973	1972
Country	million m3 solid volume under bark
Argentina *	0.6		0.6	2.5	8.0	10.5	11.5	12.7
Australia	2.7		2.7	8.7	0.7	9.4	12.1	14.1
Austria	9.6	0.29	9.8	1.4	0.7	2.1	11.9	12.5
Brazil*	11.1	15.0	26.1	12.7	125.0	137.7	163.8	163.8
Canada*	111.4	1.2	112.6	9.4	2.2	11.6	124.2	119.7
China*	28.5	54.0	82.5	18.5	82.2	100.7	183.2	179.0
Columbia*	0.0		0.0	4.9	20.0	24.9	24.9	26.8
Czechoslovakia	10.6	0.7	11.3	3.0	0.8	3.8	15.1	14.7
East Germany	7.2	0.8	8.0	1.4		1.4	9.4	7.9
Ethiopia*	0.2	2.8	3.0	1.1	20.2	21.3	24.3	24.2
Finland*	29.9	1.4	31.3	5.7	5.9	11.6	42.9	42.9
France*	13.7	1.0	14.7	4.8	4.8	19.2	33.9	33.9
Great Britain	1.6	0.2	1.8	0.8	0.1*	0.9	2.7	2.5
India*	1.4	3.2	4.6	9.2	102.8	112.0	116.6	116.6
Indonesia*	0.1		0.1	27.1	104.0	131.1	131.2	120.0
Italy	1.1*	0.3*	1.4*	5.0	5.3	10.3	11.7	13.1
Japan*	25.6	0.0	25.6	17.5	1.5	19.0	44.6	46.2
Mexico	5.0	2.6	7.6	0.5	6.0	6.5	14.1	14.2
New Zealand*	8.0	0.5	8.5	0.2	0.1	0.3	8.8	8.8
Nigeria*				3.0	56.8	59.8	59.8	59.8
Norway	7.5	0.2	7.7	0.4	0.5*	0.9	8.6	8.3
The Philippines*	0.0		0.0	13.8	21.1	34.9	34.9	33.1
Poland	15.6	0.9	16.5	4.5	0.9	5.4	21.9	18.8
Portugal	4.9	0.3	5.2	2.0	0.2	2.2	7.4	7.0
Romania	6.5	0.4	6.9	9.6	4.9	14.5	21.4	21.2
South Africa	4.4	0.1*	4.5	5.1	0.9*	6.0	10.5	9.6
Spain	5.6	2.1	7.7	3.2	5.9*	9.1	16.8	16.0
Sweden	50.5	1.2	51.7	4.4	1.9	6.3	58.0	58.0
Tanzania*	0.1	0.0	0.1	1.1	31.5	32.6	32.7	32.7
U.S.A.*	269.7	2.5	272.2	72.5	10.9	83.4	355.6	355.7
U.S.S.R.*	263.5	55.5	319.0	34.1	29.9	64.0	383.0	383.0
Venezuela*				0.5	6.9	7.4	7.4	7.4
West Germany	22.6	0.8	23.4	6.1	1.1	7.2	30.6	23.8
Yugoslavia	4.1	0.0	4.1	5.3	3.8	9.1	13.2	13.2
Zaire*				1.9	12.8	14.7	14.7	14.7
WORLD 1973	949.7	174.1	1123.8	401.5	972.2	1373.7	2497.5
1972	935.3	172.9	1108.2	378.0	967.4	1345.4		2453.6
*Estimations made by the FAO secretariat.

Bolin (1977) estimates the rate of deforestation and fuel wood production to about 1.1 x 10¹⁵ g/year. However, there is an increase in biomass due to reforestation of about 0.3 x 10¹⁵ g/year, which will, to some extent, counteract the increased CO₂ output to the atmosphere. Due, for example, to fertilization, the wood production has increased during the last few years. In Finland, for example, forest fertilization is at present (1976) carried out on well over 200 000 ha annually. The effects of the synthetic fertilizers on accumulation of carbon in the world's forests can, however, be discussed. The increased growth of the trees will lead to an earlier cutting. Thus, the real accumulation will not increase to any larger extent. Nitrogenous fertilizers also result in an increased mineralization of the organic matter in the soil, which decreases the carbon pool in the soil. However, if organic fertilizers (e.g. compost or sewage sludge) are used, this will make up for the loss of organic material by supplying humus. This will, for example, increase the water- and nutrient-holding capacity (Bramryd, 1976). Also, in subarctic forests the soil conditions and nutrient status are made more favourable. This can probably lead to more rapid productivity in these forests (Bramryd in manuscript).

