P.N. Duinker; S. Nilsson and M.E. Chipeta
Peter N. Duinker is a professor with the Faculty of Forestry, Lakehead University, Ontario, Canada.
Sten Nilsson is head of the Boreal Forest Study, International Institute for Applied Systems Analysis, Laxenburg, Austria.
Mafa E. Chipeta is Senior Forestry Officer (Economic Analysis), Forestry Policy and Planning Division, FAO, Rome.
Exploring the possible implications of sustainable forest management for future global fibre supplies.
Forest management concepts have evolved significantly in recent decades. Not long ago it was still fashionable to speak of timber management, where forests were managed primarily for timber and other values were satisfied as circumstances permitted. The 1970s and 1980s witnessed an increasing desire to integrate non-timber values in forest management under paradigms such as multiple-use management and integrated resource management. However, even then the non-timber values were decidedly utilitarian in nature, such as recreation, tourism and control of water flows to prevent downstream flooding. In the early 1990s, ecosystem sustainability climbed to the top of the forest management agenda. As we stand on the threshold of the twenty-first century, the prevailing paradigm is in most instances called sustainable forest management (FAO, 1995).
The purpose of this article is to explore some possible implications of sustainable forest management, across a range of interpretations, for global fibre supplies during the twenty-first century. A secondary objective of the article is to discuss factors that need to be incorporated in the development and implementation of future global fibre supply analyses to make them more relevant to this form of management. A first and fundamental step is, therefore, a basic consideration of the concept of sustainable forest management.
In general, boreal forest management has been extensive and relatively insensitive from an environmental point of view
At the 1992 United Nations Conference on Environment and Development, a number of forest principles were stated, among which:
· the subject of forests is related to the entire range of environmental and development issues and opportunities, including the right to socioeconomic development on a sustainable basis; and
· the guiding objective of these principles is to contribute to the management, conservation and sustainable development of forests and to provide for their multiple and complementary functions and uses.
Long-term thinking historically has been a fundamental element of forestry in many countries. But the concepts and goals underlying this long-term thinking have evolved over time, owing to many factors including changing societal values, economic growth and population increases on a global level. A fundamental aspect of "new" long-term thinking is the concept of sustainability.
Nilsson (1991) and Seip (1996) discussed three concepts of sustainability:
· sustainable management of forest resources;
· sustainable development of forest resources; and
· forestry for sustainable development.
The first concept is an old one, concentrating on the maintenance of supply of forest products, although not considering aspects external to the forest and forestry itself. The second deals with deforestation, forest cover, forest quality, etc., but is also disconnected from ongoing transitions in a changing world. As an aside, and to be more correct, these two concepts should refer to sustainable management or development of forest resource use. A resource per se can be neither sustainable nor unsustainable (Zwahlen, 1995).
The third concept encompasses all human requirements, with a goal of increasing human welfare and aggregate benefits from the forests. This concept implies that the production of wood, the maintenance of biological diversity and the sequestration of greenhouse gases, etc. are means and not objectives, with human welfare being the overall objective (Nilsson, 1991; Seip, 1996). This also corresponds to FAO's Strategic Plan for Forestry (FAO, 1997a) the aim of which is "to enhance human well-being through the sustainable management of the world's trees and forests". A useful definition of sustainable forest management comes from the Canadian Standards Association (CSA, 1996): sustainable forest management is "management to maintain and enhance the long-term health of forest ecosystems, while providing ecological, economic, social and cultural opportunities for the benefit of present and future generations".
In terms of practical application, sustainable forest management can be approached from two basic perspectives. These perspectives represent the extremes on a continuum - on-the-ground application will undoubtedly fall somewhere in the middle.
The two interpretations have several common characteristics. It is assumed that both interpretations include a reasonable minimum of forest area put into permanent protection from industrial development, and both foresee a substantial area under intensively managed industrial plantations. Both interpretations, as discussed here, apply to large tracts of land (e.g. states, provinces or countries)' not to individual woodlots. This is not to deny, however, that salvage wood from forest clearing for agriculture or other land uses, woodlots and trees on farms and in human settlements are also significant contributors to overall fibre supply.
"Natural", sustainable forest management
In this interpretation, all forest management activities are gentler - this is forestry with a supposedly softer footprint (Booth et al., 1993). It is characterized by environmentally sound harvesting, where damage to soil and remaining vegetation is low. Harvest areas are carefully laid out to avoid sensitive ecosystems and animal populations, and trees are selected for harvest so that genetic diversity in remaining tree populations is conserved. Rates of timber removal are kept well within natural rates of wood accrual.
