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