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Solar Energy Bioconversion at the Ecosystem Level
Abstract
In nature, the importance of the bioconversion of Solar energy depends on the type of ecosystem in which this bioconversion is taking place. But the concept of ecosystem is a wide one, covering a large scale of ecological units differing in extent, structure and functioning. Adequate definitions are necessary.
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The biogeochemical cycle of carbon constitutes the basic mechanism for the production of renewable resources such as food, fibre, and fuel and for the removal of organic waste material through mineralization. Man is increasingly affecting the carbon cycle by combustion of fossil fuels, by intensifying agriculture, and by destroying segments of the vegetation cover of the earth. In particular, the increasing CO2 content of the atmosphere causes grave concern because of the climatic changes that may result from it. Thus, while having to rely to an increasing extent of renewable resources, man at the same time is now modifying in an as yet unpredictable manner the very process that is supplying these renewable resources.
In this introductory chapter, an atmospheric researcher (B. Bolin), a chemical oceanographer (E. T. Degens), an ecologist (P. Duvigneaud), and a geologist (S. Kempe) have pooled their talents: (i) to assess the air water life rock interactions as they determine the global biogeochemical cycling of carbon; and (ii) to define problem areas in the field of carbon where man's impact will be felt in the immediate future. The chapter also serves as a springboard for more detailed discussions on special topics presented in the follow-up chapters of the book.
Current knowledge of the world carbon budget is reviewed with special emphasis on the question of whether the biota is a source or a sink for CO/sub 2/. The analysis shows through convergent lines of evidence that the biota is not a sink and may be a source of CO/sub 2/ as large or larger than the fossil fuel source. The issue is important because of the potential that changes in the CO/sub 2/ content of air have for changing climate worldwide. Various analyses suggest that human activities in the near future could release large additional amounts of CO/sub 2/ into the atmosphere with results that are substantially unpredictable.
False bad news about population growth, natural resources, and the environment is published widely in the face of contradictory evidence. For example, the world supply of arable land has actually been increasing, the scarcity of natural resources including food and energy has been decreasing, and basic measures of U.S. environmental quality show positive trends. The aggregate data show no long-run negative effect of population growth upon the standard of living. Models that embody forces omitted in the past, especially the influence of population size upon productivity increase, suggest a long-run positive effect of additional people.
“Primary production” refers to energy fixed by plants. The total amount of energy fixed is usually called “gross production.” A certain fraction of gross production is used in respiration by the plants; the remainder appears as new biomass or “net primary production.” Thus for a single plant or a community of green plants:
Net Primary Production = Gross Production − Respiration (of Autotrophs)
Similar relationships occur in ecosystems except that the organic matter and respiration of heterotrophs must be included. The increase in total organic matter is “net ecosystem production”; respiration is the total respiration of the green plants (autotrophs) and the animal community and decay organisms (heterotrophs). Gross production is of course identical to that of the plant community. Thus for an ecosystem:
Net Ecosystem Production = Gross Production − Respiration (of Autotrophs and Heterotrophs)
Study of these attributes of terrestrial ecosystems is difficult, both because of the complex interrelations of the processes involved, and because of the problems of working with systems as large as whole forests. Three approaches are in use: (1) Harvest techniques measure weight increase (and caloric equivalent and chemical composition) of net production. A refinement ot this approach based on “dimension analysis” has made possible important recent advances in the study of forests. Other techniques approach gross production and respiration through measurement of exchange of gases, especially CO2. These include: (2) Enclosure studies, involving measurements of CO2 exchange in plastic enclosures of parts of ecosystems and (3) Flux techniques based on measurement of CO2 levels in the environment. All three approaches are being applied to a forest at Brookhaven National Laboratory to determine the production equation of this ecosystem.
Results to date have established general ranges of such parameters of ecosystems as total biomass, total surface area of leaves and of stems and branches, rates of decay of organic matter in soils, rates of production of roots, and rates of photosynthesis and respiration under different environmental conditions. In the Brookhaven forest net primary production is 1124 dry g/m²/yr (with an energy equivalent of 492 cal/cm²/yr), and gross production is about 2550 dry g/m²/yr; the producers or green plants thus respire 56% of their gross production. Net ecosystem production is 422 dry g/m²/yr in this young forest. The ratio of total respiration to gross production is a convenient expression of successional status; a value of 0.82 for the Brookhaven forest indicates that this is a late successional community, but not in steady-state or climax condition (1.0). A leaf surface area of 3.8 m² per m² of ground surface intercepts sunlight energy, and the ratio of net primary production to incident visible sunlight energy gives a net efficiency of primary production of 0.0088.
These and other functional characteristics of ecosystems are currently important topics of research—involving understanding of communities as biological systems, evaluation of the potential of environments to support life and man's harvest; and understanding of the fundamental meaning and consequences of man's alteration, exploitation, and pollution of ecosystems.
