SCOPE 13 - The Global Carbon Cycle
5
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Terrestrial Primary Production and Phytomass
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G. L. AJTAY, P. KETNER, and P. DUVIGNEAUD |
ABSTRACT
The pools and fluxes of carbon in terrestrial biota form important links in the total biogeochemical cycle. This study evaluates some of the earlier estimates while an attempt is also made for a new assessment in the light of many recent data on production and phytomass becoming available through the efforts of the International Biological
Programme. The problems of classifying ecosystem types and of obtaining their real surface areas are being discussed. Recent surface areas are evaluated and used for calculating total production and
phytomass. Total net primary production of terrestrial biota is estimated to be 60 x 1015 g C/year produced by 560 x 1015 g C living
phytomass. The total amount of standing dead is estimated as 30 x 1015 g C, and the total amount of litter as 60 x 1015 g C. An attempt is made to assess the total
litterfall, being the flux from living to the dead compartment in the biota. The value of total litterfall is estimated to be between 45 and 50 x 1015
g C. Soil organic matter amounts to 1600
2000 x 1015 g C, a value much lower than the most recent one published, but higher than any of the earlier estimates. Man's activities influencing the carbon cycle are discussed, including urbanization and industrialization. Potential production of ecosystem in relation to world population and food demands is briefly discussed.
5.1 INTRODUCTION
Primary productivity is an important link in the carbon cycle, since it is the main flux from the atmosphere to the biota. Primary or basic productivity of an ecological system, community, or part can be defined as the rate at which radiant energy is stored by photosynthetic and chemosynthetic activity of
producerorganisms, chiefly green plants, in the form of organic substances which can be used as food materials
(Odum, 1971). In the process, CO2 from the atmosphere is converted into glucose (C6H12O6), the foundation stone for any further biochemical synthesis of new complex compounds. A distinction must be made between gross primary productivity, which is the total rate of photosynthesis including the organic matter (mainly
C6H12O6) used up in respiration (RA) during
the measurement period, and net primary productivity (NPP), which is the rate of storage of organic matter in plant tissues in excess of respiration; it is also called
,apparent photosynthesis' or `net assimilation' (Odum, 1971). The terms 'productivity' and `rate of production' are interchangeable here. The term `net production' is used to designate a total of accumulated organic matter. Although not strictly necessary, a time element is always used as
a reference scale, such as one year when speaking of agriculture. In this sense the term `production' is employed in this study. It should be noted that in forestry the terms `net productivity' and
'net production' are only used to designate the annual wood growth or even the usable above-ground wood increase. Sometimes the increase in phytomass during a certain period of time is called net production, not taking into account loss in material caused by dying off or grazing.
The assimilation of CO2 can be estimated, to a certain extent, in terms of the reserves of
phytomass, i.e. the total reserve of living organic matter in the aboveground and underground spheres of plant communities. For the estimates, a distinction should be made in the structural elements of the
phytomass, such as perennial and annual above-ground parts, non-green and green assimilating parts, etc. Account must also be taken of dead organic matter in the ecosystem in the form of litter or peat deposits (see
Bazilevich, 1974). For an estimate of the total amount of carbon in terrestrial ecosystems, the soil organic matter (humus) should also be considered. A measure of the returned organic matter is the annual litter fall, including dead parts of plants or entire plants above and below ground in the communities, and the decomposition rate (the rate at which the litter disintegrates through microbial activities and becomes incorporated in the soil as humus). Decomposition causes the main flux of carbon from the biosphere to the atmosphere. In the process of the life activity and of dying off of plants, chemical elements are returned to the atmosphere and lithosphere. This is the essence of the biogeochemical work of the living matter, or the so-called small biological cycle of chemical elements.
The magnitude of the world's terrestrial production and phytomass can be estimated in two different ways, either by classifying the biosphere into ecosystem types and estimating averages and total values for each of these, or by modelling the effects of environmental factors on productivity and phytomass and integrating the results of the model for the earth surfaces. The first approach is used in this study (see Whittaker and Likens, 1973a, 1973b, 1975). Details on the second approach can be found in Lieth (1972, 1975).
Numerous estimates have already been made of the total world net primary production.
Table 5.1 lists the various estimates in chronological order, with land and marine biota treated separately
(Bazilevich
et al., 1971; Whittaker and Likens, 1973b). One of the most recent estimates of total terrestrial production is 53 x
1015 g C/year (117.5 x 1015 g dry matter) (Whittaker and Likens, 1975). The highest estimate of production comes from Bazilevich
et al. (1971, 1974), who give 72x 1015 g C.
Table 5.1 Estimates of world net primary production. (Adapted from Whittaker and Likens, 1973b)
Phytomass estimates are less numerous. The following values are found in the literature and are often quoted: 450 x 1015 g C (Bolin, 1970); 480 x 1015 g C
(Garrels
et al., 1973); 518 x 1015 g C (Bowen, 1966); 680 x 1015 g C
(Baes
et al., 1976), 700 x 1015 g C (Waksman, 1938), 826 x 1015 g C (Whittaker and Likens, 1975); 1000 x 1015 g C
(Garrels and Mackenzie, 1972); and 1080 x 1015 g C (Bazilevich
et al., 1971; Rodin et al., 1975). The values given by Waksman and by Garrels and Mackenzie refer to all living matter in the biosphere (i.e. including marine organisms). Bazilevich
et al. (1971) calculated the plant mass reserves and annual production for a reconstructed plant cover of the earth, i.e. without
correction for agricultural lands, cut forest land, etc., whereby a climax vegetation was assumed for each of the bioclimate zones distinguished. They were primarily interested in evaluating the potential biological resources of the earth. More realistic are the estimates by Lieth (1975) and by Whittaker and Likens (1973a, 1973b, 1975), whereby measured production and phytomass values of various ecosystem types are used for extrapolation.
Table 5.2 Net primary production and other characteristics related to productivity (After Whittaker and Likens, 1973b, 1975, and Whittaker, 1975)
An increased interest in the production of natural ecosystems, particularly stimulated through the various projects carried out within the framework of the International Biological Programme
(I.B.P.-News, 1967, 1969), has resulted in a flow of publications, a number of national and international meetings, which in turn resulted in various
Proceedings and compilation works, such as Wiens (1972), of the biosphere. Worldwide totals by ecosystem types and for the earth's surface. Young (1968), Human Ecology (1973), Woodwell and Pecan (1973), Reichle
et al. (1975), Lieth and Whittaker (1975), Cooper (1975), and works on more specified
ecosystem types, such as Eckardt (1968), Reichle (1970), Duvigneaud (1971), Golley and Golley (1972), Farnworth and Golley (1973), Golley
et al. (1975), Golley and Medina (1975), Ulrich et al. (1974), Young (1974, 1976). In the study by Lieth and Whittaker (1975) of the history of productivity, methods of measurement as well as patterns in productivity and some application in research are dealt with.
This study is an attempt to evaluate the various data on NPP and phytomass of terrestrial ecosystems found in the literature and to use this data for a new assessment of total production and
phytomass, whereby recent results of production studies are taken into account. In particular, more attention is paid to
realistic surface area measurements and the relation of organic matter with the environment.
Table 5.3 Various estimates of surface areas of world terrestrial ecosystems.
Updated from Golley (1972). (Areas in 106 km2)
The studies by Whittaker and Likens (1973b, 1975) have formed the basis for the new assessment.
Table 5.2 gives the principal data on
NPP, phytomass, and related characteristics as presented by these authors. In addition to NPP and
phytomass, attention is paid to dead organic matter. Changes in some ecosystem types, induced by man's activities, are discussed in relation to productivity and
phytomass. This part is dealt with in much more detail by Bramryd (Chapter
6, this volume).
5.2 CLASSIFICATION OF ECOSYSTEM TYPES
The earth's surface is naturally a mosaic of different kinds of vegetation, associated with different environmental features. All these differences are expressed
in net primary productivity variations, which are of vital importance for the self-maintenance or management of the respective ecosystems. The understanding of these variations and their causes is therefore of prime importance for the optimal use of individual types of ecosystems
(Lieth and Whittaker, 1975).
In the earlier estimates of total production and phytomass, only a very few vegetation types were distinguished
(Lieth, 1975), due to the limited amount of data available. This changed when more data were published as a result of an increase in the number of vegetation maps and an increased interest in the productive dimensions of the biosphere. For example, Bazilevich
et al. (1971), and Rodin et al. (1975) give NPP and phytomass data for 106 different vegetation units. Usually, only about 20 units are distinguished
(Lieth, 1972, 1975; Whittaker, 1970; Whittaker and Likens, 1973a, 1973b, 1975).
Various classification systems have been designed, based on different criteria, and remain to some degree subjective
(Rübel, 1930; Ellenberg and Müller-Dombois, 1967; Schmithüsen, 1968; Schmidt, 1969; Walter, 1973, 1976; UNESCO, 1973.
The terms `vegetation unit', or `ecosystem type', are applied to any grouping of plants and are not limited. They are, therefore, perfectly safe terms to use to designate a band of tropical forest or a marsh vegetation. However, in studying any of these, it is desirable to recognize criteria for comparison. Therefore, in this study, the same classification was adopted as that used by Whittaker and Likens and presented in
Table 5.3, which is, in fact, the UNESCO scheme in its modified form. However, some subdivisions have been made in a few ecosystem types, when considered necessary in the light of recent production of surface area data, for instance, the division in several savanna vegetations, and the splitting of tundra biome into polar desert, high arctic, and low arctic tundra. It is beyond the Scope of this paper to give detailed descriptions of each of the ecosystem types. The reader is referred to Ellenberg and Müller-Dombois (1967), Schmidt (1969), Schmithüsen (1968), and Walter (1973, 1976).
It is beyond doubt that, although a classification has been adopted, it still remains almost impossible to draw a sharp distinction between any of these ecosystem types or their subdivisions. Each division is an arbitrary separation.
Figure 5.1 represents the distribution of the main biomes of the biosphere. Going from the equator to the North or South Poles there is a zonation of vegetation units, chiefly based on climatic factors. Gradually, one unit is replaced by another (Walter, 1973). Only the tundra and the boreal forest have some continuity throughout the northern hemisphere.
Other biomes of the same type (e.g. tropical rain forests, temperate grassland) are isolated in different biogeographical regions, and therefore may be expected to have ecologically equivalent, but often taxonomically unrelated species.
