Despite the vast destruction caused by recent fires in Sumatra's forested areas, there have been few analyses of their spatial variability and causes. Various combinations of predisposing and land-use factors in space and time may cause fires. For each variable, fire densities were calculated to assess the spatial and temporal association between natural and cultural landscape variables and fire occurrence in four provinces in Sumatra, Indonesia for a non-dry year (1999) and a dry year (1997). This was complemented by a single multivariate logistic regression for the two years over the four provinces with fires as the dependent variable and land use and predisposing factors as independent variables. The provincial analyses showed that fires are determined by multiple, interacting factors and that these factor interactions are not the same in all provinces. In the non-dry year, the factors were only weak determinants of fires and only few determinants were common to all provinces (presence of undisturbed forests, elevation, smallholder area, land allocation to production area). In the dry year, more determinants of fires were found to be common to all provinces (presence of undisturbed forests, elevation, land allocation to production or conversion areas, presence of plantations, distance from roads). This led to the identification of pathways that increased fire probability. The first step was the allocation of land to different land-use types and stakeholders by national policies. If allocated to large-scale landowners, fires were more likely to occur in plantations, especially in the dry year. Logging concessions did not play a major role in increasing the incidence of fire. However the strongest increase in fire probability was outside the areas under use, in undisturbed forests. We conclude that areas not yet used by large-scale landowners were more prone to fire, indicating a serious threat to the remaining forests in this region.
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