"Zoned" sustainable forest management
In this interpretation, more of the forest landscape would be protected from industrial development as a more secure way to conserve biodiversity (Binkley, 1997). At the same time, though, there would be much more focus on high-intensity timber production in the areas that were not protected. In essence, the area of soft-touch timber management becomes much reduced compared with natural forest management, and the areas at opposite ends of the naturalness spectrum - intensive timber and protected areas - are expanded.
How these two options for implementation (and the range of alternatives along the continuum) would influence future global fibre supply is an open and hotly debated question. Williams, Duinker and Bull in FAO (1997b) concluded that, based on current literature, natural sustainable forest management seems to lead to reductions in sustainable harvesting volume, at least in the short term, with concomitant increases in delivered wood costs. There have been very few studies carried out on the effects of implementing zoned sustainable forest management. Ask, Dahlin and Sallnäs (1997) studied two areas in Sweden on this issue. They ranked the forest landscapes with respect to the suitability for intensive industrial wood production and to the ecological values. Based on these rankings, a zoned management programme was identified and modelled, and the consequences were evaluated for a 100-year period. The zonation programme generated, on average, a somewhat higher fibre supply during the 100-year period than the current forest management regimes in Sweden. But the economic result of the current management regimes was superior to the zonation programme, owing to the heavy investment required in the latter.
The following sections of the article make some general observations as to the potential implications of the two interpretations of sustainable forest management outlined above on long-term wood-fibre supplies in the boreal, temperate and tropical forests of the world.
At the risk of overgeneralizing, up to recent decades North American and Russian boreal forest management was characterized by extensive, relatively environmentally insensitive forest management which focused on the exploitation of natural forests for timber. More recently, North American boreal forest management has taken a strong turn towards environmental sensitivity, with only limited increases in silvicultural intensity. Russian forest management has changed little except for dramatically reduced harvest levels during the past decade as a result of economic and political factors outside the forest sector. In contrast, boreal forest management in the Nordic countries, particularly Sweden and Finland, was characterized by intensive but environmentally more sensitive forest management. Nordic forest management has also taken a strong turn towards even greater environmental sensitivity, but silvicultural intensity remains relatively high.
Given constant silvicultural intensity, the first environmental gains in boreal forest management (achieved simply through better implementation of conventional practices) had relatively low impacts on timber production. However, the incorporation of additional environmental concerns in the 1990s is significantly reducing long-term wood supply and raising wood costs.
It appears that, all other things being equal, the application of "natural" sustainable forest management in the boreal forests would at best maintain long-term sustainable fibre supplies and possibly could reduce them substantially. A key issue here will be timber-harvest economics. If sustainable management means a more widely distributed, selection-oriented timber harvest rather than an area-concentrated clear-cut system, harvest costs may increase substantially (Gooding and van Damme, 1996). "Zoned" sustainable forest management, as Binkley (1997) argued, has the potential to achieve better biodiversity conservation and maintain or increase timber production, but only if: i) large areas are zoned for timber production and are managed intensively for wood production (with the necessary additional financial inputs and at the expense of biodiversity considerations in these areas); and ii) areas zoned for protection (i.e. for biodiversity conservation) are expanded severalfold.
A well-managed forest in Austria. Most temperate forests are far from their original "natural" state
The world's temperate forests have been altered by people for a much longer period and at much greater intensities than the veal forests. Only minor pockets of original natural forest remain and most of these are in areas that are unavailable as sources of industrial fibre for policy, physical or economic reasons. Timber growth rates are generally higher than in the boreal zone, proportions of non-coniferous species are higher, gap dynamics are more prevalent (as opposed to broad-scale disturbances) and a much higher proportion of the forest land is owned privately.
The biodiversity status of temperate forests is much more eroded than that of boreal forests and, because of high human densities, the possibilities of establishing large protected areas with strong assemblages of natural biota are small.
The world's temperate forests are, by and large, far from natural. If "natural" sustainable forest management is pursued, fibre production can be kept high and/or restored from a degraded situation, but it must be with the recognition that biodiversity standards would be more or less arbitrary. Zonation for sustainable forest management does not appear to have applicability, at least at a regional level.