Whenever possible, every effort has been made to conform to the adopted classification. However, because of incompleteness of the available information, mistakes in classifying local ecosystem terminologies could not be avoided. Within each main biome there exist various opinions as to its subdivisions. The terms `forest', `woodland', `grassland', and `savanna' have proved especially difficult to specify, as each author has his own concept of what they constitute. For example, `woodland' may be applied to a vegetation which lacks a continuous tree canopy, but the total vegetation coverage is continuous, while it can also be applied as a general term for forest
(Ovington, 1965). On the other hand, the term `forest' is used for real closed forests as well as for more open woodlands. In this study, the term `woodland' has only been used for a group of unclassifiable, woody types of vegetation in temperate zones.
The definition of savanna is elusive. It involves a discontinuous canopy of trees and a continuous cover of herbaceous plants. The savanna grades into woodland savanna and parklands on the one hand, and into open grassland on the other. The question of how many trees make a savanna, or how many woody plants form a woodland, cannot be answered
(Laubenfels, 1975). The savanna is characteristically a tropical grassland, often disturbed by fire, with gallery forest along streams and with scattered groves. Palms are not infrequent, and the scrubby growth tends to be
thorny. Typical thorn scrub and thorn forest usually appear adjacent to either savanna, or light tropical forest and often develop out of this type after disturbance, especially overgrazing.
Figure 5.1 Schematic map of the major biomes of the world. (After Odum, 1971. Reproduced by permission of Saunders Co., London.)
The term `grassland' has only been applied to temperate grasslands, which include such vegetation as prairies: Great Plains in North America, steppes in Inner Asia, pampas in South America, and the velds in South Africa. However, it is difficult to separate semi-natural grasslands from the permanent pastures established and managed by man.
`Tundra' is defined as the treeless regions beyond timberlines in the north (Arctic tundra), and on high mountains (alpine tundra) (Webber, 1974). This is the usual definition, which is less limited than the original meaning (Lapland treeless plain). Polar desert, which to some authors is not part of the tundra zone, is also included. Oceanic moorland areas in cool temperate climates, which are sometimes included in the definition, are not treated as tundra in this study. For the various opinions on the arctic and alpine zonations in North America and Eurasia refer to Alexandrova (1970), Barry and Ives (1974), and Blüthgen (1970).
Bogs (peatlands) and `human area' are ecosystem types which have never yet been treated as separate entities. They have been included because for bogs an estimate of their surface area could be made, while it was realized that human area can no longer be neglected, due to the rapid expansion of human settlements, cities, and industries. Urbanization mainly takes place in areas which have high biological productivity, replacing it with low-productive sites, thus diminishing NPP and
phytomass.
5.3 ASSESSMENT OF SURFACE AREAS OF THE VARIOUS ECOSYSTEM TYPES
Various estimates of surface areas of the main ecosystem types of the world have been made in the past.
Table 5.3 represents a summary of these estimates. The great variation reflects the difficulties in classification, as discussed above, and also the fact that no biome has ever been measured on a global scale. The estimates are often based on figures given by FAO in their various statistic publications, which, in the absence of more accurate information, are the best available source.
The values given by Whittaker and Likens (1973, 1975) are derived from Lieth (1972, 1975) and are assumed to apply to the situation in 1950. In the present study, these values have been updated by making use of recent reports and detailed vegetation maps, as well as various assumptions on human interference in natural vegetation.
All data are given in round figures to avoid the pretention that the new assessment is complete and consistent. Although many new sources have been consulted, some of the sources were incomplete in themselves. In addition, it would have taken a long period of library study to trace and understand all background data of the sources, particularly concerning classification and methodology.
The reassessment of surface areas was mainly based on the following considerations and assumptions: as a result of clearing and fire, the tropical humid forests are gradually replaced by secondary forests, derived savanna, cultivated plots, and roads and settlements. Likewise, dry forest is giving way to savanna-like vegetations, settlements and cultivation, while temperate rain forests are being replaced by mountain grasslands and meadows. Destruction of savanna by fire and overgrazing leads to degradation and a semidesert-like plant-cover, while semideserts turn into deserts through human and climatic influences.
Taking recent estimates for surface areas of forests, such as those given by Persson
(1974), Brünig (1977), Synnott (1977), Reinbek-Weltforstatlas, and using many other references, the areas of the remaining ecosystem types were estimated. The new surface areas are presented in
Table 5.5.
Persson (1974) has estimated the area of the closed forests in the world as
28 x 1012 m2 or 22% of land area, while the area of open woodlands of different types is closer to 10 x 1012 m2. The area of closed boreal forests is given as
6.7 x 1012 m2. Although definitions of closed forests and open woodland are given, they are rather flexible and Persson
(1974) stated that they need to be improved. According to Brunig (1977), the area covered by tropical closed forests and open woodland is roughly
20 x 1012 m2, of which half is closed forest. The term `tropical closed forest' is used synonymously with `tropical moist forest' and excludes the dry tropical forest.
Brünig's estimate was adopted in this study. Other estimates for tropical moist forests are:
16 x 1012 m2 (Budowski, 1956 for humid and monsoon forests together); approximately
9 x 1012 m2 (Persson, 1974); 7.75 x 1012 m2
(UNESCOMAB,
1977); and 9.35 x 1012 m2 (WWF cited in Woodwell et al., 1977).
Although it is difficult to judge to what extent the values given by Whittaker and Likens
(1975) for 1950 reflect the real situation, they nevertheless form a basis for comparison.
If the present estimate of 10 x 1012 m2 for tropical moist forests is close to the truth, this would mean a decrease of
42% since 1950. Assuming the same rate of decrease for tropical dry forests, the present area can be estimated at
4.5 x 1012 m2, a value which coincides with the estimate of Persson
(1974). Our surface area of closed boreal forests is slightly higher than that given by Persson
(1974). The surface area for open boreal forests, or forest tundra, is similar to that given by Mikola
(1970).
It is difficult to judge the presented surfaces for temperate forests. Both are based on approximately the same rate of decrease since
1950 as for tropical forests. Compared with the data given by Persson (1974), the values appear to be too high.
Figures on surface areas of savanna vegetations are scarce. Malaisse
et al. (1975) gives 3.8 x 1012 m2 for savanna woodland in Africa
(forêts claires); this includes Miombo (which is thorny forest). Synnott
(1977) gives 1.7 x 1012 m2 for Miombo. Hueck (1966) reports
0.85 x 1012 m2 for thorny `forests' (caatinga) in Brazil. Our figure of
3.5 x 1012 m2 does not seem an overestimate.
The total surface area of permanent pastures and meadows given by FAO (1974) is 30 x 1012 m2 , but probably does not include all savanna from Whittaker and Likens (24 x 1012 m2). The present value is close to that given by
FAO. As grasslands, in the widest sense of the word, form the world's second most important ecosystem type, it is evident that more attention should be paid to the classification and mapping of grasslands.
The total peatland area of predominantly temperate regions is 2.31 x 1012
m2 (Moore and Bellamy, 1973), which the authors consider a low estimate. Peatland areas in the tropics cover 0.38 x 1012 m2
(Soepraptohardjo and Driessen, 1976; Pons, 1977). If a correction is made for the overlap in both estimates, the total peatland area is 2.56 x 1012 m2. The largest areas are located in the boreal zone of North America, the
U.S.S.R., and of some countries in Northern Europe. The distribution of tropical peatlands is limited
(Pons, 1977); they are concentrated in South East Asia, around the Sunda Flat in Indonesia and Malaysia where a wet climate prevails
(Pons, 1977).
Many of the peatlands are drained for agricultural use, afforestation or exploitation of the peat for horticultural purposes or fuel (see
Chapter 6, this volume). Therefore, it was assumed that 1.5 x 1012 m2 are still unexploited, and mainly consist of
bogland.
It might well be possible that there is an overlap between peatlands and the category of swamps and marshes. The surface area for the latter was taken from the literature (Whittaker and Likens, 1975). This value probably needs revision as many marsh areas are being drained, particularly in the tropics.
The estimate of mangrove area was based on area data given by Chapman (1976) for about one-third of the total mangroves. The remainder was calculated by measuring the length of coastlines where mangroves are found and assuming an average width of 80 m.
The subdivision into annual and perennial crops has been made using FAO Production Yearbook data
(FAO, 1974). Their figures indicate that perennial crops, such as fruit trees, coffee, tea, and rubber plantations, cover less than 8% of the total cultivated areas. The chosen figures remain rather subjective, as is the division between temperate and tropical.
Compared with the surface area of cultivated land, which is usually given as 14 000 x 109 m2, the present area reflects an increase of less than 0.5% per year. This seems very low considering the fact that the increase in food production is chiefly a result of expanding cultivated land and partly a result of increased yield per unit area. However, at the same time, a decrease in cultivated area due to desertification, erosion, or other factors has to be taken into account. According to FAO statistics
(FAO, 1974) cultivated land covers approximately 15 000 x 109 m2. Much of FAO data is out of date. In addition, it refers only to those plots of which the crops reach the national or international market; smallholdings are probably not included.
The term `human area' has been introduced, defined as the area occupied by man for housing, schooling, utilities, industries, transport, etc., and is not restricted
to urban areas. Because of the rapid expansion of urban-industrial development and the large diversity of its impact on the environment, an attempt has been made to assess the extension of total human area. Published data are scarce and, if available, mainly for developed countries. Evdokimova
et al. (1976) report that 10% of the forest zone of the U.S.S.R. is occupied by cities, roads, and settlements. In the forest-steppe zone this area is
4%, while in the steppe zone it amounts to 20%. According to Maier-Bode (1959) 7.7%
of the land surface in West Germany was already being used as cities, villages, roads, etc., in the
1950s. In the Netherlands, cities, villages, roads and industries occupy 9.2%
of the country (Statistisch Zakboek, 1977). In Japan, 48% of the total population lives in built-up areas, which cover
1.25% of the total land area. Of course no extrapolation can be made globally, but these high figures indicate that the surface of human area might be rather high and can no longer be neglected. Population data of about
3000 metropolitan areas with known surfaces (Rand McNally, 1972) has revealed that about
25% of the world population live on 617 x 109 m2. Roads and railways probably cover some
180 x 109 m2 (World Road Statistics, 1975). Total human area was assumed to be
1.5% of the land surface (without perpetual ice) or 2000 x 109 m2. This might be too low an estimate, as a recent and more detailed study gives
1.82% (Ajtay, unpublished). Such a relatively small area has not, of course, a great influence on our estimate of NPP and
phytomass.
