Vegetational history of the Late glacial-Holocene transition in the grasslands of eastern Argentina

Palynological records from eight sites located in the eastern plains of Arg entina, from the humid Pampa in the east to the xerophytic woodland-dry Pam pa ecotone in the west, allow a regional reconstruction of the vegetation o f Late glacial-Holocene transition. Fossil pollen records were interpreted in terms of palaeoenvironmental information using the modern pollen-vegetat ion-climate relationship as an analogue. Although an exact analogue does no t exist today, Late glacial vegetation consisted mostly of psammophytic ste ppe in large areas of the central and southwestern Pampa grassland which, i n the south west, is associated with a shrub steppe, suggesting subhumid-dr y to semi-arid climatic conditions. The replacement of dry steppe by humid grassland and the rapid evolution of pond environments during the early Hol ocene (starting during the Late glacial-Holocene transition) suggests a shi ft towards subhumid to humid climate conditions. The Late glacial-Holocene transition is indicated in the pollen records by repeated abrupt changes in the proportions and concentrations of some taxa, such as Brassicaceae, Ast eraceae and Carduus-type. In the modern vegetation they represent species o f common occurrence in human-induced disturbance habitats, suggesting an en vironment that was subject to frequent natural disturbance such as rapid an d repeated flooding episodes that inundated the depressions along the valle y floodplains, caused by unusual rainfall events, Some species of Carduus m ay have been native and not introduced by the Europeans, suggesting that La te glacial vegetation of the Pampa grasslands may account for the abundance of ruderal species. Of special interest is the presence of Myrtaceae in so me pollen assemblages between 11,000 and 9000 yr B.P. At present, Myrtaceae are not part of the natural vegetation in the Pampa grassland. Their prese nce in the fossil pollen records may be interpreted as local deposition ins tead of extra-regional origin. Myrtaceae formed part of the psammophytic st eppe, and after Late glacial-Holocene transition they disappeared from the regional flora, perhaps due to the climatic change that occurred at the beg inning of the Holocene. It is the most significant cause of extending the a rea of some Pampa grassland taxa, but it may have reduced the area for othe rs. Of the Late glacial vegetation, at least Myrtaceae were probably not ab le to adapt to the new habitats and hence disappeared from the regional flo ra at the Pleistocene-Holocene transition. Although other plants may have d isappeared as well, these cannot be identified because pollen from these pl ants cannot be differentiated morphologically. During the Late glacial-Holo cene transition, communities with no modern counterparts probably formed an d disappeared. (C) 2000 Elsevier Science B.V. All rights reserved.