ABSTRACT
A review of previously published palaeo-ecological data reveals a complex biogeographical history of seasonally dry tropical forests (SDTF) in South America over the Late Quaternary. Some dry forest communities (e.g. Colombian Andes) have undergone considerable re-assortment of species through the Holocene, whilst others (e.g. south-eastern Brazil) appear to have been more stable. In contrast to prediction from the dry forest refugia hypothesis, the key dry forest species, Anadenanthera colubrina, has only been an important component of the Bolivian Chiquitano dry forest since the early Holocene, and appears to have been absent during the last glacial period. Thorn-scrub caatinga vegetation probably dominated north-eastern Brazil over most of the Late Quaternary period, although it diminished during wetter intervals, e.g. towards the end of the last glacial period (c.15,500–11, 800 carbon-14 yr BP (18,500–13,800 cal yr BP)), when semi-deciduous dry forest expanded. Although the distribution of SDTF across South America during the last glacial period remains unclear, there is consistent evidence from most dry forest regions for a mid–late Holocene peak in SDTF cover, centred around 6000–3000 carbon-14 yr BP (6800–3200 cal yr BP), due to widespread reduction in precipitation across tropical South America. Rather than assuming that the current disjunct distribution of SDTF taxa across South America is attributable to vicariance (i.e. fragmentation) of once continuous populations spanning these dry forest areas, I propose population migration/dispersal as an alternative explanation.