5.4 ASSESSMENT OF NET PRIMARY PRODUCTIVITY AND LIVING PHYTOMASS
All data on productivity, phytomass, litter, and soil organic matter are commonly expressed in grams of dry matter, grams of carbon, or sometimes both. The relationship of dry matter to carbon is variable. Woody parts and roots normally have a higher carbon content than foliage. During the process of decomposition, the carbon content can change. Kimura
(1963) found that the carbon content in the needle litter remained fairly constant with the progress of decomposition
(52.7%), while for branch litter it gradually increased up to 58%. Table
5.4 shows the variation in carbon content of plant parts, and of different components in forest ecosystems. For reasons of comparison, the following conversion factors were used in this study:
NPP, phytomass, and litterfall 45% (Lieth and Whittaker, 1975; Larcher, 1976);
litter 50% (Kimura, 1963); and humus 58% (Waksman, 1938; FitzPatrick, 1974).
For the assessment of net primary productivity and
phytomass, the values given by Whittaker and Likens (1975) were evaluated and adjusted in the light of much new data (mainly from IBP-studies) published in various forms. Earlier and recent compilation works dealing solely or partly with productivity were consulted, such as Lieth
(1972), Rodin and Bazilevich (1967), Young (1968), Wiens (1972), Cavé (1974), Van Dobben and Lowe-McConnell
(1975), Cooper (1975), Lieth and Whittaker (1975), Reichle et al. (1975),
as well as many studies on specific ecosystem types. For tropical ecosystems, the following references are mentioned:
Table 5.4 Average carbon content of plants and parts of plants on DM-basis. (Number of samples given in parentheses)
|
| Ecosystem, plants, plant organs |
Carbon |
|
| and soil organic matter |
% |
Main references |
|
| Zea mais (total plant) |
43.6 |
Brouwer (1966) |
| Herb leaf |
45.0 |
Woodwell and Whittaker (1968) |
| Tree foliage |
41.5 (7) |
Kimura (1963), Klinge and Rodriques (1968), |
|
|
Ulrich et al. (1974), Woodwell and Whittaker (1968) |
| Tree stem |
47.0 |
Woodwell and Whittaker (1968) |
| Forest standing dead |
49.6 |
Ulrich et al. (1974) |
| Forest standing dead* |
51.0 (17) |
Ulrich et al. (1974) |
| Forest litter (above ground) |
50.0 (46) |
Klinge and Rodrigues (1968) Ulrich et al. (1974) |
| Forest living roots |
50.6(9) |
Ulrich et al. (1974), Woodwell and Whittaker (1968) |
| Forest living roots* |
52.4 (17) |
Ulrich et al. (1974) |
| Forest dead roots* |
50.6 (17) |
Ulrich et al. (1974) |
| Forest ecosystem (above-ground |
48.0 (14) |
Minderman (1967), Ulrich et al. (1974), |
| phytomass) |
|
Whittaker and Likens (1972), |
|
|
Woodwell and Pecan (1973) |
| Forest ecosystem (above-ground |
51.0 (17) |
Ulrich et al. (1974) |
| phytomass)* |
|
|
| Meadow tundra |
42.0 |
Wielgolaski (1975) |
| Average of ecosystem values |
45.0 |
Larcher (1976), Lieth and Whittaker (1975) |
| Humus (soil organic matter) |
58.0 (10) |
FitzPatrick (1974), Kimura (1963), |
|
|
Kononova (1970), Ulrich et al. (1974), |
|
|
Waksman (1938) |
|
| *Dry organic matter basis. |
Misra and Gopal (1968), Golley and Golley (1972), Farnworth and Golley (1973), Golley and Medina (1975), and Golley et al. (1975). Important studies on forest production include Reichle (1970), Duvigneaud (1971), Ellenberg (1971), Ulrich et al. (1974), Young (1974, 1976), and Brünig (1977). Valuable data on grasslands can be found in Coupland and Van Dyne (1970), French (1971), Breymeyer (1971), Rychnovska (1972), Coupland (1973a, 1973b), Whyte (1974), César and Menaut (1974), Numata (1975) and Pandeya et al. (1977). Useful data on tundra ecosystems are available in Alexandrova (1970), Webber (1974) and Wielgolaski (1975a). It is impossible to list all the smaller publications which were consulted, but a few should be mentioned, such as Klinge (1973a, 1973b, 1973c, 1976), Klinge and Rodrigues (1973), and Lemée and Huttel (1975).
Since the data were drawn from so many different sources, a considerable
variation in methodological approach was to be expected. All data from literature was converted to dry matter and to total production and
phytomass, i.e. above plus below ground. When there was a questionable figure, the orginal paper from which data were quoted were consulted; locations were checked on vegetation maps when there was any doubt about classification.
The adjusted means for NPP and phytomass per ecosystem type are given in
Table 5.5, together with total production and total
phytomass; the latter values were calculated for the new estimates of the surface areas. In some cases, the means are approximate averages of published values (forest, grassland, savanna forest, tundra), but in many other cases they have been chosen subjectively as possible values (Whittaker and Likens, 1975). It proved particularly difficult to place the typical local vegetations, of which NPP and phytomass data were available, into the adopted classification. Compared with the values of Whittaker and Likens, the main differences in NPP data are for savanna, temperate grasslands, and tundra, which are all higher. Forest productivity seems only slightly higher than was previously reported.
Human area is not entirely unproductive, for parks, small woodlands, gardens, or other quarters with vegetation still fix carbon by photosynthesis. The nonproductive site is an energy-consuming system in the form of fossil fuels, thereby releasing large quantities of carbon to the atmosphere, rivers, and oceans. It was assumed that 40% of the total human area is still productive. According to Abrams (1965), 18% of the central-city part of some North American cities was open space. He found a positive relation between increasing population, occupied area, and open space for some of the cities. Duvigneaud
et al. (1977) found for Brussels that up to 50% was open space, with a relatively high productivity.
The phytomass values per unit area are meant to be annual average values. However, due to limited data on phytomass and its fluctuation per year, the presented values are probably less accurate than the NPP values. In the case
of annual crops of cultivated land, an average phytomass was calculated by dividing the NPP by 12 (the number
of months per year). Maximum standing crop usually applies to the time of harvest and is slightly lower than NPP due to dying of plant parts as the growing season proceeds. Even if maximum standing crop was taken as an average, it would not have changed the total phytomass by very much. The new estimates for total net primary production and total living phytomass are 133 x
1015 g dry matter and 1243.9 x 1015 g respectively (i.e. 60 and 560 x 1015 g C).
The main share in the total production has the forest ecosystems, followed by the different savanna systems, cultivated lands, and temperate grasslands. The total production value is more than 10% higher than the estimate of Whittaker and Likens, and higher than any of the earlier estimates (see
Table 5.1), except that of Bazilevich
et al. (1971). The much higher values of savanna/grassland for NPP are explained by the fact that in the past years more attention has been paid to root production in these ecosystems. Root production appears much higher than was
originally supposed (Numata, 1975; Pandeya et al., 1977). However, no conclusion may be drawn about changes. Not only do the surface areas differ, but the estimates for NPP and phytomass sometimes differ considerably from author to author; in addition, there is the difficulty of classification.
Table 5.5 Surface areas, net primary productivity, and
phytomass of terrestrial ecosystems of the biosphere. Adopted conversion factor
from DM to carbon is 0.45
|
|
|
Total production
|
Total living
phytomass
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Living |
|
|
|
|
Surface |
NPP |
|
|
phytomass |
|
|
|
|
area |
DM |
DM |
Carbon |
DM |
DM |
Carbon |
|
Ecosystem type |
x 1012 m2 |
g/m2 yr |
x 1015 g |
x 103 g/m2
|
x 1015 g |
|
| 1. |
Forests |
31.3 |
|
48.68 |
21.9 |
|
950.5 |
427.73 |
|
|
Tropical humid |
10 |
2300 |
23 |
10.35 |
42 |
420 |
189 |
|
|
Tropical seasonal |
4.5 |
1600 |
7.2 |
3.24 |
25 |
112.5 |
|
|
|
|
|
|
|
|
|
|
50.62 |
|
|
Mangrove |
0.3 |
1000 |
0.3 |
0.14 |
30 |
9 |
4.05 |
|
|
Temperate evergreen/conif. |
3 |
1500 |
4.5 |
2.02 |
30 |
90 |
40.5 |
|
|
Temperate deciduous/mixed |
3 |
1300 |
3.9 |
1.76 |
28 |
84 |
37.8 |
|
|
Boreal coniferous (closed) |
6.5 |
850 |
5.53 |
2.49 |
25 |
162.5 |
73.13 |
|
|
Boreal coniferous (open) |
2.5 |
650 |
1.63 |
0.73 |
17 |
42.5 |
19.12 |
|
|
Forest plantations |
1.5 |
1750 |
2.62 |
1.18 |
20 |
30 |
13.5 |
| 2. |
Temperate woodlands (various) |
2 |
1500 |
3 |
1.35 |
18 |
36 |
16.2 |
| 3. |
Chaparral, maquis, brushland |
2.5 |
800 |
2 |
0.9 |
7 |
17.5 |
7.88 |
| 4. |
Savanna |
22.5 |
|
39.35 |
17.71 |
|
145.7 |
65.56 |
|
|
Low tree/shrub savanna |
6 |
2100 |
12.6 |
5.67 |
7.5 |
45 |
20.25 |
|
|
Grass dominated savanna |
6 |
2300 |
13.8 |
6.21 |
2.2 |
13.2 |
5.94 |
|
|
Dry savanna thorn forest |
3.5 |
1300 |
4.55 |
2.05 |
15 |
52.5 |
23.63 |
|
|
Dry thorny shrubs |
7 |
1200 |
8.4 |
3.78 |
5 |
35 |
15.75 |
| 5. |
Temperated grassland |
12.5 |
|
9.75 |
4.39 |
|
20.25 |
9.11 |
|
|
Temperated moist grassland |
5 |
1200 |
6 |
2.7 |
2.1 |
10.5 |
4.72 |
|
|
Temperated dry grassland |
7.5 |
500 |
3.75 |
1.69 |
1.3 |
9.75 |
4.39 |
| 6. |
Tundra arctic/alpine |
9.5 |
|
2.12 |
0.95 |
|
13.05 |
5.87 |
|
|
Polar desert |
1.5 |
25 |
0.04 |
0.02 |
0.15 |
0.23 |
0.10 |
|
|
High arctic/alpine |
3.6 |
150 |
0.54 |
0.24 |
0.75 |
2.7 |
1.22 |
|
|
Low arctic/alpine |
4.4 |
350 |
1.54 |
0.69 |
2.3 |
10.12 |
4.55 |
| 7. |
Desert and semidesert scrub |
21 |
|
3 |
1.35 |
|
16.5 |
7.42 |
|
|
Scrub dominated |
9 |
200 |
1.8 |
0.81 |
1.1 |
9.9 |
4.46 |
|
|
Irreversible degraded |
12 |
100 |
1.2 |
0.54 |
0.55 |
6.6 |
2.97 |
| 8. |
Extreme deserts |
9 |
|
0.13 |
0.06 |
|
0.78 |
0.35 |
|
|
Sandy hot and dry |
8 |
10 |
0.08 |
0.04 |
0.06 |
0.48 |
0.22 |
|
|
Sandy cold and dry |
1 |
50 |
0.05 |
0.02 |
0.3 |
0.3 |
0.14 |
| 9. |
Perpetual ice |
15.5 |
0 |
0 |
0 |
0 |
0 |
0 |
| 10. |
Lakes and Streams |
2 |
400 |
0.8 |
0.36 |
0.02 |
0.04 |
0.02 |
| 11. |
Swamps and Marshes |
2 |
|
7.25 |
3.26 |
|
26.25 |
11.81 |
|
|
Temperate |
0.5 |
2500 |
1.25 |
0.56 |
7.5 |
3.75 |
1.69 |
|
|
Tropical |
1.5 |
4000 |
6 |
2.7 |
15 |
22.5 |
10.13 |
| 12. |
Bogs, unexploited peatlands |
1.5 |
1000 |
1.5 |
0.68 |
5 |
7.5 |
3.37 |
| 13. |
Cultivated land |
16 |
|
15.05 |
6.77 |
|
6.64 |
2.99 |
|
|
Temperate annuals |
6 |
1200 |
7.2 |
3.24 |
0.1* |
0.6 |
0.27 |
|
|
Temperate perennials |
0.5 |
1500 |
0.75 |
0.34 |
5 |
2.5 |
1.12 |
|
|
Tropical annuals |
9 |
700 |
6.3 |
2.83 |
0.06* |
0.54 |
0.24 |
|
|
Tropical perennials |
0.5 |
1600 |
0.8 |
0.36 |
6 |
3 |
1.35 |
| 14. |
Human area |
2† |
500 |
0.4 |
0.18 |
4 |
3.2 |
1.44 |
|
|
|
|
|
|
|
|
|
|
|
|
TOTAL |
149.3 |
895 |
133.0 |
59.9 |
3.75 |
1243.9 |
559.8 |
|
| *Annual average values. |
| †Of which only 40% (or 0.8
x 1012 m2) productive. |
To make a better comparison between the new assessment and that of Whittaker and Likens, the values for NPP of
Table 5.5 were multiplied with the surface areas used by Whittaker and likens (column 2,
Table 5.2), assuming NPP of natural ecosystems has not changed since 1950. For the agricultural systems, only a slightly higher average NPP was taken (800 g/m2 ). Calculated in this way, total net production amounts to 142 x 1015 g dry matter or 64 x 10 15 g C. Although there still remain many incongruencies which bias any comparison, e.g. some ecosystem types are not listed by Whittaker and
Likens, it could be concluded that total net primary production has declined since 1950. However, this cannot be proved.
