CHANGES OF THE FOREST-SAVANNA BOUNDARY IN BRAZILIAN AMAZONIA DURING THE HOLOCENE REVEALED BY STABLE-ISOTOPE RATIOS OF SOIL ORGANIC-CARBON

The possibility of ecosystem boundary changes in northern Brazilian Am azonia 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 natural C-1 3 isotope abundance, expressed as delta(13)C, in soil organic matter ( SOM). SOM C-13 analyses and radiocarbon dating of charcoal fragments w ere carried out on samples derived from soil profiles taken along tran sects perpendicular to the ecotonal boundary. SOM delta(13)C values in the upper soil horizons appeared to be in equilibrium with the overly ing vegetation types and did not point to a movement of the boundary d uring the last decades. However, delta(13)C values obtained from deepe r savanna and forest soil layers indicated that the vegetation type ha s changed in the past. In current savanna soil profiles, we observed t he presence of mid-Holocene charcoals derived from forest species: fir e frequency at that time was probably greater, and more extensive sava nna may have resulted. Isotope data and the presence of these charcoal s thus suggest that the forest-savanna boundary has shifted significan tly 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 deve lopment of the savanna vegetation. At the beginning of the late Holoce ne, the forest may have invaded a part of this savanna, and fires occu rred again.