In many parts of the world, the forest areas have increased due to drainage of mires. In Finland alone, almost 300 000 ha of mires are drained every year. For example, between 1967 and 1973 the forest area in Finland increased by 0.5 x 10⁶ ha, owing to swamp drainage and reforestation of farming lands. Forest land in Finland now covers over 19.2 x 10⁶ ha or 63% of the total Finnish land area (Kuusela, 1976). Drainage of mires usually increases the decomposition of peat and thus, to some extent conteracts carbon accumulation in the trees (Holmen, personal communication). However, this means that the input of organic matter from the new forest would make up for this decomposition, The new litter layer could also prevent the peat layer from a further rapid degradation.

The forest plantations planned for energy extraction will increase the flow of carbon into the forests in a short-term view, but their total effects on carbon accumulation are hard to predict. At least the liberation of carbon from fossil fuels may decrease. One million tons of treedry substance could equal about 0.5 million tons of oil in thermal value (Kuusela, 1976). The wood is then, however, burned and the CO₂ returns to the atmosphere. The plans for whole-tree utilization including stumps, bark and other logging residues as alternative to fossil fuel, would markedly decrease the amount of organic material available for humus production. An increased use of fertilizers would be needed to compensate for nutrients, which are often bound up in needles and bark. Thus, this would also result in an increased energy demand for production of fertilizers.

B. Case Studies

Sweden: The total land area of Sweden is about 41 x 10⁴ km². Of this area, about 57% or 23.5 x 10⁴ km² are covered by productive forests. The forest covered area has increased by about 1015 x 10³ km² during the last few decades, mainly at the expense of agricultural area. The total wood supply in Swedish forests in 1970 was about 2360 x 10⁶ m³. If we estimate the density of wood at 0.5 g/cm³, the water content at about 50% and the content of carbon in dry wood at 50% (Nihlgård, 1972), we find that, in 1970, the pool of carbon in above-ground parts of standing trees in Swedish forests was about 590 x 10¹² g C. Since 192329, the supply of wood has increased by almost 700 x 10⁶ m³ (175 x 10¹² g C). About 67% of the increase derives from the southern parts of Sweden. Due to fertilization and modern forestry, the wood supply has increased from 7200 m³/km² in 192329, to 10 000 m³ /km² in 196872. The annual total growth has increased from 57 x 106 m3, to 77 x 10⁶ m³ during the same period (Jordbruksdepartementet, 1974; Skogsstyrelsen, 1976) (see Table 6.4).

Table 6.4 The increase of wood supply in Sweden (Skogsstatistisk årsbok, 1974, from Skogsstyrelsen, 1976)

		Annual growth 106	Annual growth
Part of Sweden	Year	m3 solid volume	m3 per ha
Norrland	19381952	24.5	1.8
	19531962	32.6	2.6
	19641968	33.0	2.5
	19681972	30.2	2.3
Svealand	19381952	19.3	3.7
	19531962	21.6	4.1
	19641968	20.2	3.7
	19681972	18.1	3.4
Götaland	19381952	19.1	4.3
	19531962	23.5	5.2
	19641968	24.0	5.0
	19681972	21.6	4.5
Total Sweden	19381952	62.9	2.7
	19531962	77.7	3.5
	19641968	77.2	3.3
	19681972	69.9	3.0

The volume of fellings in 1972/73 was 73.6 x 10⁶ m³. This was an increase of about 1.4 x 10⁶ m³ compared with previous years. The annual flow of carbon from the Swedish forests due to wood and pulp production is, therefore, about 18.4 x 10¹² g carbon (Table 6.4) (Skogsstyrelsen, 1976). Most of this wood was used for paper production (about 52%) and other products, which are used rather quickly and then turned into garbage (Table 6.5, Figure 6.2). Hence the organic matter in most timber is decomposed and turned into carbon dioxide within a few years. Only a small percentage is stored for centuries in furniture, buildings, and other construction works.

Prognoses from the Swedish State Department of Agriculture estimate that the draining and fertilization of mires and moist woodland growing on peat could increase the annual wood production by about1015 x 10⁵ m³ (Virkesbalansutredningen, 1968). This would correspond to an amount of about 2.53.8 x 10¹² g C/year.

Table 6.5 Consumed and exported quantities of roundwood in 1974 in Sweden (Skogsstyrelsen, 1976)

The total clearcut area in Sweden has not increased during the twentieth century. By clear cut area we mean all forest ground which is not sufficiently covered by trees or is only covered by seed plants. Forest fertilization means a speeding up of the production of organic matter in forests, but this is mostly compensated by more rapid felling and, hence, more rapid mineralization of the assimilated carbon. Fertilization also tends to stimulate mineralization of litter and organic matter in soil.

U.S.S.R.: The total forest area in the U.S.S.R. is about 7.65 x 10⁶ km² (5.53 x 10⁶ km² coniferous and 1.75 x 10⁶ km² broadleaved trees) (Persson, 1974).