The total amount of phytomass (560 x 10 15 g C) is much lower than the estimate by Whittaker and Likens. Major differences exist between the estimates of phytomass per unit area of the forest, the savanna, and the tundra. The former are somewhat lower than those of Whittaker and Likens, while the latter are much higher. Forests contribute up to 75% of the total
phytomass, followed by woodlands and savanna. The above comments on comparison of the data also apply to the phytomass data. Nevertheless, it seems reasonable to conclude that, compared with the data of Whittaker and Likens, total phytomass is likely to have decreased over the past decades. This decline can chiefly be traced to the continuous decrease in
forest-phytomass as a result of intensive exploitation and clearcutting. The cleared areas are replaced by systems with a much lower phytomass per unit area, so that the loss is only partly compensated for. Compared with other estimates, the present estimate for total phytomass is higher than that given by Bolin (1970) (450 x 1015 g C), and by Bowen (1966) (510 x
1015 g C), which makes it all the more difficult to draw conclusions about changes.
The values for total NPP and phytomass depend mainly on the extrapolation on single data or sets of average values of relatively small research plots to large uncertain surface areas, and they are, therefore, burdened by a great degree of inaccuracy. Existing vegetation maps of the world or individual continents present the opitmal situation, without taking human interference into account. On the other hand, large-scale vegetation maps, representing detailed local vegetation units, are not suitable to be used for extrapolations to world scale but could be used for NPP and phytomass estimates of regions (Sharp, 1975; Sharp
et al., 1975). Without a good international usable classification of ecosystem types, and without maps representing the real situation which can be used for obtaining real surface areas, it remains impossible to make estimates about NPP production and phytomass with a certain degree of accuracy.
As the plant cover of the earth surface is rapidly changing due to man's interference, continuous monitoring is needed, which could be done by satellite and ground observations. In addition, more field observations about NPP and
phytomass of natural ecosystems are needed. In preparing the present study, we mapped each location of which NPP and phytomass data were available. We must conclude that, despite the efforts of the IBP, there still exist large gaps of knowledge on productivity of ecosystem types, particularly in South America, Asia, Africa, and Australia.
5.5 DEAD ORGANIC MATTER
The dead organic matter in land biota consists of the total amount of organic matter incorporated in trees and shrubs that are dead but still standing, of dead organs (dry but not yet fallen branches of trees and shrubs, dry stems of herbaceous plants), the matter accumulated as litter, steppe matting or turf horizon of the soil, and the organic matter accumulated in the soil as humus
(Rodin and Bazilevich, 1967).
5.5.1 Standing Dead
Limited data are available on the amount of dead plant parts, still attached to living plants. Data are mainly available on forests and tundra ecosystems.
Table 5.6 lists some values of above-ground dead phytomass in various ecosystems. Compared with the values for total living phytomass
(Table 5.5) the standing-dead component is only very small, except for savanna forest where it can equal about 25% of the living
phytomass. The data are too scanty to make any extrapolation, but it seems reasonable to assume that total standing dead, on average, equals about 5% of the total living
phytomass, i.e. about 60 x 1015 g DM. With an average carbon content of 50%, this would mean 30 x
1015 g C, a value which is much lower than that given by Bazilevich (1974) for standing dead and dry trees, namely 75 x 1015 g C.
5.5.2 Litter and Litterfall
The term `litter' is used in ecology with the following two meanings: the layer of dead plant material, which may be present on the soil surface; and dead plant materials which are not attached to a living plant. These are not, however, satisfactory as definitions for the ecologist concerned with the functioning of ecosystems. The litter layer may be clearly distinguishable from an underlying mineral layer or there may be no sharp boundary between a layer containing recognizable plant structures and a layer containing only amorphous organic material.
The presence of tree trunks, in the English Pennines, 7000 years old, under peat 4 m deep, illustrates the problem of defining a litter layer by the same criteria in all habitats. Also, tree trunks above ground are a form of litter, but are often treated separately.
The problems are no less severe when litter is defined as dead plant materials
which are not attached to a living plant. Plant organs neither die instantly nor, when dead, fall instantly. Abscission of a leaf follows a more or less prolonged senescence when much of the mineral content is withdrawn to the stem and the phylloplane fungi are already decomposing the carbohydrates. The argument may be extended to recognize that there is a turnover of molecules in all living matter, and that death begins
prenatally.
Table 5.6 Average standing crop of dead phytomass in some ecosystem types. Values in
g/m2 dry weights. (Number of measurements in parentheses)
|
|
|
|
Standing |
|
|
Ecosystem types |
Location |
dead |
References |
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Salas cited by Klinge (1976) | |
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Westman and Rogers (1977) | |
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Westman and Rogers (1977) | |
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500(17) |
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Kjelvik and Kärenlampi (1975) | |
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Kjelvik and Kärenlampi (1975) | |
|
Understorey moist birch f. | |
|
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|
Kjelvik and Kärenlampi (1975) | |
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Kjelvik and Kärenlampi (1975) | |
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Kjelvik and Kärenlampi (1975) | |
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Kjelvik and Kärenlampi (1975) | |
| Temperate dec. woodland |
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| Grassland (Miscanthus sp.) |
|
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|
| Grassland (Miscanthus sp.) |
|
167(4) |
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Kjelvik and Kärenlampi (1975) | |
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Kjelvik and Kärenlampi (1975) | |
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Kjelvik and Kärenlampi (1975) | |
|
In the soil, formation of litter occurs by the dying off of roots and below-ground organs. This litter is termed root-litter. As it has proved very difficult to separate dead and living roots, root-litter is usually not measured in biomass studies, and the term `litter' usually refers to above-ground litter only. `
Whittaker and Likens (1973b) gave for total littter-mass on the soil surface of land communities 111
x 1015 g DM (= 55 x 1015 g C), which is equal to 6% of their estimate of living
phytomass. Bazilevich (1974) gives for litter, steppe, or desert-matting 193.8 x 1015 g DM (=97 x 1015 g C), equalling 8% of the living
phytomass. Bazilevich, however, does not state whether or not the litter includes root-litter. Applying the same living
phytomass/litter ratio as Bazilevich to the new estimate for phytomass, the amount of litter would be 100 x 1015 g DM or 50 x
1015 g C.
Litter-mass data are mainly available for forest ecosystems (Volobuev, 1963;
Duvigneaud, 1971; Rodin and Bazilevich, 1967; Ulrich et al., 1974; Kononova, 1975), but are scarce for savanna, grasslands and deserts. Nevertheless, an attempt was made to make an assessment of the litter per ecosystem type based on the scanty data from the literature; the result is given in
Table 5.7.
The ecosystem types are less subdivided. It should be noted that from published data, it is not always clear which definition of litter was adopted or whether litter included standing dead or not. Generally, litter accumulation takes place in cool areas, in biomes such as scrub tundra, taiga, and temperate forests, and can amount to 8000 g/m2,
30004500 g/m2, and 1500 g/m2 respectively. The highest values, more than 10 000 g/m2 , were found for temperate coniferous forests
(Volobuev, 1963). In wet tropical lands, where mean temperatures are above 30 °C, litter is destroyed faster than it is supplied. Between 25 and 30
°C, litter supply and decomposition are about equal (cf. also 5.5.4).
Based on the values given in Table 5.7, the total amount of litter is now estimated at 119 x 1015 g DM or 60 x 1015 g C. The estimate for cultivated land might be too high, but does not influence the total value. Estimates for peatland swamps and marshes, however, might be too low. By adding the amount of standing dead to the litter mass, dead phytomass amounts to
8090 x 1015 g C. Another 5 x 1015 g C should perhaps be added to include dead wood, dry trees, etc., which might have been neglected in the calculations. Total dead phytomass will thus be 95 x
1015 g C, or equivalent to approximately 15% of living phytomass. This percentage coincides with the estimate of Bazilevich for stand dead and litter together.
