Late Quaternary Araucaria forest, grassland (Campos), fire and climate dynamics, studied by high-resolution pollen, charcoal and multivariate analysis of the Cambara do Sul core in southern Brazil

摘要

Late Quaternary vegetation, fire and climate dynamics have been studied based on high-resolution dated pollen and charcoal samples and multivariate data analysis. The samples were taken from a 212-cm-long sediment core of a bog in the Cambará do Sul region on the highlands of northeastern Rio Grande do Sul State, Brazil. The records, including seven AMS radiocarbon dates, span 42 840 ~(14)C years, for the first time extending the reconstruction of past environmental changes on the southern Brazilian highlands back to the Last Glacial Maximum (LGM) and pre-LGM times. The last 1100 years provide a decadal resolution. Initially the site was a permanent shallow lake which became seasonally dry after 26 900 ~(14)C yr BP. Seasonal climate with a long annual dry period prevailed until the late Holocene. The climate was somewhat wetter from 42840 to 41470 ~(14)C yr BP and from 41470 to 26900 ~(14)C yr BP than during the LGM and the late-Glacial period. Natural fires were rare, but became very frequent after 7400 cal BP, suggesting human occupation of the southernmost Brazilian highlands since that time. The records suggest that a species-rich Campos (grassland) vegetation existed in the area under a relatively dry and cold climate during glacial times under possibly as low as -10 ℃. The record also suggests that small populations of Araucaria were probably only present in refugia of deep and protected valleys and/or on wetter coastal slopes. Campos vegetation existed through the early and mid-Holocene until 4320 cal yr BP, after which Araucaria forest expanded into the network of gallery forests along the streams. By 1100 cal yr BP the Araucaria forest replaced the Campos vegetation reflecting the onset of the wettest period with no marked annual dry season. The marked expansion of the Araucaria forest coincided with the reduction in fire. Between AD 1520 and 1770 Weinmannia became a common taxa in the Araucaria forest, suggesting a shift to warmer climatic conditions on the highlands. This interval was synchronous with a cool phase within the Little Ice Age known from North Atlantic land records. After about AD 1780 human activities changed the original forest composition, first by introducing cattle into the forest and than by selective logging of Araucaria trees. Multivariate analysis of the pollen data shows compositional changes that follow a trajectory alternating undirectional, random phases and phases with directional, sometimes fast transitions. The results also show that compositional changes in the vegetation are slower during cool periods (LGM compared to pre-LGM) and faster in warm periods (Holocene).