The carbon isotope record in soils along a forest-cerrado ecosystem transect : implications for vegetation changes in the Rondonia state, Southwestern Brazilian Amazon region

This paper presents carbon isotope data on soil organic matter (SOM) collected along an ecosystem transect that includes a wooded savanah (cerrado), a tropical semideciduous forest (cerradao), a forest transition type and a tropical forest. The study area is located in the Rondonia state, southwestern Brazilian Amazon region. 14C data of total soil organic matter and charcoal indicate that the organic matter in these soils is a least Holocene in age. The forest and forest transition sites are characterized by delta 13C soil depth profiles generated typically by C3 plants, indicating no major changes in plant communities have occurred in this region during the time period represented by the isotope data. In contrast, the cerrado and cerradao have experienced significant vegetation changes during the Holocene. The delta 13C data (-3% to -2.7%) obtained in the deepest part of the profile at the cerradao site show the expansion of the C3 forest vegetation into this region during early Holocene. A vegetation change consisting of increased C4 plant influence is reflected in the 13C-enriched values (-1.9% to -1.6%) at both sites during the middle Holocene. The recent part of the 13C record shows a clear expansion of C3 vegetation, particularly at the cerradao site. The regression/expansion of the forest and savannah vegetation documented at the cerradao and cerrado sites is probably related to changes from a humid to a drier climate and a return to more humid conditions and is in agreement with palaeoclimatic information reported for Brazil and the Bolivian Altiplano. This study suggests that large areas in the Amazon basin have been affected by vegetation changes during the Holocene and that soil organic matter in the transition areas between savannah and forest ecotones contains a valuable palaeorecord of vegetation changes in the Amazon region. (Résumé d'auteur

The carbon isotope record in soils along a forest-cerrado ecosystem transect : implications for vegetation changes in the Rondonia state, Southwestern Brazilian Amazon region

This paper presents carbon isotope data on soil organic matter (SOM) collected along an ecosystem transect that includes a wooded savanah (cerrado), a tropical semideciduous forest (cerradao), a forest transition type and a tropical forest. The study area is located in the Rondonia state, southwestern Brazilian Amazon region. 14C data of total soil organic matter and charcoal indicate that the organic matter in these soils is a least Holocene in age. The forest and forest transition sites are characterized by delta 13C soil depth profiles generated typically by C3 plants, indicating no major changes in plant communities have occurred in this region during the time period represented by the isotope data. In contrast, the cerrado and cerradao have experienced significant vegetation changes during the Holocene. The delta 13C data (-3% to -2.7%) obtained in the deepest part of the profile at the cerradao site show the expansion of the C3 forest vegetation into this region during early Holocene. A vegetation change consisting of increased C4 plant influence is reflected in the 13C-enriched values (-1.9% to -1.6%) at both sites during the middle Holocene. The recent part of the 13C record shows a clear expansion of C3 vegetation, particularly at the cerradao site. The regression/expansion of the forest and savannah vegetation documented at the cerradao and cerrado sites is probably related to changes from a humid to a drier climate and a return to more humid conditions and is in agreement with palaeoclimatic information reported for Brazil and the Bolivian Altiplano. This study suggests that large areas in the Amazon basin have been affected by vegetation changes during the Holocene and that soil organic matter in the transition areas between savannah and forest ecotones contains a valuable palaeorecord of vegetation changes in the Amazon region. (Résumé d'auteur

Millenial-scale climatic and vegetation changes in a northern Cerrado (Northeast, Brazil) since the Last Glacial Maximum

In the Southern Hemisphere, lacustrine sediments started to be deposited with the beginning of the deglaciation at ca 19,000 cal yr BP. At this time the region of Lake Caco was dominated by sparse and shrubby vegtation with dominance of steppic grasses in a poor sandy soil. The landscape did not present any ecological characteristics of a modern Cerrado. However single pollen grains of two Cerrado indicators, Byrsonima and Mimosa, suggest that some Cerrado species were able to survive under the prevailing and climate, probably as small shrubs. After 15,500calyrBP, a sudden increase in the moisture rates is evidenced with the progressive expansion of rainforest showing successive dominance of various associations of taxa. The development of the forest stopped abruptly at the end of the Pleistocene between 12,800 and 11,000 cal yr BP, as attested by strong fires and the expansion of Poaceae. In the early Holocene an open landscape with a relatively high level of water in the lake preceded the progressive expansion of Cerrado species towards a denser forested landscape; fires are recorded from then on, resulting in the physiognomy of the Cerrado we know today. Late Pleistocene paleoenvironmental records from northern Brazil reflect the interplay between insolation forcing of two hemispheres with the local components represented by the interannual shift of the Inter Tropical Convergence Zone and the influence of seasonal equatorwards polar air incursions. (c) 2005 Elsevier Ltd. All rights reserved