An important part of the biological cycle is litterfall, which is the flux from
living to dead matter, and has attracted a great deal of interest for a long
time. Litterfall can be defined as the organic matter incorporated in all plant
elements of the above-ground and underground parts of the community that die
annually, and in plants or parts of plants that die in the course of ageing or
natural thinning (Rodin and Bazilevich, 1967). The term `leaf litterfall',
however, is often used only to denote the organic debris (leaves, flowers,
glumes) shed by the vegetation upon the surface of the soil. This definition
frequently does not include the timber of dead trunks and the larger branches or
the litterfall of plants in the ground flora. Many western investigators have
studied leaf litter only, whether or not they included big branches. An
extensive work on litter production in forests of the world has been prepared by
Bray and Gorham (1964). Root-litter is not considered in this study. For other
ecosystem types there is little information available on litterfall.
Table 5.7 Estimated litterfall and litter in various ecosystem
types. Adopted conversion factor from DM to carbon for litterfall 0.45 and for litter 0.50
|
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Litter- |
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600 |
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850 |
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600 |
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 |
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550 |
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Chaparral, maquis, brush land | |
|
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800 |
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900 |
|
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550 |
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|
1.43 |
0.644 |
|
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|
0.014 |
30 |
0.05 |
|
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|
145 |
|
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200 |
|
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125 |
|
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100 |
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15 |
0.14 |
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600 |
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50 |
|
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150† |
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300 |
|
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|
| *50% of net production loss to detritus. |
|
|
|
| ‡Of which only 40% productive. |
|
In steady-state ecosystems, total litterfall should equal total net
production, and often the same value is taken for total litterfall as for NPP
when a model of the carbon cycle is prepared (Bolin, 1970). On the other hand,
part of the net production is consumed by herbivores or lost through other
causes, so that total litterfall should be less than net production.
Reiners (1973a) was the first to make estimates of total litterfall, applying
four different approaches. The first approximation was based on estimates of
world annual net production values per ecosystem type, derived from Whittaker
and Likens (in Whittaker, 1970), together with his own rough estimates of the
percentage of production that enters the detritus pathways. The second
approximation used the phytomass estimates of Whittaker and Likens (in
Whittaker, 1970), together with averages of Rodin and Bazilevich (1967),
estimates of total litter as percentage of biomass. The third approximation of
total litter measurements is based on a few observations in single stands (Rodin
and Bazilevich, 1967); the values were multiplied by Whittaker and Likens'
estimate of areal cover. A fourth approximation was developed by Reiners of more
extensive data than that of Russian work, and rests on a number of assumptions
regarding root contributions to litter. The total amounts for litterfall
estimated by Reiners were 45.9 x 1015 g C, 63.9 x 1015
g C, 62.2 x 1015 g C and 37.5 x 1015 g C for the four approximations
respectively. The second and third values exceed the estimate of terrestrial
production given by Whittaker and Likens. According to Reiners, the fourth
approximation most closely represents the carbon input that under current
conditions of land use will be returned to the atmosphere by decomposition
alone.
We have applied approximations one, two, and four using the new surface
areas, total production values, and phytomass values presented in Table
5.5. The
values thus obtained are 98 x 1015 g DM, 88 x 1015 g DM
and 73 x 1015 g DM. Using a carbon content of 50%, as did Reiners,
the values obtained are 49 x 1015 g C, 44 x 1015 g C, and
36.5 x 1015 g C.
Finally, a new attempt was made to assess litterfall using
data by Ulrich et al. (1974) in addition to the numerous values reported by
Rodin and Bazilevich (1967), and by Bray and Gorham (1964). For the annual crops of cultivated land and for the plantations, it was assumed that 50% of the net production goes to detritus
(Reiners, 1973a;b). According to Reiners, this might be too high for agricultural land. For all other ecosystem types, average sets of values were prepared from literature data. Only a limited number of data was available for savanna, deserts, chaparral, and bogs/marshes.
The total litterfall calculated in this way amounts to 94.7 x 1015 g DM or 47 x 1015 g C (or 43 x 10 15 if conversion factor 0.45 is applied), a value which coincides with our approximations one and two. It is rather a subjective choice as to whether to use a factor 0.45 or 0.50 for the conversion of DM litterfall to carbon, because there is a gradual change in carbon content from living phytomass to litter-mass.
It is very difficult to draw any conclusions from the various values of total
litterfall. It can tentatively be said that total litterfall is somewhere in the order of
4550 x 1015 g C per year. However, due to the lack of
accurate data on the contribution of roots to
litterfall, as well as to the different interpretations of what the terms litter and litterfall include, more studies are needed.
It is, therefore, also premature to conclude that the difference between total net production and total litterfall is the amount of phytomass actually eaten by the herbivores plus (in the case of non-steady-state) an eventual increase in the form of wood.
Total herbivore consumption is estimated to be 3.3 x 1015 g C per year (see
Table 5.2). Added to this should be the consumption by livestock and man. In 1975 there were 1500 x 106 livestock units in the world
(FAO Production Yearbook, 1975) (one livestock unit = 500 kg live weight of domesticated animals, such as camels, horses, mules, asses, cattle, sheep, and goats). Assuming that one livestock unit consumes 5 kg C per day, the total annual consumption is 2.7 x 1015 g C, which is almost as high as the total herbivore consumption. Subtracting total animal consumption from the difference between total production and annual
litterfall, it would still mean that total phytomass increases annually by a least 7 x 1015 g C. This is rather high, and is contrary to the opinion that total living phytomass is decreasing.
5.5.3 Peat
Peat can be considered as a special kind of litter, with a position between litter and soil organic matter (humus). Waksman (1938) has defined peat as `a layer of the earth's crust', largely organic in nature, which has orginated in water basins and in a water-saturated condition, as a result of incomplete decomposition of the plant constituents, due to the prevailing anaerobic conditions (cited by
Allision, 1973). The nature of the peat depends upon the plant association which has produced it. Peat always contains mineral matter, but usually less than 35% on the dry weight basis.
According to Moore and Bellamy (1973), the 2310 x 109 m2 of peat resources are covered with 330 x 109 t of organic matter, representing 1700 x 1016 kcal of potential energy, holding 180 x 109 1 of water and, if oxidized, capable of producing 500 x 1015 g CO2 or 136 x 1015 g C. To this can be added the 270 x 109 m2 of tropical
peatlands, which, if calculated on the same basis as above, contain 29 x 1015 g C. Thus, the total amount is 165 x 1015 g C. This has to be considered as a minimum value, because for some countries only the exploitable peat areas are given (Moore and Bellamy, 1973). This value is much higher than the value given by Bazilevich (1974) (110 x 1015 g C), while according to Waksman (1938) the carbon content of all forms of peat is
1123 x 1015 g C. The latter seems an overestimate, even when an increased exploitation of peatlands is taken into account.
5.5.4 Soil Organic Matter
Soil organic matter or humus is a very important component of the carbon cycle. Accurate data are still lacking. The assessment of the amount of humus below the Ao and A1 horizon is especially difficult. Waksman (1938) estimated the humus content of the biosphere as being 400 x 1015 g C; Bolin (1970) gives 700 x 1015 g C (for all dead organic matter of the terrestrial biota). Bazilevich (1974) gives 1392 x 1015 g C (excluding peat) and Baes et al. (1976) 1080 x 1015 g C, including newly found peat and non-humus dead organic matter. The spread in data may be explained in part by differences in the various depths considered. Waksman used in his assessment a depth of 0.30 m, whereas Bazilevich assumed a depth of 1 m.
The most recent and detailed estimate was made by Bohn (1976). Based on
FAO-UNESCO soil data (FAO-UNESCO, 1971, 1974), FAO-UNESCO soil maps (1974), and Ganssen and Hädrich (1965) soil maps, Bohn's estimate of soil organic carbon was 2946 x 1015 g, to a depth of 1 m, which he considered as a somewhat conservative figure, open to considerable refinement, as 122 x 106 km2 of land area have been considered. Bohn calculated the error of his estimate to be 500 x 1015 g. No specific reference to peat is made by Bohn; therefore it is not clear whether it is included in the estimate or not.
We made a similar estimate of soil organic carbon using FAO-UNESCO data and maps for the whole of America
(FAO-UNESCO, 1971), as well as detailed data on soils of America presented by Soil Survey Staff (1975). For North America (including Canada), a total soil organic carbon content of 248 x 1015 g C was found, or 135
x 1015 g C for the United States alone. For Central and South America, the figures were 48 and 305 x 1015 g C respectively. As
FAO-UNESCO data for other continents are still lacking, an extrapolation was made by assuming the same accumulation of soil carbon per unit area for Europe, a 30% higher content for the
U.S.S.R. and Asia, and a 30% lower content per unit area in Africa and Oceania.
Table 5.8 represents our estimates and those of Bohn. There is a great
discrepancy between Bohn's value for North America and our calculations. The discrepancy between the two sets of data rests principally on the different values for total soil carbon used by Bohn and us.
Table 5.8 Quantities of organic carbon accumulation in the soil. (Number of samples in parentheses)
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2070
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Although more data on soils and its carbon content are being published, comparisons are becoming more and more difficult and complicated because of the various classifications and nomenclature systems applied (Simonson, 1967;
FAOUNESCO, 1971; Buringh, 1974; Gieseking, 1975; Kononova, 1975; Soil Survey Staff, 1975).
A second approach to estimating total soil carbon is via the various ecosystem types. A soil type, however, does not always correspond to one vegetation type, nor is one vegetation type strictly restricted to one and the same soil type. This makes extrapolation of single data very difficult, particularly as large surface areas are involved.
There is a close relationship between environment and humus accumulation, as shown in
Figure 5.2. This relationship is generally reflected in the following pattern: in cold climates, net primary production exceeds the rate of decomposition in the soil, but as temperature increases decomposition will be stimulated. At about 25
°C, with good soil aeration, there is a balance between production and breakdown, and above this temperature organic carbon does not accumulate.
Data on the humus content per ecosystem type are still very scarce and are mainly found in Russian literature. For tropical rain forests values from 13 000 up
to 25 000 g/m2 dry matter are reported (Yoda and
Kira, 1969; Klinge and Rodrigues,1973), subtropical forest soils (70 cm deep) in Japan contain 21 000 g/m2
(Kira and Shidei, 1967, cited by Odum, 1971), dry monsoon forest 15 500 g DM/ m2 (Yoda and
Kira, 1969), rendzinoids or peaty soils of temperate deciduous forests in Belgium contain 15
60018 600 to 31
60038 000 g/m2
(Duvigneaud et al., 1971), and savanna in Zaire 28 00035 000 g/m2 dry weight (Kellog and
Davol, 1949).