A final point is that people in temperate areas, where so much commercial agriculture is practiced, may well find tree-fibre plantations to be a strong improvement (ecologically and aesthetically) to their otherwise annually cropped landscapes. In this case, sustainable "land" management, as opposed to sustainable forest management, could lead to higher availabilities of industrial fibre. In this respect, it is noteworthy that the temperate zone is already home to large expanses of tree plantations, examples being China, Chile, Japan, New Zealand and the United States (the latter mostly in the subtropical belt).
The situation in tropical forests, both humid and arid, vis-à-vis sustainable forest management is decidedly complex. For one thing, these forests are much more biologically diverse and their utilization for fibre has always been a challenge. Thus, the focus up to now has been on selected primary species that are often a small fraction of gross fibre volumes. It is also complex because of the relatively underdeveloped status of economies and the generally weak institutional infrastructure for forest utilization and management. In a few countries, India, the Philippines, and Thailand being notable examples, environmental concerns have led to logging being banned or severely restricted from natural forests, thus effectively withdrawing them from industrial raw material provision. As prosperity increases in the developing countries, a growing environmental consciousness could lead to more of this and preference for more conservation.
Beyond these commonalities, it is helpful to separate issues of the tropical humid forests (a favourite of the global environmental community) and the more open forests and woodlands. The remaining major blocks of humid forest are limited to the Amazon and Congo basins and the Indonesian archipelago. In all of these forests and particularly in the Amazon and Congo, forest-dwelling people are still relatively sparse. The forest is basically protected by its inaccessibility and by the poverty of its few inhabitants. Where roads and other transport infrastructure develops, however, the evidence shows that rapid forest destruction is possible, especially if policy incentives favour settlement and the replacement of forests with large-scale agriculture.
Of relevance to fibre supply is the fact that, in the process of land clearing, large volumes of wood that are potentially available for industrial use are currently burnt (this being the cheapest land clearing method for moist forest). Forest industries could, with proper scheduling and cooperation with land planning and agricultural agencies, salvage and capture the wood that is at present wasted; however, the volumes of fibre produced in this way would not be sustainable.
The potential to extract industrial fibre from natural forests in the dry zone is constrained by three main factors: the quality of the forest has low volumes of industrially usable fibre for existing technologies; population pressure is already high and climbing rapidly, with consequent rapid deforestation; and, in many countries, protected areas are already large. In certain countries (e.g. Botswana, Kenya, the United Republic of Tanzania, Zimbabwe and Zambia), large tracts have already been reserved as protected areas for wildlife to support economically important tourism. Thus, as in temperate forests, the creation of additional large protected areas is unlikely.
There have been interesting responses to fibre shortage in the tropics, most notable being industrial plantations of exotic high-yield trees. Clonal pulpwood Eucalyptus sp. plantations in Brazil and the Republic of the Congo are already well publicized; nearly as well known are subtropical pine plantations in East and southern Africa. In Indonesia and Malaysia and a few other moist tropical countries, the greater convenience of Acacia sp. and other single-species plantations for pulpwood has led to investment in extensive plantations as an alternative to pulping mixed hardwoods.
In the same countries, agricultural tree crops, particularly rubberwood and coconut, are becoming significant sources of sawntimber - the former particularly for furniture. Recently, oil-palm fibre, of which vast quantities are wasted annually, has proved suitable for panel products (MDF) and pulping tests are advanced. Trees growing around homesteads and on farmlands are an important complement to forests for fibre supply - in South Asia, as much as two-thirds or more of fuelwood comes from such sources. Some industrial roundwood is also produced from outside forests.
Having considered the potential implications of sustainable forest management on future global fibre supply, a related question arises: How, if at all, do current methods for analysing global fibre (both supply and demand) take sustainable forest management issues into consideration, and what areas are there for improvement?
According to Seip (1996), "several analyses have been published on future trends in the production and consumption of wood, in particular industrial wood, but there are difficulties in drawing conclusions about sustainability from existing analyses". He continues, "the problems cannot be postponed or ignored if a policy for sustainable development is to be formed. A closer analysis of this problem is an important task for international organizations and institutes." Nilsson (1997) claims that the most recent wood supply analyses carried out by international organizations neglect the concept of forestry for sustainable development.
There are basically two different approaches employed today for global fibre analyses in an aggregated form:
· a general econometric equilibrium framework (e.g. FAO, 1997c);
· sustainable yield analyses (e.g. WRI, 1997).