Origin and dynamics of soil organic matter and vegetation changes during the Holocene in a forest-savanna transition zone, Brazilian Amazon region

Carbon isotope data on soil organic matter (SOM) were collected along an ecosystem transect 90 m in length that includes a tropical forest on the plateau, a transitional forest-savanna and savanna in a depression. Total organic carbon data show a significant increase in carbon content from sites representing forest ecosystem to sites representing savanna ecosystem. It was hypothesized that carbon accumulation in the depression is controlled by flooding conditions that slow down carbon decomposition and in part by carbon transport from the upper part of the transect (the savanna and the transition forest-savanna areas) into the depressions by water during the rainy season. The origin of the carbon was confirmed by using soil 13C analysis. The savanna sites located in the depression showed theta13C values between -19.5% and -22.5% indicating a mixture of C3 and C4 plants. The vegetation cover in the depression is predominantly C3 grasses with theta13C values of about -27% and -26%. In the site under savanna located at an elevation slightly higher, the theta13C value was more enriched (-16%) showing the predominance of C4 plants (theta13C of -13.6%). At the forest-savanna transition and in the forest ecosystem the theta13C values were characteristic of C3 plants (-25% and -28.1%). 14C and 13C data indicate that the organic matter of mixed origin has been deposited for at least the last 7000 years in the savanna depressions. The 13C pattern observed in the soil organic matter profiles indicate a predominance of C3 plants in the early part of the Holocene. About 7000 to 4000 years ago, the data show the influence of C4 plants, indicating forest regression associated with a drier climate than at present. The most recent 13C records suggest forest expansion, and the return to a climate similar to the present. (Résumé d'auteur

Millenial-scale climatic and vegetation changes in a northern Cerrado (Northeast, Brazil) since the Last Glacial Maximum

In the Southern Hemisphere, lacustrine sediments started to be deposited with the beginning of the deglaciation at ca 19,000 cal yr BP. At this time the region of Lake Caço was dominated by sparse and shrubby vegetation with dominance of steppic grasses in a poor sandy soil. The landscape did not present any ecological characteristics of a modern Cerrado. However single pollen grains of two Cerrado indicators, Byrsonima and Mimosa, suggest that some Cerrado species were able to survive under the prevailing arid climate, probably as small shrubs. After 15,500 cal yr BP, a sudden increase in the moisture rates is evidenced with the progressive expansion of rainforest showing successive dominance of various associations of taxa. The development of the forest stopped abruptly at the end of the Pleistocene between 12,800 and 11,000 cal yr BP, as attested by strong fires and the expansion of Poaceae. In the early Holocene an open landscape with a relatively high level of water in the lake preceded the progressive expansion of Cerrado species towards a denser forested landscape; fires are recorded from then on, resulting in the physiognomy of the Cerrado we know today. Late Pleistocene paleoenvironmental records from northern Brazil reflect the interplay between insolation forcing of two hemispheres with the local components represented by the interannual shift of the Inter Tropical Convergence Zone and the influence of seasonal equatorwards polar air incursions. © 2005 Elsevier Ltd. All rights reserved.This research benefited from a CNPq (Brazil)–IRD (France) convention and is part of Paléotropique UR 55 at IRD.Peer reviewe

Holocene palaeoenvironmental reconstruction in northeastern Brazil inferred from pollen, charcoal and carbon isotope records

Soils in the Barreirinhas region, Maranhao State, were sampled for delta(13)C analysis and buried charcoal fragments in the soils were radiocarbon dated. Three soil profiles collected in forested areas around the Lagoa do Caco and one in a woody savanna (mixture of non-arboreal and arboreal species) located approximately 10 km southeast of the Lagoa were studied. A high-resolution pollen record was obtained from lake sediments, showing that forest vegetation was predominant in the area in the early Holocene. From approximately 10 000 C-14 yr BP the pollen spectrum gradually changed, suggesting the dominance of open savanna communities, these were transformed to a more forested landscape ( woody savanna) from approximately 7500 yr BP. The lake sediments also record evidence of fire ( indicated by buried charcoal particles at several soil depths) during the Holocene. The delta(13)C analysis of soil organic matter (SOM) indicates that from between approximately 10 000 yr BP and 9000 yr BP to similar to 4000 yr BP, a woody savanna prevailed at two sites around the lake, probably reflecting a drier climate. From similar to 4000 - 3000 yr BP to the present, the results indicate a moderate and progressive increase in arboreal vegetation around the lake as a result of the return to more humid climate conditions probably similar to the present-day. The carbon isotope results from the site located 10 km from the lake indicate the presence of an open vegetation from the early Holocene. In general, there is agreement between the palaeovegetation patterns inferred from the pollen and carbon isotope data. However, a much less uniform landscape, with a mosaic of different ecosystems at any given time, is inferred from the carbon isotope record