Figure 5.2 Relationship of soil organic matter accumulation to wetness and temperature. (After Mohr and Van
Baren, 1954. Reproduced by permission of W. van Hoeve, The Hague.)
The organic carbon content of the extensive arctic tundra soils
(gelic regosols in the FAO-nomenclature) varies greatly over short horizontal distances, and ranges from 270 g C/m2, on arctic beaches and recent floodplains, to 200 000 g C/m2 on shallow peats (cited by Bohn, 1976). Very high values are reported for tundra ecosystems in Norway and Finland, namely 7663 g C/m2 for low alpine heath, 6020 g C/m2 for a lichen heath, and 32 506 g C/m2 for a wet alpine meadow (all to a depth of 35 cm), and 2861 g C/m2 for an oligotrophic mire, 30 cm deep. By using some additional data on soil types (Bohn, 1976), an attempt was made to extrapolate to a global scale
(Table 5.9). For
peatland, the carbon content as given by Moore and Bellamy (1973) was taken and adjusted to the tropical peatland as given by Pons (1977). For conversion of dry matter to carbon, a carbon content of
58% was assumed. These calculations lead to a global amount of soil organic carbon of 1600 x 1015 g. The only conclusion that can be drawn is that earlier estimates of soil organic carbon
(Waksman, 1938; Rubey, 1951) have been too low. Soil organic carbon (humus) represents a much larger reservoir than the carbon fixed in dead organic matter such as litter, standing dead or dead wood, and also much larger than the amount of carbon in living
phytomass. However, the distinction between the various components is gradual and, because of this, an estimate of each component remains difficult. The study of dead organic matter including humus, has been neglected in the study of biological productivity of natural ecosystems and needs further attention, particularly as the impact of man on the soil carbon is increasing. Through increased decomposition as a result of clearing and cultivation, soil carbon content is decreasing in many parts of the world
(Chapter 7, this volume).
Table 5.9 Estimate of soil carbon content per ecosystem type
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| *Calculated after data from Moore and Bellamy (1973)
and from Pons (1977). |
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5.6 ANIMAL BIOMASS
Very limited data are available on total animal biomass in terrestrial ecosystems; animals represent only a small percentage of total living biomass in the biosphere.
Whittaker and Likens (1973) give a total animal biomass of 2002 x
1012 g dry weight, of which 1005 x 1012 g is for terrestrial ecosystems and 997 x 1015 g total marine
(Table 5.2). Converted into carbon, these are approximately 457 x 1012 g and 449 x 1012 g respectively (carbon content 45% of DM; Bowen, 1966). The total value for animal biomass does not include the biomass of man and domestic animals, for which Whittaker and Likens (1973b) give the values 23.6 x 1012 g C and 120 x 1012 g C respectively. A division is made for each ecosystem type, with tropical rain forests having the highest animal biomass per unit area, 9 g C/m2 ; followed by temperate forests, 7 g C/m2 ; savanna, 6.8 g C/m2 ; and marshes and swamps, 4.5 g C/m2. The value for tropical rain forests seems rather high. Usually, grasslands have a higher animal biomass than forests. The highest values are found for savanna ecosystems in East Africa, with zoomasses of up to 35 g C/m2
(Dasmann, 1964; Lamprey, 1964; Curry-Lindahl, 1971; Harris, 1972).
Duvigneaud (1974) presents similar figures for total animal biomass (terrestrial and marine), namely 2000 x 1012 g DM for animals, and 100 x 1012 g for human beings. The latter value seems rather high; with a population of 3.5 x 109 people it would mean an average weight per person of 28 kg dry weight, or 80 kg total weight. With a world population of 3.8 x 109, and assuming an average weight of 50 kg, a dry weight of 30% and a carbon content of 45%, the total human biomass can be calculated as 25.6 x 1012 g C. Doubling of the world population during the next few decades, gives an accumulation of another 25.6 x 1012 g C, which is only a very minor part of the extra carbon
released to the atmosphere by human activities.
Bowen (1966) estimated animal biomass per taxonomic group. His values are presented in
Table 5.10, which also contains the values for plant biomass as well as for decomposers. His total value for animal biomass is 3930 x 1012 g DM or 1768 x 1012 g C, which is much higher than the above-mentioned values. No explanation can be given for this discrepancy, but it might be that the values from Whittaker and Likens and from Duvigneaud do not include invertebrates and protozoa.
5.7 MICROORGANISMS
It is even more difficult to make a global estimate for the total amount of carbon fixed in the microorganisms (bacteria, yeasts, fungi), which are responsible for the decomposition of dead organic matter, thus releasing CO2 to the atmosphere. Stockli (1940, cited in
Duvigneaud, 1974) was probably the first to publish data on numbers and weights of microorganisms in soils under cultivation; his values
were:bacteria, 600 x 106 per g soil or 1000 g/m2 fresh weight; fungi,
400 x 103 per g soil or 1000 g/m2 ; protozoa, 1500 x 106 per dm
2 or 37.9 g/m2.
Table 5.10 Number of species, mean biomass in g dry matter per m2 habitat and total biomasses in 1012 g, estimated for various groups of organisms (after Bowen, 1966)
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species |
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1012 g DM |
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cont. shelf |
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2.313 |
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land; wild |
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land; tame |
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FitzPatrick (1974) states that bacteria amount to 100-600 g/m2 live weight in the top 15 cm of the soil. According to him, this weight is slightly less than fungi biomass but greater than that of the other microorganisms together. Dry weight is usually only about 10% of fresh weight. On average, a mixed deciduous forest can contain up to 30 g/m2 microorganism in the soil
(Duvigneaud, 1974). Various quantitative data on decomposers in tundra ecosystems can be found in Wielgolaski (1975) and Rosswall and Heal (1975); for wet grasslands, see Jacubczyk (1971); for dry grasslands, Clark and Paul (1970) and Andrews
et al. (1974).
As a world total for microorganisms, Duvigneaud (1974) gives 1 x 1015 g DM. By taking a mean value of 100 g/m2 C dry weight of microbial biomass (Rosswall, 1976), with a carbon content of 50%, a global figure can be calculated of 6.6 x 1015 g C for terrestrial area not covered by ice. This value is much higher than the estimates of Duvigneaud and of Bowen (1966) of 3.4 x 1015 g C (see
Table 5.10). It should be noted that autotrophic and heterotrophic microorganisms are taken together. No data could be found on quantities of autotrophic microorganisms. Their share in the decomposition processes is much more important than in the production process.
5.8 AN ATTEMPT AT COMPILATION OF DATA ON FLUXES AND POOLS
In the previous section, estimates of carbon in the various compartments of the biota were presented, whereby averages were calculated on the basis of numerous data. It has proved to be far more difficult to make estimates for the fluxes between the different spheres and the different compartments, except for the flux from the atmosphere to biosphere, the NPP. The figures for litterfall (= flux living compartment
dead compartment) are based on a smaller number of data, but might still be in the right order of magnitude (i.e. approximately
4050 x 1015
g C). In contrast to this are the data on decomposition rates of dead organic matter as the basis for an estimate of the main part of the carbon flux from land biota to the atmosphere. Extrapolation of the scattered data available is not possible. In steadystate (mature) ecosystems, however, decomposition rates, usually calculated from soil respiration data, are a reflection of net primary productivity of the system and can, therefore, serve as a basis for an indirect estimation of NPP or vice versa, if there is little herbivore consumption. Secondly, the decomposition rate is a function of litterfall, so that the value for total litterfall is often taken as the value for soil respiration (Woodwell and Pecan, 1973; Baes
et al., 1976). This, of course, is only valid in a steady-state situation. If it is assumed that this is still the case for all land biota, annual decomposition might release
4050 x 1015 g C.
It should be mentioned that CO2 escaping from soil not only comes from decomposition of accumulated organic matter by heterotrophic microorganisms, but also from root respiration. It is difficult to separate these two components of
soil respiration. However, while root respiration proper should be deducted from soil respiration, there are some components of primary production which are apt to be lost if soil respiration is used as an indirect estimate of productivity
(Wanner, 1970).
Estimates of the percentage of CO2 released by root respiration of the total soil respiration varies considerably, e.g. 30% for agricultural crops in summer (Lundegårdh, 1927; Wanner, 1970), 50% in forests under experimental conditions and 70% in summer in birch forests, and
3050% in winter in the same stand (Minderman and Vulto, 1973). Lower values are usually found for grasslands (MacFadyen, 1971). In temperate climates, in general, the quantity of escaping CO2 is strongly correlated with temperature, high values are found in summer, and low values between October and May (see for references: Minderman and Vulto, 1973). During active growth, root respiration can be many times greater than respiration of the soil organisms. On the other hand, decomposition of organic matter in the soil is subject to fluctuations depending on environmental conditions which are mainly determined by soil moisture and temperature. Moreover, the activity of microorganisms can decrease or increase because the organisms have their own definite rhythm of activity, for example each period of high activity is followed by one of low activity (Witkamp, 1963; Jacubczyk, 1971). For an extensive overview of plant litter decomposition, see Dickinson and Pugh (1974).
Table 5.11 lists some decomposition rates in different ecosystems, arranged according to latitude. The rates clearly show a relationship with temperature and moisture supply. Different temperatures at various latitudes result in different decomposition rates. The respiration figures given by Svensson et al. (1975) are, in fact, only for the period without snow. The authors compared the soil respiration with primary production at the same site. For the lichen heath and wet meadows, the soil respiration data were low in relation to primary production while those for the dry meadow and birch forest were considerably higher. When total soil respiration is lower than NPP, there is an increment in organic matter in the system. High soil respiration compared with net primary production can be explained by a high contribution from root respiration to total soil respiration.
The decomposition rate of litter in a temperate coniferous forest in France was found to be 20% in the first year, and 50% in the second year (Millar, 1974), where total mineralization is 22 years. In West Germany, the mineralization time for a coniferous forest was found to be 75 years, and for a deciduous forest 20 years.
Waid (1974) found a decomposition rate of 40% for roots of some temperate agricultural crops, and 70% for the shoots of the same crops. The yearly turnover of the surface litter, in a woodland in southern Sweden, was calculated to be 52% and, in a meadow at the same site 75% (Andersson, 1970). In weight of organic matter these give 6.50 and 7.20 g/m2 respectively. With regard to the total amount of humus present in this woodland and meadow
21 800 g/m2 and 30 400 g/m2 DM respectively
the annual turnover was 3.9% and 4.4%, corresponding to 900 and 1340 g/m2 DM (= 450 and 675 g C/m2 ; see
Table 5.11).