In the general equilibrium approach, demand and supply are treated simultaneously with the help of econometric functions. The model currently being used in FAO to apply the general equilibrium theory is the Global Forest Products model. The purpose of the model is to analyse and project world forest products consumption, production and trade. It was developed with the Price Endogenous Linear Programming System (PELPS III) (Zhang et al. in FAO, 1997c). PELPS is a general modelling tool for sector analysis. It finds a spatial, competitive equilibrium by maximizing, for each year, the sum of producer and consumer surplus subject to material balance and capacity constraints in each country. Because material flows throughout the system must balance, it helps check the consistency of data and ensures the coherence of projections.
The general principle of the PELPS system is that markets optimize the allocation of resources in the short term (within one year). Long-term resource allocation is partly governed by market forces, as in capacity expansion and trade, and also by other forces such as the wood supply potential determined by forest policy, the wastepaper recovery rates induced by environmental policy, the trade inertial that depend in part on international agreements and production techniques determined by exogenous progress.
In the sustainable yield approach, there are no links to the demand side and no econometric analyses involved. This approach is a form of sustainable biological supply of fibre under current land use and management regimes (WRI, 1997). The sustainable yield is adjusted to a "probable supply" for industrial purposes. In essence, the forest undisturbed by humans is harvested over a time and at a rate set by policy-makers. Once this initial forest is changed into a forest disturbed by humans, the driving variable to determine the sustainable rate of harvest is the net annual growth (which in some countries is substituted with actual volumes removed until growth information becomes available). The growth of trees is a biological constraint to short-term economically efficient solutions. This ensures that many other forest values are protected and that there is a security of supply for industrial production in the long term. Foresters have long used the sustainable yield approach as a central theoretical framework for forest management. However, although foresters and ecologists have now expanded on the principle of sustained yield to allow for the more explicit inclusion of other resource values such as biological diversity, these elements have been given virtually no consideration in most global fibre analyses, under either of the theoretical frameworks described above.
Humid forest in Honduras. Growing conservation consciousness regarding tropical forests is leading to reductions in exploitation
Land use issues
Future fibre studies need to consider probable and possible changes in land use. Sustainability needs to be achieved not only within a specific forest landscape, but also within a wider land use context. Land use allocation is a key driving force with respect to poverty alleviation and economic development in many parts of the world. For example, based on information produced by Fischer and Heilig (1996), Nilsson (1997) calculated the forest area that is threatened by increased population and associated increased food demands as well as the areas required for housing and other infrastructure. These estimates show that by the year 2030, some 157 million ha of forests are at risk of permanent conversion to other uses, even under the assumption that other suitable land will be used before forest land for food production and habitation.
Global food supply
The global food supply issue is closely linked to the land use issues presented above. Hoskins (1990) and Ogden (1990) have illustrated the important role of existing forests to food security. They noted that trees and forests contribute to food security by providing a direct source of food supply, by providing essential nutrients and medicines that increase the natural impact of other foods, and by filling food gaps by supplying food during seasonal shortages. However, there are no regional estimates of forest food requirements and potential food supply from available trees and forests. In certain regions, the food value of the forests greatly overshadows the timber value, for example Peters, Gentry and Mendelsohn (1989) indicated that the food value is six times higher than the timber value.
Energy is a crucial factor for sustainable socio-economic development in many regions. In 1995 it was estimated that 4 billion to 4.5 billion people did not have the energy required for sustainable development. Nilsson (1996) estimated the requirements for fuelwood and charcoal at 4.5 billion m3 by the year 2020. This varies significantly from conventional demand estimates based on consumption trends; for example, Apsey and Reed (1995) estimated 2.5 billion m3 for the same year. These conventional estimates mainly deal with household demands and the developing world's energy requirements. In contrast, wood fibres are also expected to play a more important role in large-scale energy systems in the future. The World Energy Council (WEC/IIASA, 1995) estimated that between 700 million and 1350 million ha of land are needed for biomass energy production by the middle of the next century to close the global energy balance. This estimate was made prior to the Conference on Climate [Ed. note: the Third Conference of the Parties to the UN Framework Convention on Climate, held in Kyoto, Japan in December 1997], the outcome of which postulates an even greater importance of wood for energy in the future.
Soil and water resources
During the past 45 years, more than 1.2 billion ha of land (an area the size of China and India combined) has suffered moderate to extreme soil degradation (Oldeman, Hakkeling and Sombrock, 1990; WRI, 1992). Stabilizing soil, preventing erosion, controlling water runoff in catchment areas, providing shelter from wind and heat and against sandstorms and dust are all roles for which trees are much needed (Evans, 1992). Wiersum and Ketner (1989) calculated the need for some 120 million to 170 million ha of plantations in the tropics to fulfill these functions. The data are somewhat outdated and probably underestimate the actual requirements. Moreover, they do not include requirements in subtropical, temperate and boreal zones.
