A dynamic spatial model of shifting cultivation in the highlands of Guinea, West Africa

A dynamic spatial model of tropical deforestation and land-use change was developed from remotely sensed data for the Fouta Djallon mountain range in the Republic of Guinea, West Africa. The objective was to simulate patterns of forest clearing for shifting cultivation in terms of farmers' selection behavior for new fields based on topography and proximity to villages. Data describing the current and historic condition of the vegetation cover and land use for a watershed in Guinea were derived from aerial photography and ground sampling. Maps of these conditions were prepared and entered in a geographic information system (GIS) together with topographic data. From these data, maps of secondary variables (slope, village proximity, site productivity, and labor) were derived using the spatial operators contained in the GIS. These variables were ranked for agricultural preference and then combined following a pair-wise hierarchy to generate a composite agricultural site-preference map. Shifting agriculture was simulated by running the model through 18 iterations on a two-year time increment which corresponds to the typical duration of cultivation for any one field. An empirically derived number of cells from the agricultural site-preference map were converted from forest to active farms for each iteration to simulate deforestation. Different variable combinations and underlying assumptions of model logic were tested to determine influence on simulation results. To validate the model, the spatial characteristics of the projected landscape were compared with land-use data collected in 1989, using tests for (1) size and distribution of agricultural sites, (2) image similarity (Kappa test), and (3) physical characteristics (slope and distance from population centers) of the site. Although the model did not simulate the farmers' selection behavior for topography and village proximity successfully, test results with individual variables suggest that site productivity as determined by the length of fallow is a critical variable in the site selection process. Thus, aside from documenting the dynamics of shifting cultivation, this model allows planners to evaluate alternative strategies of land-use conversion with a graphic display of zones of potential hazards. Finally, the data contained in the GIS serve as a structure for monitoring long-term change in the region.