1. For elucidation of phytoecologial and -sociological problems the key function is the dry-matter production in plants or plant communities, which is maintained only through the dry-matter reproduction. The essential processes in the latter are matter production by photosynthetic system and transformation of the product into new production system. The increase of photosynthetic system is the indispensable factor for expanded reproduction in the vigorously developing plant.
2. The characteristics of reproduction systems are discussed mainly in relation to longevity of leaves, formation of seeds or tubers, enlargement of non-photosynthetic system. Accordingly five schemata were presented for typical dry-matter reproduction systems, i. e. annual herb, perennial herb, permanent herb, annual-leaf tree and perennial-leaf tree systems (cf. Table 1).
3. The shade tolerance of Pinus silvestris and Picea excelsa was elucidated in the light of dry-matter reproduction, on the basis of the data of Stålfelt and others. It was clearly demonstrated thereby that the longevity of leaves and the low respiration in non-photosynthetic system are cardinal factors for shade tolerance, besides the characters of leaves in photosynthesis.
Annual plant production and total plant mass are calculated for 106 types of soil and plant formations of the world, grouped in turn by major thermal belts and bioclimatic regions (humid, semiarid, arid) within these thermal belts. Aggregate magnitudes of annual growth and total plant mass by thermal belts and their bioclimatic regions follow a sequence of alternating high values in humid regions and low values in arid regions that bears a similarity to the periodic law of geographical zonality. Maximum plant production is associated with the Tropical belt, where combinations of heat and moisture favor maximum intensity of the physical-geographic process. A related paper, by Bazilevich and Rodin, appeared in Soviet Geography, January 1971.
Increases in atmospheric COâ, risks of climatic change, and impacts from such change are reviewed. Needs for closer scrutiny of changes in organic carbon and in the patterns of the biosphere are explained. The organic carbon pool is considered here as a moderate source of COâ, perhaps 1 to 3 Gtons/yr in recent years. The biosphere could become a net sink for COâ if it were managed optimally to minimize the future peak of atmospheric COâ and to maximize storage as well as production rates of organic carbon. A preliminary global computer model explores the implications of tropical and subtropical forest clearing as a nonfossil source of COâ as well as some limits on the biosphere's role as a potential sink. Because of these limits and the oceanic factors of carbonate buffering, slow physical circulation, and limited net sedimentation of carbon, models confirm that 4- to 7-fold increases of COâ could occur if projections of high fossil energy consumption were to materialize. Review of climatic changes does not support beliefs that cooling would become great enough to counteract the mean surface temperature rise of 2 to 9°C in 100 years. Changes in climate pattern, temperature, drought, and waterlevel, and in their varied biological effects, would interact to change success of crops, other ecosystems, and many social institutions.
If the amounts of wood consumed in deforestation to increase agricultural land and as firewood in underindustialized countries are added to the amount consumed by the money economies as forest products, the estimates of the amount of wood removed from the biosphere in this century should be revised upwards. The per capita ratio of the weight of carbon from net wood burned to the weight of carbon from fossil fuel burned in this century has been at least 0.1 and may have approached 1.0. The annual per capita consumption of fossil fuels and wood in the US and Brazil are tabulated and a model for short term (10 to 100 yr) circulation of CO2 into and out of the atmosphere is presented.
The development of fuels from biomass can lead naturally to dispersed facilities that incorporate food or materials production
(or both) with fuel production, forming adaptive systems that can be modified to meet evolving needs and constraints. The
technology that is appropriate to each system needs to be worked out, taking into account associated food and materials opportunities
in order to decrease the ultimate cost of energy delivered to the consumer. I analyze possible systems based on sugarcane,
corn, and guayule.
I have listed some of the ways in which the lowland tropics are not such a warm and wonderful place for the farmer, some of the reasons why it may be unreasonable to expect him to cope with the problems, and some of the ways in which the temperate zones make his task more difficult. The tropics are very close to being a tragedy of the commons on a global scale ( 69, 103 ), and it is the temperate zone's shepherds and sheep who are among the greatest offenders ( 31 ). Given that the temperate zones have some limited amount of resources with which they are willing to repay the tropics, how can these resources best be spent? The first answer, without doubt, is education, and the incorporation of what is already known about the tropics into that education. Second should be the generation of secure psychological and physical resources for governments that show they are enthusiastic about the development of an SYTA. Third should be support of intensive research needed to generate the set of site-specific rules for specific, clearly identified SYTA's.