Forest sustainability issues need to be considered in a broad land use context. In the photo: deforestation or agriculture in Mexico
The climate change issue has been raised even higher on the global agenda since the Kyoto Conference on Climate. At this conference, the countries involved, individually or jointly, agreed to reduce overall emissions of greenhouse gases (in CO2 equivalents) to at least 5 percent below 1990 emission levels. This corresponds to some 130 million tonnes of carbon, calculated on basic figures from Bolin (1998). The prospects for improving the carbon balance through plantations, carbon-directed silviculture and replacement of fossil fuels by wood are positive (Nilsson, 1996) but management for increased carbon sequestration (longer rotation periods, increased harvestable minimum diameter, etc.) and replacement of fossil fuels by wood may reduce industrial fibre supply potential.
Currently there is no consensus on which indicators to use to measure biodiversity, and data that would aid in identifying where biodiversity is threatened and what local pressures it faces are missing (World Bank, 1995). However, this lack of data must not be used as an excuse to avoid the question of biodiversity in fibre supply analyses. The integration of biodiversity conservation goals into forest management will almost certainly result in a decrease in sustainable timber supply. A pilot study in Sweden (Lundström, Nilsson and Ståhl, 1997) analysed the impact on the industrial wood supply of agreed Forest Stewardship Council certification standards in Sweden (a significant component of which is biodiversity conservation). The long-term supply would decrease by some 12 to 15 percent under certified forest management in comparison with the current Swedish forest management.
Amenity and well-being
There are limited statistics with respect to the current existence and future requirements of forests related to the provision of amenity and well-being. Nilsson (1996) attempted to estimate the possible requirements for these functions in Europe, based on UN statistics for 1980 and 1990. Areas with a high to medium demand for different amenity and well-being functions increased by 8 to 29 percent over the time period. Continued development in this direction is expected in most parts of the world and may have impacts on the amount of forest area available for future fibre supply.
This article has attempted to examine the implications that the implementation of sustainable forest management would have for global fibre supply. Raising long-term sustainable timber harvests on all areas of forest land is physically possible through management intensification but clearly undesirable from a general ecological viewpoint and specifically from a biodiversity perspective - while perhaps also being socially and economically unfeasible. Protecting more large forest areas from industrial development has clear benefits for ecological aspects of forests, including biodiversity conservation, but clear negative consequences for industrial fibre supplies. Environmentally smarter extensive forest management generates strong environmental gains for small economic pain. In some areas of the world, for example the Russian Federation and some tropical areas, these gains are yet to be made. In most places, however, these gains are behind us and the future demands tougher choices to be made.
In a world that wants more industrial wood fibre (arguably the most environmentally benign raw material for both structural materials and pulp-based products), more fuelwood, more forest cover, stronger conservation of biological diversity and more non-timber forest goods and services, our view is that sustainable forest management will have to be characterized by increased overall forest area, an increased demarcation of fully protected areas and increased intensification of timber production, including industrial plantations. Moving sustainable forest management from the realm of theory into that of practical application will require technical, political and financial commitments. Of the three, perhaps the financial side is the one where most still remains to be done. Sustainable forest management has a price tag and, in the final analysis, the only way that it can be implemented without significant reductions in global fibre supply is for the users of fibre to agree to pay significantly more for their supplies, whether they be fuelwood, raw materials for industry or finished products.
This article has also attempted to illustrate elements needed to be taken into consideration in future global wood analyses in order to respond to concepts of forestry for sustainable development and sustainable forest management and to live up to international agreements. The following articles in this issue of Unasylva illustrate the first efforts by FAO on a difficult but necessary road to new global fibre analysis and global sustainability. One point that emerges consistently throughout these articles is the fundamental lack of data needed to fuel any form of fibre analysis. The limited value of trying to implement forward-looking analyses based on current, unreliable data and, on the basis of these, to form global policies on forestry for sustainable development (including fibre supply) and sustainable forest management would seem obvious. Until this data gap is filled, the hope for implementation of sustainable forest management is slim indeed.
Apsey, M. & Reed, L. 1995. World timber resources outlook, current perceptions. Discussion Paper. Vancouver, Canada, Council of Forest Industries.