Table 5.11 Some decomposition rates in different ecosystem types. (All values expressed in g C/m2.)
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per hour |
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In a tropical rain forest, a daily decomposition rate of 0.6% for litter was found in the dry season (Klinge, 1973a). The decomposition of litter into humus in tropical forests can be highly influenced by termite activity; decomposition rates of up to 359 g C/m2 per year have been found near termite mounts (Maldague, 1964, 1976, cited in Jensen, 1974). Much slower rates have been found in subalpine forest
litter, namely 25% in 2 years and only 3% in the third year, with an annual decomposition in the soil of 4% (Kimura, 1963).
Some studies have been conducted on leaching of organic substances from tree canopies, but information on total amounts under natural conditions is scarce. In orchards in Denmark, it was found that 50 g C/m2 per year was lost through leaching; in deciduous forests in England this amounted to 20 g C/m2 annually. For general reviews on leaching from plants see Tukey (1970, 1971).
In a steady-state ecosystem, there exists a balance between the total input
flux and the outflux, so that NPP equals total heterotrophic respiration (= respiration of animals, man, and decomposers) plus any other output. A second main outflux is constituted by the release of
CO2, as a result of fire (man induced or naturally initiated) in natural ecosystems. Non-fuel burning has a long history and is increasing rapidly under population pressure. The ecological effects of burning on ecosystems were reviewed by Kozlowski and Ahlgren (1974). See also Bramryd
(Chapter 6, this volume) and Hampicke (Chapter
7, this volume).
Baes et al. estimate the flux from fires to be 7 ± 4 x
1015 g C per year. Reiners (1973b) gives 13 x 1015 g C/year, which combines fire and animal respiration. excluding microbial respiration. This value is considered an upper limit. Assuming that there is still a balance between input and output, and applying the NPP and decomposition values from this study and the value of Baes
et al. (1976) for fire efflux, the heterotrophic respiration, excluding microorganisms, can be estimated to be between 2 and 8 x
1015 g C annually. However, there is no proof that the balance exists and, in view of the growing scale of man's activities, more work is needed to get reliable data on input and output from various terrestrial ecosystems.
Turnover rates, i.e. the ratios between ingoing or outgoing fluxes to poolsize, are also important parameters reflecting stability or instability of the various ecosystems or the biosphere as a whole. According to Odum (1969), the ratio between NPP and total biomass is a measure of the maturity of a system, low values indicating mature, stable ecosystems, while high values are mostly found in young and less stable systems with short residence times (residence time is the reciprocal of the turnover rate). Baes
et al. (1976) discuss this topic at length. They make a division between biomass in the northern and southern hemispheres as well as between components with a rapid turnover (such as leaves, twigs, small roots, surface litter, most invertebrates) and components with a slow turnover rate (such as big tree stems, thick roots, thick row humus, peat). For further details see Baes
et al. (1976) and Harris et al. (1975).
Harris et al. (1975) present turnover rates and residence times for carbon in the various compartments of a temperate deciduous forest in Tennessee, calculated from the ratio of total carbon efflux to total ecosystem carbon pool. For the forest as a whole, a 10-year residence time is calculated, but there is a great variation in turnover rates (residence times) per compartment. This ranges from turnover rate of 0.0064 (i.e. residence time 156 years) for above ground woody components, to
0.89 (i.e. residence time 1.1 year) for rapidly decomposable litter fall. The turnover rate for soil organic matter was 0.009 31 (residence time 107 years). The comparatively long residence times of the pools of soil carbon and of above-ground woody components suggest that forests have a large capacity to act as a carbon `sink', and could effectively buffer increases in atmospheric
CO2 content. However, this can only be for a relatively short period of time, as decomposition of a larger detritus pool would release an increasing amount of
CO2 to the atmosphere (Harris
et al., 1975).
5.9 MAN'S ACTIVITIES
5.9.1 General
A major concern is the effect of human activities on the biogeochemical cycle of carbon. The release of carbon from fossil fuels, and changes in the terrestrial carbon pool caused by agricultural and forestry practices, are significant in natural global processes (Baes
et al., 1976; Woodwell and Pecan, 1973; Whittaker and Likens, 1975).
Regarding the anthropogenic effect on the global primary productivity, it has been assumed that: (i) the NPP and the phytomass should be increasing because of such factors as the use of industrial fertilizers and atmospheric
CO2 enhancement; or (ii) it could be decreasing because of pollution effects, cutting of forests and the extension of deserts. However, measurements carried out in the northern hemisphere did not show significant changes (Hall
et al., 1975). This could imply, of course; that the potentially adverse effects are approximately balanced by the stimulatory effects. More research is necessary to prove whether (i) or (ii) is correct. In
Chapter 8 (this volume) possible effects of atmospheric
CO2 increase on rates of photosynthesis are more fully discussed. Concerning man's impact through changes in land usage, see Whittaker and Likens (1973b, 1975), Bolin (1977), Woodwell and Houghton (1977), and
Chapters 6 and 7 (this volume).
5.9.2 Forest
Mainly because of the economical importance of forest ecosystems, there are numerous publications on world forest resources, covering area, production, and other aspects (Paterson, 1956; Haden-Guest
et al., 1956; Weck and Wiebecke,1961; Reinbek Weltforstatlas, 19671973;
Persson, 1974; Synnott, 1977). In fact, all these authors use FAO statistics on area and other characteristics. According to the FAO statistics, the forest area of the world has decreased from 44 x
1012 m2 to approximately 38 x 1012 m2 at least, in the period from 1952 to 1972. This decrease is continuing, with the greatest changes taking place in tropical areas (Brünig, 1977; UNESCO-MAB, 1977). In no way can it be checked how accurate the figures are. Earlier values of forest areas seem less reliable. In addition, classification and terminology cause problems in the comparison of the different sources.
The above-mentioned figures would mean an annual decrease of 270 000 km2. The current estimate of forest destruction, in the tropics alone, varies from 120 000 km2 (Bolin, 1977) to 300 000 km2 (Brünig, 1977) annually.
Between 1960 and 1970, at least 27 x 109 m2 of tropical forest was converted into cultivated land, 6 x 106 m2 into roads and railways, and 20 x 109 m2 was lost through war. Against these losses there was an increase of 9 x 109 m2 through natural reafforestation, and 3 x 109 m2 through managed reafforestation (Fittkau
et al., 1968; Persson, 1974; and Boerboom, 1976). It is estimated that the annual rate of forest destruction in tropical Africa is approximately 40 x 109 m2
(Synnott, 1977).
Boerboom (1976) reported a 40% decrease of forest area in the Philippines in the period of 1960 to 1970, as result of intensive wood cutting. He found in the same period a cut forest area enlargement from 0.2 to 0.8 x 109 m2 in North Borneo. At the same time, there was an increase in felled forest area from 1.2 to 3.5 x 109 m2 in Indonesia, as a consequence of increase of annual wood export from 14 to 17 x 106 m3 in the period of 1968 to 1974.
Forest losses can also be expressed through figures on exploited wood quantities. FAO statistics (FAO, 1950, 1953, 1954, 1972,
Yearb. Forest Prod. Stat.) show a more than ten times increase in felled wood in the world in the period of 1950 to 1970 (1435 x 1012 g and 15 820 x 1012 g respectively) with the highest increases in Asia and Africa. However, it should be mentioned that the FAO figures for
19481949 are probably not representative of all countries of each continent (see
Chapter 6, this volume). Temperate deciduous forest regions represent one of the most important biotic regions of the world, because `white man's civilization' has achieved its greatest, development in these areas. This biome is, therefore, greatly modified by man, and much of it is replaced by cultivated and forest-edge communities. The increase in cultivated areas by deforestation in the tropics only gives temporary economic successes, followed by a rapid decrease in soil fertility. The only way to preserve soil fertility here is to retain forests, which produce great amounts of litter annually, which, through rapid decomposition, is the source of nutrient which is the condition for the existence of vegetation.
As already stated, there is a close relationship between climate and accumulation of carbon in vegetations.
Figure 5.3 gives an illustration of the distribution of accumulated carbon over the different compartments in some forests. The total amount of carbon accumulated in the different forests in the form of soil organic matter, live phytomass and dead phytomass, is approximately the same. In contrast, in temperate and cold climates, the accumulation takes place in the form of humus, and in tropical forests the largest amount of carbon is found in the form of living phytomass, mainly wood, and only a small amount in the form of litter. Con,sequently, the clearing of tropical forests diminishes accumulated carbon more rapidly than in temperate forests. Moreover, an extra diminishing of carbon takes place through increased decomposition of soil organic matter after clearing, particularly in tropical areas. The cleared areas are being replaced by
agro-ecosystems, which are pools of carbon with a rapid turnover rate. According to Baes et al. (1976), the general diminishing of phytomass in the tropics is approximately 1.2 x
1015 g C per year.
Figure 5.3 Distribution of organic carbon accumulated in abiotic (soil, litter) and biotic (wood, leaves) components of forest ecosystems in different climatic zones. (1) subalpine coniferous forest, (2) boreal coniferous forest, (3) evergreen forest of warm temperate zone, and (4) tropical rain forest. (After Kira and Shidei (1967), in Odum, 1971; and Yoda and Kira (1969), in Larcher, 1976. Reproduced by permission of Saunders Co., London, and Ulmer Verlag, Stuttgart.)
The biogeochemical consequences concern not only the carbon cycle, but also the more indirect contribution of phytomass reduction and erosion to the increased transfer of nutrients from land surface to rivers and oceans. The diminishing vegetation cover, followed by erosion of the top soil, will ultimately lead to a noticeable decrease in productivity, for water conditions and nutrient content are no longer optimal for photosynthesis.
5.9.3 Grasslands
It can be expected that a decrease in forest areas is reflected in an increase in grassland area, in addition to an increase in cultivated area. The destroyed forests might turn partly into woodland area, whatever the definition of the latter may be. As a result of increasing world population, the demand for grazing land increases in order to feed domesticated animals. Due to the different concepts of what constitutes grassland, the comparison of data on surface areas is rather difficult (see different data on surface area of grassland in
Table 5.2).
According to FAO data, grassland area around 1950 amounted to 23 450 x 109 m2 and around 1974 to 30 307 x 109 m2, which would be an increase of 29.2% (see
Table 5.12); also depicted is the number of livestock units for approximately the same periods. From these figures, it can be noted that the increase in the number of livestock units is much larger than the increase in the
surface area of grassland. The high increase in the surface area in the U.S.S.R., compared with the low consumption increase, might be explained by the incompleteness of earlier data on surface area. Only North America and Europe show a slight decrease in grassland area, while livestock numbers and consumption have increased. This is mainly due to the fact that, in these continents, cattle no longer feed on grass only, but have additional food supplies. Other continents show a much higher percentage increase in livestock units than in surface areas. Thus density per unit area has increased. It is beyond doubt that certain areas are being overgrazed, particularly in Africa and Asia, which finally might lead to disastrous results.