The total supply of wood has been calculated at about 73.3 x 10⁹ m³ (Skogsstyrelsen, 1976). The pool of carbon in above-ground parts of standing trees in the U.S.S.R. is about 9.16 x 10¹⁵ g. The annual clearing in the U.S.S.R. in 1973 was about 0.38 x 10⁹ m³. This corresponds to an annual output of carbon from the Soviet forests of about 47.5 x 10¹² g. Of this, about 0.021 x 10⁹ m³ is pulp (2.6 x 10¹² g C) (Skogsstyrelsen,1976).

The production of roundwood in the U.S.S.R. has increased from about 0.37 x 10⁹ m³ in 1963 to 0.38 x 10⁹ m³ in 1973, and pulpwood from 0.019 x 10⁹ m³ to 0.035 x 10⁹ m³ during the same period. This is probably, in part, due to the increased use of fertilizers in U.S.S.R. Fuel wood and charcoal production, however, have decreased from 0.10 x 10⁹ m³ to 0.085 x 10⁹ m³ (FAO, 1975).

U.S.A. and Canada: The total forest area in the U.S.A. and Canada is about 6.30 x 10⁶ km². The total supply of wood has been calculated as 58.5 x 10⁹ m³ (Skogsstyrelsen, 1976). From this we can roughly estimate the total carbon pool in the trees in North America at 7.31 x 10¹⁵ g. The annual clearing in the U.S.A. and Canada was about 0.48 x 10⁹ m³ in 1973. About one-quarter of this is pulpwood (Skogsstyrelsen, 1976). The net loss of forest land for agricultural purposes amounted to 2000 km² per year between 1962 and 1970 (Spurr and Vaux, 1976).

Figure 6.2 Quantities of pulpwood, sawtimber, and fuel wood in Sweden from 1955/56 to 1974/75 (after Skogsstyrelsen, 1976. Reproduced by permission of the National Board of Forestry, Sweden.)

The tropics Most nutrients in the tropical rain forests are bound in the aboveground biomass (De las Salas and Fölster, 1976) (Table 6.6). Therefore, an increased exploitation of these forests will lead to the removal of nutrients, which, ultimately, can lead to impoverishment of the soil and a fundamental disturbance of the ecosystem. The most successful permanent crops in the tropics are those which make relatively small demands on the soil, for example, rubber, teak, and cocoa, because only a limited amount of the nutrients is removed from the ecosystem during harvest (Richards, 1973).

Tropical rain forests serve as large assimilators of CO₂, converting it into organic substances. A disruption of this process, together with the large CO₂ liberation resulting from burning and mineralization of products (wood, paper, etc.), can lead to severe consequences for the ecosystem. For Brazil, Adams et al. (1977) have estimated the annual consumption of wood per capita at about 3 x 10⁶ g C. At least 75% of the cutting is for firewood, while per capita use of other wood products is low. Only about 20% of the cuttings in the Sao Paulo area are replaced. The total reforestation is much lower and is estimated to be approximately 1.0 x 10⁶ g/year per capita (Adams et al., 1977).

Table 6.6 Total carbon and bioelement stores of the primary forest, and the percentage of their distribution between vegetation, organic layer and soil. Data obtained from Middle Magdalena Valley, Columbia, U.S.A. (De las Salas and Fölster, 1976)

In tropical regions, forests are often cut down and the trees are burnt to give new land for agriculture. The land is frequently left fallow after only two or three crops and a new patch of forest is cleared. In many tropical areas, the land is exploited again before fertility has recovered and this can lead to impoverishment of the soil. Tropical forest clearings, if not kept under continuous cultivation, soon become covered with weeds, shrubs, and young trees. Re-establishment of the primary forest might be possible, but this process will probably take centuries (Richards, 1973; Goodland and Irwin, 1974). This, however, is only possible if enough primary forest is left to serve as a refuge for the flora and fauna, which is seldom the case in Brazil, Indonesia, and many other tropical areas.

Among the different types of fallows in warm humid tropics, the early stages of a secondary forest are most capable of restoring the soil productivity potential. Trees have deep roots and therefore they can take up nutrients from deeper soil layers (Van Wambeke, 1974). After about 115 years, the original level of organic matter is reached in a rain forest. At later stages the older regrowths immobilize the nutrients almost exclusively in the woody parts (Laudelout, 1961).

Kellman (1969) has studied the effects of clearings for agricultural purposes in Mindanao in the Philippines. He found that the carbon content of the topsoil had decreased from 8-10% obtained under primary forest, to about 3% after being used for agriculture and then left fallow. In addition to accelerated decomposition of organic matter in soil, a loss of organic matter through erosion also occurs (Table 6.7) (Kellman, 1969).

Gosden (1956) showed that 60 g/m². of soil was lost per cm rainfall on a 10-year-old teak plantation in Trinidad, compared with 47 g/m² per cm lost from secondary evergreen forests (Beard, 1946). Many scientists recommend that burning and cultivation should be controlled or banned completely on high altitudes with steep slopes in order to prevent erosion (Cornforth, 1970). Thus, large amounts of soil organic matter will be transported from rain forests