The Pleistocenic Arc, which must have originated from the climatic
shifts in South America during the late Pleistocene, ranges from the Caatingas
of north-eastern Brazil through south-eastern Brazil to the Paraguay and Paraná
rivers confluence, into south-western Bolivia and north-western Argentina,
and extends sporadically into dry Andean valleys of Peru or coastal western
Ecuador. It is regarded as a new phytogeographic unit for South America, here
named the Tropical Seasonal Forests Region, as it is characterized by a considerable
number of endemic plant taxa at both generic and species level. The paradigm
of the arc is the distribution pattern of Anadenanthera colubrina
(Fabaceae), which, when superimposed on those of other woody species
of seasonal forests, permits a reasonably accurate mapping of the new region,
which might also have biogeographic implications for endemic bird taxa. The
plant communities of this newly recognized unit are unique in their nature
and floristic composition, when compared with other floristic areas of the
continent by means of classic phytosociological and numerical analyses (PCA).
These ecosystems have remained submerged within other vegetation units in
South American phytogeography (such as the Chaquenian or Amazonian regions),
and have been particularly neglected in conservation policies. An urgent call
is made for these areas to be preserved before they disappear, because they
are located in areas with some of the best agricultural soils of tropical
South America, and therefore subject to clearing for farming.
