Land use is obviously constrained by environmental factors such as soil characteristics, climate, topography, and vegetation. But it also reflects the importance of land as a key and finite resource for most human activities including agriculture, industry, forestry, energy production, settlement, recreation, and water catchment and storage. Land is a fundamental factor of production, and through much of the course of human history, it has been tightly coupled to economic growth (Richards 1990). As a result, control over land and its use is often an object of intense human interactions.
Human activities that make use of, and hence change or maintain, attributes of land cover are considered to be the proximate sources of change. They range from the initial conversion of natural forest into cropland to on-going grassland management (e.g., determining the intensity of grazing and fire frequency) (Schimel et al. 1991; Hobbs et al. 1991; Turner 1989).
Such actions arise as a consequence of a very wide range of social objectives, including the need for food, fibre, living space, and recreation; they therefore cannot be understood independent of the underlying drivingforces that motivate and constrain production and consumption. Some of these, such as property rights and the structures of power from the local to the international level, influence access to or control over land resources. Others, such as population density and the level of economic and social development, affect the demands that will be placed on the land, while technology influences the intensity of exploitation that is possible. Still others, such as agricultural pricing policies, shape land-use decisions by creating the incentives that motivate individual decision makers.
Interpretations of how these factors interact to produce different uses of the land in different environmental, historical, and social contexts are controversial in both policymaking and scholarly settings. Furthermore, there are many theories regarding which factors are the most important determinants. Particular controversy arises in assessing the relative importance of the different forces underlying land-use decisions in specific cases (e.g., Kummer 1992). For example, apparent dryland degradation could be the result of: overgrazing by increasingly numerous groups of nomadic cattle herders; an unintended consequence of a "development" intervention such as the drilling of bore holes which increases stress on land close to the wells; or the political clout of groups that, through governmental connections, are able to over-exploit land belonging to the state or local communities (Pearce 1992; NERC 1992). Identifying a particular cause may have implications for the rights of competing user groups or the formulation of policy responses.
Candidate Driving Forces of Land-Use Change
The possible forces driving land-use and land-cover changes can be grouped into six categories: population; level of aMuence; technology; political economy; political structure; and attitudes and values (e.g., Turner and Meyer 1991; Stern et al. 1992).
The first three have been linked to environmental change in the I = PAT relationship that considers environmental impact (I) to be a function of population (P), affluence (A), and technology (T) (Commoner 1972). The relationships of these three categories of driving forces with environmental change have been statistically analyzed. Some of this work specifically addresses land-use and land-cover change (Ambio 1992; Meyer and Turner 1992) and suggests measures for each category: respectively, population density, GNP or GDP per capita, and energy consumption per capita.
Of these three categories of driving forces, population produces the most controversy. It is, however, one of the few variables for which worldwide data of reasonable accuracy are available, providing a basis for statistical assessments of its role in various kinds of environmental change (e.g., Ambio 1992). At the global level of aggregation, the neoMalthusian and "comucopian" positions use the same data to reach opposite conclusions: that population growth is or is not a cause of environmental damage (Boserup 1965, 1981; Ehrlich and Ehrlich 1990; Ehrlich and Holdren 1988; Simon 1981). At the regional scale, several studies relate population growth and deforestation in developing countries in the tropics (e.g., Allen and Barnes 1985; Palo 1990; Rudel 1989), although their findings and methods have been questioned (Kummer 1992).
Comparative assessments of population and land use suggest that: (i) population growth is positively correlated with the expansion of agricultural land, land intensification, and deforestation, but (ii) these relationships are weak and dependent on the inclusion or exclusion of statistical outliers (Bilsborrow and Geores 1991; Bilsborrow and Okoth-Ogendo 1992). Sub-continental comparisons for Africa have led Zaba (1991) to conclude that population density and growth ranked below environmental endowment and economy as factors in environmental degradation. Population density was found to be related to agricultural expansion and intensification everywhere, but only in some regions to deforestation. Detailed studies of specific regions for example, modelling exercises with Amazonian data likewise indicate subtle and varying relationships aones and O'Neill 1992; Skole 1992).
