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

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Abstract

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 14C 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 14C yr BP. Seasonal climate with a long annual dry period prevailed until the late Holocene. The climate was somewhat wetter from 42 840 to 41 470 14C yr BP and from 41 470 to 26 900 14C 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°C. 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).

Introduction

Earlier palaeoenvironmental studies from the Araucaria forest and Campos regions of the southern Brazilian highlands documented large-scale environmental changes during the late Quaternary. Paleoenvironmental data from Paraná (Serra Campos Gerais; cf. Behling, 1997), Santa Catarina (Serra do Rio Rastro, Morro da Igreja, Serra da Boa Vista; cf. Behling, 1993, Behling, 1995) and Rio Grande do Sul (Aparados da Serra; cf. Roth and Lorscheitter, 1993; São Francisco de Paula; cf. Behling et al., 2001) showed that extensive areas of Campos vegetation existed on the highlands throughout the late-Glacial and early to mid-Holocene times. The dominance of Campos vegetation was attributed to drier climates, cold and dry during the late-Glacial, and warm and dry during the early Holocene (Behling, 1997). An annual dry season lasting about three months was probably characteristic of the early and mid-Holocene period (Behling, 1997). About 3000 years BP expansion of the Araucaria forests started by migration from the gallery forests along the rivers, indicating a shift to a somewhat wetter climate. A marked expansion of Araucaria forests on the highlands, replacing Campos vegetation in Santa Catarina State, began about 1000 years ago, and in Paraná State (Serra Campos Gerais) about 1500 years ago. This is a period well within the late Holocene’s very humid climate without a marked annual dry period. Araucaria forest apparently continues to expand today on the highlands under natural conditions (e.g. Klein, 1960, Klein, 1975, Rambo, 1956a, Rambo, 1956b, Rambo, 1960).

Earlier palynological studies from highland sites relied on peat bog cores within the Campos area. In these records the Araucaria forest dynamics is muted. The present study analyzes a core from a peat bog from Cambará do Sul in the state of Rio Grande do Sul, Brazil, located within the Araucaria forest formation. It provides for the first time detailed insight into the long-term processes of Araucaria forest dynamics. Among the specific aspects affecting Araucaria forest dynamics are fires which have received little attention. The present paper reports on the last 42 840 14C years history of the Araucaria forest region.

Section snippets

Geographical setting

The study region is located on the southern Brazilian highlands, in the northeastern part of the Rio Grande do Sul State (Fig. 1). The studied bog (29°03′09″S, 50°06′04″W) is situated about 7 km east of the village of Cambará do Sul near the Aparados da Serra National Park, at a distance of 6–10 km from the steep escarpment of the Serra Geral mountains. The bog is located at 1040 m elevation in a small circular rocky basin of about 50 m in diameter.

Vegetation

The vegetation prior to post-Columbian

Sampling

The bog was cored in its deepest part using a Russian corer. Sections of 50 cm length were extruded on-site, wrapped in plastic film and aluminum foil and stored under refrigeration (+4°C) in the laboratory prior to sub-sampling. Seven 1-cm-thick bulk samples were submitted for AMS radiocarbon dating.

Pollen and charcoal analyses

For pollen analysis 190 subsamples (0.25 cm3) were taken mostly at 1-cm intervals along the 211-cm-long core and were processed by standard pollen analytical methods, including acetolysis (Faegri

Stratigraphy

The sediment core starts on the bedrock subsurface 212 cm down from the surface. The material is clay from 212 to 109 cm core depth, overlain by black decomposed peat from 109 to 56 cm, and brown decomposed peat from 56 to 0 cm (Table 1).

Radiocarbon dates and chronological control

AMS radiocarbon dates were obtained for seven levels (Table 2), suggesting continuous sedimentation during the late Quaternary. The base of the core is dated at 42 658±984 14C yr BP. Between 144 and 79 cm core depth the sedimentation rate was low. First

Last glacial period (zones CDS-I to CDS-III: 42 840–10 120 14C yr BP)

The peat bog evolved from a shallow lake during the last glacial period, shown by the occurrence of Myriophyllum and Isoetes, both shallow-water indicators. The lake started to fill up with sediment from about 41 470 14C yr BP and became even shallower suggested by the reduction in Myriophyllum. At about 26 900 14C yr BP the lake ceased to be permanent as indicated by the further reduction of Myriophyllum and abundance of Botryococcus colonies. The seasonal drying of the lake during the Last

Conclusions

The pollen and charcoal records from Cambará do Sul (Table 3) reveal for the first time insight into the palaeoenvironment of the southern Brazilian highlands during the pre-LGM and LGM periods. Seven AMS radiocarbon dates and high-resolution sampling of the lake and peat bog deposits allow a detailed analysis and decadal resolution of the last 1100 years. Furthermore, the Cambará do Sul record documents for the first time how a peat bog site became surrounded by Araucaria forest, allowing

Acknowledgements

The authors are grateful to the reviewers Drs. Vera Markgraf and Jorge Rabassa for providing constructive comments. This project was supported by the Conselho Nacional de Desenvolvimento Cientı́fico e Tecnológico (CNPq; Brazil) through a research grant given to V.D.P. and fellowships to V.D.P. and L.O.

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