Genesis and dynamics of Araucaria forest during the Holocene and climatic changes in the southernmost Brazil

The palynomorphs preserved in sediments are excellent indicators in plant succession studies, showing alterations in the vegetation throughout the times. This study is about plant succession and the climatic changes in the last millennia in the eastern plateau of Rio Grande do Sul, Brazil, using palynology of a sedimentary profile inside Araucaria forest in São Francisco de Paula (29^o29'S–50^o37'W). A core 132 cm long and samples for radiocarbon dating were obtained with the Hiller Sampler. Fifteen samples were collected from the core. For chemical processing HCl, HF, KOH, and acetolysis were used, mounting the slides in glycerol-jelly. The results indicate, between 13000-11000 BP, a water reservoir in the site where Myriophyllum, Isoetes, and algae (Zygnema and Pseudoschizaea) predominated. From 11000 BP onwards, the gradual filling up of the water reservoir originated an herbaceous plant marsh with many Cyperaceae, Blechnum, Osmunda, and Phaeoceros laevis, probably due to the rising temperature and humidity at the beginning of the Holocene. The cold and humid local temperature developed a bog with a high Sphagnum concentration, reaching its climax at about 4000 BP. Then the bog declines with Sphagnum retraction. At about 3000 BP the development of the present forest initiates in this site, with expansion of Araucaria angustifolia, Podocarpus lambertii, Myrsine, Ilex, Myrtaceae, and Dicksonia sellowiana, with posterior accentuated retraction. These results indicate the potentiality of the present bogs as predecessors of future Araucaria forests and the decline of the forest at present, probably due to global heating and the local human activities.