The interactions of population, affluence, and technology as causes of environmental change have been explored extensively (for implications appropriate for land-use, see Lee 1986), but research on the direct association of affluence or technology with landuse change is not as common. This is because of the paucity of globally comparative data for statistical assessments and because of the common assumption that level of affluence or technology do not by themselves govern human-environment relationships but must be considered within a larger set of contextual variables.
Nonetheless, some historical assessments associate high levels of affluence and industrial development (and thus the ability to draw resources from elsewhere) with the return of forest cover (Williams, 1989; Hagerstrand and Lohm 1990; Pfister and Messerli 1990). Global comparisons indicate that afforestation is largely a phenomenon of advanced industrial societies, which are both affluent and have high technological capacity (Young et al. 1990). Wealth, however, also increases per capita consumption, bringing about environmental change through higher resource demands, although these higher demands can be reduced by advanced technologies available to wealthy societies. Poverty is often associated with environmental degradation (IDRC and SAREC, 1992), although recent research shows that this relationship is strongly influenced by other factors as well (Kates and Haarman 1992). These mixed conclusions indicate the importance of further studies of the relationship between level of affluence and environmental change.
The role of technology as a potential cause of past and prospective changes in land use and land cover also requires further study. It is obvious that technological development alters the usefulness and demand for different natural resources. The extension of basic transport infrastructure such as roads, railways, and airports, can open up previously inaccessible resources and lead to their exploitation and degradation. Technological developments and their application (such as improvements in methods of converting biomass into energy; use of information-processing technologies in crop and pest management; and the development of new plant and animal strains through research in biotechnology) may lead to major shifts in land use in both developed and developing countries during the coming decades (Brouwer and Chadwick, 1991).
To these three sets of candidate forces, three others have been added: political economy, which includes the systems of exchange, ownership, and control; political structure, involving the institutions and organization of governance; and attitudes and values of individuals and groups. The candidate driving forces grouped within these categories have received much less attention than population growth. They do not yet encompass clearly defined variables and causal relationships, but comprise similar explanations of relationships of societal and environmental change (Blaikie and Brookfield 1987). Changes in land tenure (an institution in socio-economic terms) have direct impacts on land use, as does the move from non-market to market exchange of resources (political economy). Changes in attitudes and values may add a dimension to environmental change that cannot be explained otherwise, such as impact on land use of the "green" movement. Identifying the key variables within each set of potential driving forces and developing proxy measures for them will be one of the objectives of the IGBP-HDP project on land-use and land-cover change.
Detailed examinations link all of these candidate forces (e.g., Scott et al. 1990). For example, the model of socio-economic and environmental interactions with land use developed at Oak Ridge National Laboratory explores the interrelated effects of changes in technology, political economy, and political structure in Amazonia (Jones and O'Neill 1992). Improved transport facilities are expected to exacerbate land degradation if the region in question is small, but its impact on larger regions will vary by circumstance. Additional comparative studies are needed to address the interactions of different driving forces with their environmental context.
Finally, environmental transformations - whether potential climate change or localized impacts such as soil depletion - themselves affect land use. Assessments of the impacts of climate change on land use and land cover (e.g., Glantz 1989; Parry, Carter and Konjin 1988; Riebsame 1991) rely on assumptions about land-use change that can only be improved through studies of the dynamics of land use. For example, a study in Oaxaca, Mexico, indicates that local deforestation has caused a drop in the local water table and/or a reduction in local rainfall, and that the local population has responded by expanding the area under cultivation to maintain production (Liverman 1990). Crosson (1990) calls for a better understanding of the interactions of soil depletion, landuse systems, and environmental changes.
