Vegetation dynamics in the southern Brazilian highlands during the last millennia and the role of bogs in Araucaria forest formation

For the first time in the highlands of southern Brazil, a palynological study was performed on sedimentary records from two Araucaria forests. Radiocarbon dates were used. The sites were Alpes de São Francisco (29°27′26″S, 50°36′57″W) and Banhado Amarelo (29°18′48″S, 50°08′13″W), located in the Eastern Plateau region of Rio Grande do Sul in São Francisco de Paula municipality. Between 13,000 and 10,800 BP, Alpes de São Francisco was cold and dry, and contained a shallow water reservoir surround by marsh, near sparse grassland, with forests probably in refuges. Between 10,800 and 9400 BP, the climate was warmer and the moisture increased. The reservoir began to be filled with herbaceous plants, regional grassland developed, and forests probably began to expand from the refuges. Between 9400 and 5600 BP, the climate was dry, grasslands decreased in size and forests became more isolated and were probably located in refuges. From 5600 BP onward the moisture increased, forming a local bog at 5000 BP. The bog was colonized by pioneer arboreal taxa and Araucaria forest that expanded until ca. 3300 BP. Between 4300 and 3300 BP, Banhado Amarelo consisted of an incipient herbaceous marsh with adjacent sparse grassland. A delay in the arrival of moisture at the site was probably due to geographic conditions. Between 3300 and 1600 BP, the humidity increased in Banhado Amarelo, forming a bog with forest cover. After 3300 BP in Alpes de São Francisco and after 1600 BP in Banhado Amarelo, the data suggest a reduction in the reproductive capacity of the forest taxa, probably due to temperature elevation. Therefore, these forests need urgent environmental conservation programs and the present-day bogs are very important as possible generators of Araucaria forest patches in Eastern Plateau depressions.