Constraints to seedling success of savanna and forest trees across the savanna-forest boundary.

Tropical savannas and closed forests are characterized by distinct tree communities, with most species occurring almost exclusively in only one of the two environments. The ecology of these two groups of species will largely determine the structure and dynamics of the savanna-forest boundary, but little is known about the ecological and physiological differences that might control their distributions. We performed field and nursery experiments to compare seedling establishment success, predawn leaf water potential, biomass allocation, and root carbohydrate concentration of congeneric species, each composed of one savanna species and one forest species. Seedling establishment of savanna and forest species responded differently to vegetation cover, with forest species having lowest establishment success in the open savanna and savanna species having lowest success in forest. Subsequent survival followed similar patterns, resulting in even greater differences in cumulative success. The low survival of forest species in the savanna appears related to drought stress, as seedlings of forest species had lower predawn leaf water potential than savanna species. Seedlings of savanna species had greater root: shoot ratios and root total nonstructural carbohydrate (TNC) concentration, particularly among evergreen genera. Among evergreen genera, root TNC per shoot mass, which may largely determine resprout capacity, was seven times higher in savanna species than forest species. Although water availability and microclimate may reduce the success of forest species, these factors appear unable to completely exclude forest seedling establishment in savanna. Fire, on the other hand, appears to be a much more absolute constraint to success of forest species in savanna.