Relatively few global aggregate or regional comparative studies have explicitly investigated the role of these proposed driving forces, either independently or as a group. Still fewer have investigated statistical relationships among them. In contrast, many regional or smaller-scale case studies have been undertaken that offer detailed insights into specific cases that cannot necessarily be generalized. Thus the literature is rich in insights and "stories", but weak in comparative assessments that illuminate the causes and courses of land cover change. As a result, research is driven by subjective interpretations and assumptions rather than by attempts to test different hypotheses.
Knowns and Unknowns
* The major categories of driving forces have been identified
The six categories (above) have both empirical bases and theoretical rationales. We are not certain, however, which specific variables would best represent themes implied in the category, nor which forces are dominant and under which conditions.
* Many candidate driving forces are associated with environmental change over the long ter?n and at a global level
Globally, population, technological capacity, and affluence have all increased, just as the Earth's land cover has been transformed. At the same time, social organization, attitudes, and values have also undergone profound changes. The specific role of any of the proposed driving forces is extremely difficult to demonstrate at this global scale of analysis, however, because of their complex interrelationships, and interactions with other factors such as social organization, attitudes, and values, which have also undergone profound changes.
* Global-level aggregate relationships are difficult to dernonstrate at sub-global scales of analysis
A number of empirical relationships between driving forces and changes at a global scale have been documented for the modern period (20th century) (e.g., Newell and Marcus 1987). Comparative regional assessments, however, show considerable variability among these variables and environmental impacts (Young et al. 1990).
* Regional studies suggest the existence of generic relationships between the causes of landuse change and changes in land cover
Comparative studies suggest that common situations exist (e.g., rapid growth in population and international commodity demand in frontier forest areas) and that classifying them will improve our understanding and modelling of land-use change (Clark 1989).
* Integrated theories of the relationships between the human causes of land-use change and resulting changes in land-cover are not suitably developed for testing or comparison
While empirical assessments should provide strong clues about driving forces, experience suggests that "inductive" assessments will not be sufficient. Further insights will follow from tests of theories that specify the processes and relationships in question.
A Schema for Relating Land Use and Changes in Land
To understand global land-cover change as an element of global environmental change, it will be necessary to specify the links between human systems generating changes both in land use and in the physical systems that are affected by the resulting changes in land covers. This understanding is facilitated through a simple systems description of the basic states, processes, and flows involved (Figure 2).
In this schema, a land cover (physical system) exists in a systemic relationship with human uses (land use) and the causes of those uses. Driving forces interact among themselves and lead to different land uses depending on the social context in which they operate. At time t,, the underlying human driving forces lead to actions precipitating demand for land use #l (# corresponds to Figure 2), which requires the manipulation of the land cover by means of technology employed in human activities such as clearing, harvesting, or adding nutrients (proximate sources of change). This manipulation is directed either to changing the existing land cover (#I to #2 or #3) or to maintaining a particular cover (#1). In the former, the existing cover is changed to a new state that must be maintained in the face of natural processes that would alter it (physical maintenance loop).
Changes to a new state of land cover are of at least two kinds: modification as in land cover #2 (e.g., fertilization of cropland or planting exotic grasses in pastures) and conversion as in land cover #3 (e.g., forest to cropland or dryland to paddy agriculture). Maintenance processes sustain the land-cover conversion (#3) or modification (#2). Therefore, proximate sources can be seen as those of conversion, modification, or maintenance.
The environmental consequences of uses of land cover (changes in the state of cover) affect the original driving forces through the environmental impacts feedback loop. Likewise these land-cover changes (#2 and #3) can be repeated elsewhere such that they reach a global magnitude that triggers climate change, which, in turn, feeds back on the local physical system, affecting land cover and, ultimately, the driving forces through the environmental impact loop. Regardless of the stimuli - local or global environmental impacts or the interaction of the driving forces in their social context - changes in driving forces at time t2 may trigger a new land use (#2), with new consequences for the land-use/cover system.
This perspective indicates that understanding of global environmental change must consider the conditions and changes in land cover engendered by changes in land use; the rates of change in the conversion-modification-maintenance processes of use; and the human forces and societal conditions that influence the kinds and rates of the processes.