Ask, P., Dahlin, B. & Sallnäs, O. 1997. Differentiated land use - what will it look like in practice. J. Royal Swed. Acad. Agric. For., 136(14) 9-22. (in Swedish)
Binkley, C.S. 1997. Preserving nature through intensive plantation forestry: the case for forest land allocation with illustrations from British Columbia. For. Chron., 73: 553-559.
Bolin, B. 1998. The Kyoto negotiations on climate change: a science perspective. Science, 279 (16 January): 330-331
Booth, D.L., Boulter, D.W.K., Neave, D.J., Rotherham, A.A. & Welsh, D.A. 1993 Natural forest landscape management: a strategy for Canada. For. Chron., 69: 141-145.
CSA. 1996. A sustainable forest management system: guidance document. CAN/CSA-Z808-96, Environmental Technology: a National Standard of Canada. Etobicoke, Ontario, Canadian Standards Association.
Evans, J. 1992. Plantation forestry in the tropics. Oxford, UK, Clarendon Press/Oxford University Press.
FAO. 1995. The challenge of sustainable forest management. Rome.
FAO. 1997a. FAO's Strategic Plan for Forestry. Rome.
FAO. 1997b. Implications of sustainable forest management for global fibre supplies. By J. Williams, P. Duinker & G. Bull. Working Paper No. GFSS/WP/OS, Global Supply Study. Rome.
FAO. 1997c. FAO provisional outlook for global forest products consumption, production and trade to 2010. By D. Zhang. J. Buongiorno, Y. Zhang & S. Zhu. Rome.
Fischer, G. & Heilig, G.K. 1996. Population momentum and the demand on land and water resources. Working Paper No. 96-149. Laxenburg, Austria. IIASA.
Gooding, T. & van Damme, L. 1996. Computer simulation comparisons between an ecosystem management strategy and clearcutting with artificial regeneration for a forest in northwestern Ontario. NODA Note 25. Sault Sainte Marie, Ontario, Canada, Canadian Forest Service.
Hoskins, M. 1990. The contribution of forestry to food security. Unasylva, 41(1): 3-13.
Lundström, A., Nilsson, P. & Ståhl, G. 1997. The consequences of certification on possible supply of industrial wood and fuelwood. A pilot study. Working Paper No. 23. Department of Forest Resource Management and Geomatics, Swedish University of Agricultural Sciences, Umeå, Sweden. (in Swedish)
Nilsson, N.-E. 1991. Forestry for sustainable development. Statement at ITTO Council, May 1991, ITTO, Yokohama, Japan.
Nilsson, S. 1996. Do we have enough forests? Occasional Paper No. 5. Vienna, IUFRO.
Nilsson, S. 1997. Critique of the FAO provisional outlook for global forest products, consumption production and trade to 2010. Paper presented (for IIASA) at the FAO Forest Sector Outlook Studies Working Group, 24-25 February 1997, Rome.
Ogden, C. 1990. Building nutritional considerations into forestry development efforts. Unasylva, 41(1): 20-28.
Oldeman, L.R., Hakkeling, R.T.A. & Sombrock, W.G. 1990. World map of the status of human-induced soil degradation: an explanatory note. Wageningen, the Netherlands, International Soil Reference and Information Centre.
Peters, C., Gentry, A. & Mendelsohn, R. 1989. Valuation of an Amazonian rainforest. Nature, 339: 655-656.
Seip, H.K. 1996. Forestry for human development: a global imperative. Oslo, Scandinavian University Press.
WEC/IIASA. 1995. Global energy perspectives to 2050 and beyond. London, World Energy Council.
Wiersum, K.F. & Ketner, P. 1989. Reforestation: a feasible contribution to reducing the atmospheric carbon dioxide content. In P.A. Okken, R.J. Swart & S. Swerver, eds. Climate and energy. Dordrecht, the Netherlands, Kluwer Academic Publishers.
World Bank. 1995. Monitoring environmental progress. Report on work in progress in environmentally sustainable development. Washington, DC, World Bank.
WRI. 1992. World resources 1992-1993. A guide to the global environment. New York, World Resources Institute and Oxford, UK, Oxford University Press.
WRI. 1997. Monitoring the global wood fibre equation. Reston, VA, USA, Wood Resources International.
Zwahlen, R. 1995. The sustainability of resources versus the sustainability of use: a comment. Int. J. Sustain. Dev. World Ecol., 2: 294-296.