Functional Biogeography of Tropical Vegetation
Tropical savannas tend to occur in lower rainfall regions than do tropical forests. Nevertheless, there are also areas of considerable overlap, the so-called “zone of tension” (ZOT) where both savanna and rainforest occur. ZOTs can be found on all four continents. Within the ZOT, soil type may play an important role in influencing vegetation type, with rainforests tending to occur on the more fertile soils.
Fire plays a role maintaining savannas in higher rainfall regions usually associated with the presence of rain forest. In many cases this is a consequence of human activity. There are also numerous examples of patches of savanna naturally occurring in areas surrounded by forest. This can be associated with unusually infertile soils or with seasonal waterlogging. Climatically, much of the Brazilian cerrado should be rainforest, at least by African standards. Fig. 1 shows the frequency distribution of woody savanna in Africa compared to South America. Not only do savannas occur over a wide rainfall range, but they also tend to be more persistent in higher rainfall areas in South America than in Africa. Current models purporting to simulate the distribution of global vegetation types as a consequence of climate cannot replicate this effect, which may be a consequence of differences in average soil properties between the two continents. Developing an improved ability to predict tropical biome distributions under current climatic conditions is essential for predicting biome and climate changes in the future. This is the cornerstone of TROBIT.
There are few species that exist both in rain forest and savanna. For example, “transitional forest” in Brazil is floristically similar to rainforest, but proximal woody savanna (cerradão) is quite different its canopy structure and species composition. Such dissimilarities also occur in other ZOTs. For example miombo woodlands in Southern Africa are structurally and genetically quite distinct from patches of rainforest growing in the same mosaic. Proximal savanna and rainforests plots in Venezuela have less than 15% of their species in common.
Ability to tolerate fire is an important determinant of whether a species tends to occur in savanna or forest. But overall there seems to have been little systematic work on phylogenetic associations and physiological differences (2) of the different species occurring on either side of the savanna/forest interface. A lack of knowledge of how species compositions may be differentially altered with future climate change, and how this may affect biome level responses to such changes is addressed in (3) AND (4).