title: Changes of the forest-savanna boundary in Brazilian Amazonia during the Holocene revealed by stable isotope ratios of soil organic carbon.
abstract: The possibility of ecosystem boundary changes in northern Brazilian Amazonia during the Holocene period was investigated using soil organic carbon isotope ratios. Determination of past and present fluctuations of the forest-savanna boundary involved the measurement of natural13C isotope abundance, expressed as δ13C, in soil organic matter (SOM). SOM13C analyses and radiocarbon dating of charcoal fragments were carried out on samples derived from soil profiles taken along transects perpendicular to the ecotonal boundary. SOM δ13C values in the upper soil horizons appeared to be in equilibrium with the overlying vegetation types and did not point to a movement of the boundary during the last decades. However, δ13C values obtained from deeper savanna and forest soil layers indicated that the vegetation type has changed in the past. In current savanna soil profiles, we observed the presence of mid-Holocene charcoals derived from forest species: fire frequency at that time was probably greater, and more extensive savanna may have resulted. Isotope data and the presence of these charcoals thus suggest that the forest-savanna boundary has shifted significantly in the recent Holocene period, forest being more extensive during the early Holocene than today. During the middle Holocene, the forest could have strongly regressed, and fires appeared, with a maximum development of the savanna vegetation. At the beginning of the late Holocene, the forest may have invaded a part of this savanna, and fires occurred again.
authors.affiliation: ORSTOM-INPA, CP 478, 69011-970, Manaus, AM, Brazil.
authors.forename: T
authors.initials: T
authors.lastname: Desjardins
authors.affiliation: INPA-Ecologia, CP 478, 69011-970, Manaus, AM, Brazil.
authors.forename: A C
authors.initials: AC
authors.lastname: Filho
authors.affiliation: Laboratoire de Biogéochimie isotopique, Université Pierre et Marie Curie-CNRS-INRA, UMR no 162, case 120, 4 place Jussieu, 75252, Paris cedex 05, France.
authors.forename: A
authors.initials: A
authors.lastname: Mariotti
authors.affiliation: Laboratoire de Biogéochimie isotopique, Université Pierre et Marie Curie-CNRS-INRA, UMR no 162, case 120, 4 place Jussieu, 75252, Paris cedex 05, France.
authors.forename: C
authors.initials: C
authors.lastname: Girardin
authors.affiliation: Laboratoire d'Ecologie des Sols Tropicaux, ORSTOM, 72 route d'Aulnay, 93143, Bondy cedex, France.
authors.forename: A
authors.initials: A
authors.lastname: Chauvel
keywords: 13C natural abundance; Brazilian Amazonia; Forest-savanna boundary dynamics; Paleoclimatic changes; Soil organic matter