The influences of tree biology and fire in the spatial structure of the West African savanna

International audience

English. Using a spatially explicit cellular automaton model we explore the effects of tree demography, fire-induced mortality, and seed dispersal on the spatial spread of a single tree species in a humid savanna at Lamto in West Africa. The model system is described by six parameters and consists of a grass-surrounded square grid of connecting cells, each being either inhabited by grass alone or by grass and an individual tree. In the baseline numerical simulations the tree can only recroit seedlings in immediately adjacent cells. These seedlings may perish from annual grass fires in their first year of life if they are not protected from the advancement of the fire by neighbouring reproductively mature trees. Based on preliminary parameter estimates from data collected at field sites at Lamto, we predict that fire slows, but does not stop, the spread of the tree. In the absence of fire the doubling rate of the tree population is about 6 years, whereas we predict that yearly fires prolong this to at least 30 years. The temporal dynamics of the tree population are fairly smooth and predictable as long as there are more than c. 100 cells in the system. As the number of cells is decreased below c. 100 the trajectories become increasingly variable from year to year. Mortalities from fire act in an inverse spatially density-dependent fashion, enhanc­ing tree aggregation. The role of fire in enhancing tree aggregation is supported by additional simulations in which dispersal of seeds to non-adjacent cells can occur. When a small amount of dispersal is possible the rate of tree population growth is greatly accelerated as compared to when no such dispersal occurs. We present several hypotheses to explain why the savanna at Lamto is not tree­dominated as would be predicted by the model, discuss how seed dispersal and fire influence tree dynamics; and make predictions for future testing.