XVI International Botanical Congess

Spatial variation in species composition has seldom been analyzed in the relatively unknown tropical dry forests. We describe patterns of tree species turnover in this system and assess some of its environmental, spatial and biotic determinants. The study was undertaken at the Chamela-Ciuxmala Biosphere Reserve, in the Pacific Coast of Mexico, that includes a very diverse and well conserved Neotropical dry forest. A total of forty seven 20 x 80 m transects were laid to explore species turnover within and among transects of a single morpho-edaphic unit, and among morpho-edaphic units. Species data base was divided into those with different present biogeographic distribution, and into groups of species with different dispersal syndromes. Results show that within morpho-edaphic unit differences in water availability played a major role in determining species turnover. Among morpho-edaphic units species turnover was mostly determined by differences in nutrient availability. Species habitat selection within altitudinal gradients was associated to their present biogeographic distribution. Species dispersal syndromes determined the nature and the strength of the spatial autocorrelation of species turnover. We conclude that relative importance of species turnover determinants change with scale.