The subject matter of youths' cultural programming is presumably determined by what they will need during the rest of their lives. A major component of this programming should be the teaching of the socioeconomic rules of a sustained-yield, nonexpanding economy, tuned to the concept of living within the carrying capacity of the country's or region's resources. Incorporating such a process into tropical school systems will cause a major upheaval, if for no other reason than that it will involve an evaluation of the country's resources, what standard of living is to be accepted by those living on them, and who is presently harvesting them. Of even greater impact, it will have to evaluate resources in terms of their ability to raise the standard of living by Y amount for X proportion of the people in the region, rather than in terms of their cash value on the world market.
For such a change to be technologically successful, it will require a great deal of pantropical information exchange. This information exchange will cost a great deal of resource, not only in travel funds and support of on-site study, but in insurance policies for the countries that are willing to take the risk of trying to change from an exploitative agroecosystem to an SYTA. For such an experiment to be sociologically successful, it will require a complete change in tropical educational systems, from emphasizing descriptions of events as they now stand, to emphasizing analysis of why things happen the way they do. This will also be very expensive, not only in retreading the technology and mind-sets of current teaching programs, but in gathering the facts on why the tropics have met their current fate.
There is a surfeit of biological and agricultural reports dealing with ecological experiments and generalities which suggest that such and such will be the outcome if such and such form of resource harvest is attempted. It is clear that human desiderata regarding a particular site are often radically different from the needs of the "average" wild animals and plants that formed the basis for such experiments and generalities. A finely tuned SYTA will come close to providing a unique solution for each region. The generalities that will rule it are highly stochastic. The more tropical the region, the more evenly weighted the suboutcomes will be, and thus the more likely each region will be to have a unique overall outcome. For example, it is easy to imagine four different parts of the tropics, each with the same kind of soil and the same climate, with four different, successful SYTA's, one based on paddy rice, one on shelterwood forestry, one on tourism, and one on shifting maize culture.
A regional experiment station working holistically toward an SYTA is potentially one of the best solutions available. As currently structured, however, almost all tropical experiment stations are inadequate for such a mission. Most commonly they are structured around a single export crop such as coffee, sugar, rubber, cotton, cacao, or tea. A major portion of their budgets comes directly or indirectly from the industry concerned. This industry can hardly be expected to wish to see its production integrated with a sustained-yield system that charges real costs for its materials. When an experiment station is centered around a major food crop, such as rice or maize, the goal becomes one of maximizing production per acre rather than per unit of resource spent; this goal may often be translated into one of generating more people. More general experiment stations tend to be established in the most productive regions of the country and, therefore, receive the most funding. Such regions (islands, intermediate elevations, areas with severe dry seasons) need experiment stations the least because they can often be successfully farmed with only slightly modified temperate zone technologies and philosophies. The administrators of tropical experiment stations often regard their job as a hardship post and tend to orient their research toward the hand that feeds them, which is certainly not the farming communities in which they have been placed.
The tropics do not need more hard cash for tractors; they need a program that will show when, where, and how hand care should be replaced with draft animals, and draft animals with tractors. The tropics do not need more randomly gathered, esoteric or applied agricultural research: they need a means to integrate what is already known into the process of developing SYTA's. The tropics do not need more food as much as a means of evaluating the resources they have and generating social systems that will maximize the standard of living possible with those resources, whatever the size. The tropics need a realistic set of expectations.
The U.S. annual biomass production for food, lumber, paper, and fiber, if used exclusively for energy, would provide 25 percent
of current energy requirements. The collection of unharvested wood residues and cull trees for direct use as fuel for small
nearby space-heating applications—especially for peak winter conditions—is an important near-term solar energy opportunity.
Improved management of hundreds of millions of acres of productive forest land is an important opportunity for the long term.
Harvest of cropland residues for energy values, new biomass production using intensive short-rotation silviculture, resubstitution
of natural products for petroleum-based synthetics, and forest management for large-scale production of electricity and synthetic
fuels are judged to be less appropriate directions for the U.S. energy system to take.
Les savanes du Bas-Congo. Essai de Phyto-sociologie topographique
- P Duvigneaud
Project Alter: Esquisse d’un régime à long terme tout solaire
- Groupe Bellevue
- De
- G Bellevue
The global Biogeochimical Carbon Cycle in Bolin et al., The Global Carbon Cycle
- B Bolin
- E T Degens
- P Duvigneaud
- S Kempe
La silva, le saltus et l’ager de garrigue
- G Kuhnholtz-Lordat
Quelques problèmes théoriques de la phyto-sociologie
- V N Sukachev
- VN Sukachev
Die Vegetation der Erde
- H Walter
Structure, Biomasses, Mineralomasses, Pro-ductivité et Captation du Plomb dans quelques associations rudérales
- P Duvigneaud
Tropical soil-vegetation catenas and mosaics
- C G T Morison
- A C Hoyle
- J F Hope-Simps On
- CGT Morison