Solving a biogeographic puzzle in the Brazilian seasonal tropics

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Solving a biogeographic puzzle in the Brazilian seasonal tropics

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Why is there no grass in Caatinga? Caatinga is a mosaic of deciduous scrubland and dry forest that covers a vast region of semi-arid (400 – 1000 mm annual rainfall), northeastern Brazil. Elsewhere in the world, such a climate zone would be dominated by savanna, but most areas of Caatinga are entirely without grass, and the flora is an ancient arid-adapted one that has evolved independently of Cerrado, Brazil’s savanna.

In early November, Alan Andersen led a team of leading Caatinga and savanna experts on an expedition across a transition between Caatinga and Cerrado to see if they could solve the puzzle. The team included Brett Murphy, along with Professors Inara Leal (Federal University of Pernambuco), Giselda Durigan (Instituto Florestal de São Paulo ) and Alessandra Fidelis (Universidade Estadual Paulista). They met in Fortaleza (Caatinga) in the State of Ceara, and over 5 days drove the 600 km to Teresina (Cerrado) in Piaui.

What did they find? First, the transition from Caatinga was extremely abrupt, with a complete biome switch occurring within a few kilometres. Second, although grass was absent from most areas of Caatinga, an annual species of Aristida became dominant in the ground layer over extensive areas near the transition to Cerrado. These areas were clearly fire prone, but still supported Caatinga vegetation, without any sign of Cerrado trees. Third, the transition from Caatinga to Cerrado coincided with the appearance of the perennial grass Trachypogon spicatus, which dominates Cerrado in the region.

Why isn’t Caatinga savanna? Brett has suggested that deciduousness allows trees to develop sufficient canopy cover over the wet season to exclude grass. Long-term collaborator and global vegetation guru Professor William Bond (South African Earth Observation Network) proposes that much of Caatinga might previously been Cerrado, before the extinction of mega-herbivores (that would have broken up the vegetation, therefore favouring grass) about 1,000 years ago.

Lots of questions remain. What is it about the transition zone that triggers the switch from Caatinga to Cerrado? Why is there no grass in drier areas of Caatinga, where the vegetation is much more open? Why don’t Cerrado trees occur in Caatinga, especially near their border? Savanna landscapes in Australia and Africa have thicket vegetation that is structurally similar to Caatinga, but these occur as small and isolated patches rather than dominating vast regions; so why is Brazil different? Many mysteries to solve – another expedition is in order!