Table 5.12 Changes in surface area per continent of permanent pastures and meadows, and the number of livestock units between
195152 and 197475
Source: FAO Yearb. of Food and Agric. Stat., vols. 7, 8 (1953, 1954). FAO Prod. Yearb., vol. 29 (1975).
|
|
Permanent pasture
x 109 m2
|
Livestock units
x 106
|
Change in %
|
|
|
|
Permanent |
|
| Continents |
1952 |
1974 |
1951-52 |
1975 |
pasture |
Livestock |
|
| Africa |
|
7 933 |
|
|
|
|
|
| Asia |
|
5 498 |
344.7 |
560.6 |
27.0 |
|
62.6 |
| North & Central America |
|
3 260 |
|
|
|
15.2 | |
|
|
| South America |
|
4 452 |
|
|
|
|
|
| Europe |
|
|
|
|
|
10.9 | |
|
|
| Oceania |
|
4 540 |
|
|
|
|
|
| U. S. S. R. |
|
3 751 |
|
|
|
|
|
|
|
|
|
|
|
|
|
| WORLD TOTAL |
23 450 |
|
|
1500.8 |
|
|
|
|
Overgrazing, together with uncontrolled burning, is one of the main threats to grassland quality. It is often not understood that natural pastures must be managed as carefully as cultivated land, because of the danger of desertification. On one side, we see an increase in grassland area all over the world, as a result of forest cutting, on the other hand there is an ever progressing desertification (better `desertization', cf. Glantz, 1977), diminishing ecosystem surfaces and vegetation cover. Much is being published on desertification, and research projects are now being set up in order to try to stop this dreadful process (Rapp et al., 1976; Glantz, 1977; De Vreede, 1977;
Chapter 6, this volume). According to De Vreede, 48 x
1012 m2 of land surface is about to become desert in the near future. The degradation process in 12 x
1012 m2 of this area is already irreversible.
Conversion of wet grasslands into cereal-crop land involves relatively few basic changes in ecosystem structure and function, which may be one reason for man's success in this area. Man's record in using grasslands as pastures, however, is rather poor.
It was stated before that biomass of wild animals in savanna ecosystems can be very high. Unfortunately, as a result of heavy poaching in Africa, the number of wild animals is decreasing at an alarming rate. How this will affect the productivity of the ecosystem cannot yet be estimated.
5.9.4 Human Area
Urbanization, in the widest sense of the word, can be considered as a special kind of desertification. As a result of increase in the population pressure and rapid industrialization in many countries, the human area is increasing at a dangerous rate.
The further spread of cities and towns, merging into giant megalopolitan centres, with large networks of roads (`megalopolis', `ecumenopilis', see Doxiadis, 1970; Papaioannou, 1971) absorbing rural space and biologically productive land, while disrupting the carbon cycle is a factor of great concern. An example is constituted by the conglomerate of Tokyo, Nagoya, and Osaka, a vast area of 76 x 109 m2 which contained 69.2 x 106 people in 1966 (Southwick, 1976). This development needs intensive study, not only from the planning, architectural, and social point of
view (cf. M & R International, 1971) but particularly for its ecological consequences.
Already in the 1950s, the metropolitan region of New York was expanding at the fantastic rate of 128 x 106 m2 per year, with an annual population increase of 200 000. Between 1940 and 1950 the fourteen largest metropolitan areas of the USA grew by no less than 19% in population (Self, 1957). At present, 73% of the population of U.S.A. live in cities of over 100 000 inhabitants. In the early 1970s, over 16 x 106 m2 of land per day in the United States passed from agricultural and biological production to urban, suburban, and commercial development. At present, about 75% of the land is now committed to intensive economic use in agriculture, industry, commerce, and housing (Southwick, 1976).
In the first 60 years of the present century, the urban surface of England and Wales doubled, rising from 5.4% to 10.8% of the land surface, representing in Great Britain an increase of 10% each decade.
In developing countries, major cities are growing at a speed comparable with that of developed countries, but at far higher densities, for instance,
São Paulo and Mexico City are among the fastest growing in the world.
This trend in land use cannot continue without serious risks. How all this will affect the biosphere, we can only guess. Heat islands are created, and pollutants will contaminate the vicinity, thus, indirectly, also influencing plant productivity and phytomass. According to Duvigneaud
et al. (1977) the emission of CO and CO2 in the Brussels agglomeration, after correction for CO2 uptake by plants, amounts to 1801 x 109 g C yearly, while garbage represented 57 x 109 g C.
Emission of toxic substances, such as sulfur, from industrial sites of densely populated areas can be transported thousands of kilometres by wind before being deposited by rains, causing significant acidification in the biota (Tamm, 1967; Bolin, 1971; Anonymous, 1972; Likens and Bormann, 1974). This can affect crop production in various ways.
5.9.5 Cultivated Land and its Potential Productivity
According to FAO figures (FAO Yearb. Food and Agr. Sta., 1953 and
FAO Prod. Yearb., 1973, 1975), cultivated land has increased from 12 220 x 109 m2 around 1950 to approximately 15 000 x 109 m2 at present, reflecting an increase of 20%. Most of these areas are used to grow annual crops, with no carbon accumulation, but a rapid carbon fixation and release within a short interval. With the increase in land area, the total production of cultivated land has also increased by a higher percentage, because in addition to an increase in cultivated land area, the productivity per unit area in many parts of the world has also been raised due to increased use of fertilizers and irrigation.
The structural change in cultivated land was studied in order to obtain some information about man's impact on the production and phytomass in cultivated land. FAO data on areas and yields was used
(FAO Yearb. Food and Agr. Stat.,1950, 1954, 1956; FAO Prod. Yearb., 1972, 1975). The crops grown on cultivated land were classified as cereals, roots and tubers, pulses and vegetables, oil crops, fibre crops, shrubs and trees. A different carbon content was assumed per group (trees
46% of DM, shrubs 45%, and all other groups 43%) and, for each group, production and phytomass were calculated for the period
194853 and 197173. Table 5.13 presents this data, which is self-explanatory.. The discrepancy between total phytomass in this table and that given in
Table
5.5 is caused by the fact that, in the latter case, average phytomass is given, while in
Table
5.13 maximum phytomass is used and a smaller area of cultivated land is considered.
Table 5.13 Changes in production and living phytomass of cultivated plants between
194853 and 197173 based on FAO yield and area data.
(FAO Yearb. of Agric. Stat., 19501955, and FAO Prod. Yearb., 1972, 1975)
(Areas in 109 m2; other figures in 1012 g/C*)
|
|
|
|
World totals |
|
|
Changes in % |
|
|
|
|
|
|
|
| Plant groups |
Period |
Area |
Production |
Phytomass |
Area |
Production |
Phytomass |
|
|
|
|
194853 | |
|
|
|
|
|
|
|
|
197173 | |
|
|
|
|
|
|
|
|
|
194853 | |
|
|
|
|
|
|
|
|
197173 | |
|
|
|
|
|
|
|
|
|
194853 | |
|
|
|
|
|
|
|
|
197173 | |
|
|
|
+135 |
|
|
|
|
|
194853 | |
|
|
|
|
|
|
|
|
197173 | |
527 |
|
|
|
27 | |
|
|
11 | |
|
|
|
194853 | |
|
|
|
|
|
|
|
|
197173 | |
137 |
|
|
|
68 | |
|
|
|
|
|
194853 | |
299 |
|
|
|
|
|
|
|
197173 | |
|
|
|
|
|
|
|
|
|
194853 | |
299 |
|
|
|
|
|
|
|
197173 | |
|
|
|
|
|
|
|
|
|
194853 | |
103 |
|
|
|
|
|
|
|
197173 | |
154 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
194853 | |
|
|
|
|
|
|
|
|
197173 | |
|
|
|
|
|
|
|
|
*Assumed carbon content of DM for trees 46%, for shrubs 45%, and for other
crops 43%. |
Figure 5.4 Changes in land use for several crops from 1950
to 1972 per continent. Based on data from FAO Production Yearbook, 1953, 1973
Figure 5.4 depicts the distribution of the various groups of crops as a percentage of total cultivated land per continent around
1950 and in 1972. After 1950, a shrinkage in the area occupied by cereals in North America and Oceania can be observed, but not in the remaining continents. The area covered by oil crops and fibre crops has decreased everywhere, which can be attributed to the introduction of synthetic fibres. The area covered by pulses and vegetables shows an important increase, reflecting the awareness of the importance of proteins as part of the human diet.
It is recognized that NPP of several natural ecosystems has been lowered by over-exploitation; however, proper
management itself might stimulate productivity. Potential production of such ecosystems could be understood in terms of original state (Bazilevich
et al., 1971). However, many natural ecosystems could probably produce still more, after addition of water and nutrients (cf. irrigation in arid zones), with the risk of radical changes in the living community. The theoretical maximum of plant production on open field depends on the supply of primary environmental factors such as: light, water, temperature, and nutrients. The concentration of carbon dioxide in the atmosphere seems to be only a secondary factor (see
Chapter 8, this volume). Several attempts have been made to estimate maximum plant productivity for the whole world and separately for cultivated land (Loomis and Williams, 1963; Schmitt, 1965; De Wit, 1968; Buringh
et al., 1975; Terjung et al., 1976).
Buringh et al. (1975) have tried to assess the absolute maximum food production by computing the potential yield of all potential agricultural land under optimal management. According to these authors, the potential agricultural land covers 34 x 1012 m2 (Revelle, 1976, gives only 26 x 1012 m2 as total area of earth suitable for agriculture) instead of the present 15 x 1012 m2. The applied reduction factors refer to those limiting properties, which are practically impossible to correct, e.g. the absence of irrigation water. The absolute maximum production expressed in grain equivalents of standard cereal crop is 32.39 x 1015 g annually, if 65% of the potential agricultural land was used to grow cereal crops. This is 30 times higher than the present production and agrees with a yield of 1400 g/m2.
This approach of the production calculation is purely theoretical and the data should not be used for comparison with real production values nor cited as possible values. It should only be used as a point of reference. In order to obtain a realistic picture, further reduction factors must be used. In this case the most important reduction results from the fact that a cereal crop does not form the closed crop canopy assumed in the above calculations during the whole of its life time. Supposing, that as an average, this ideal situation only holds for half the growing period, there still is a very large possibility for an increase in cereal production.
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