Co-ordinating Editor: V. Pillar.
Coexistence of forest and savanna in an Amazonian area from a geological perspective
Article first published online: 27 OCT 2009
© 2009 International Association for Vegetation Science
Journal of Vegetation Science
Volume 21, Issue 1, pages 120–132, February 2010
How to Cite
Rossetti, D.F., Almeida, S., Amaral, D.D., Lima, C. M. and Pessenda, L.C.R. (2010), Coexistence of forest and savanna in an Amazonian area from a geological perspective. Journal of Vegetation Science, 21: 120–132. doi: 10.1111/j.1654-1103.2009.01129.x
- Issue published online: 14 JAN 2010
- Article first published online: 27 OCT 2009
- Received 9 December 2008; Accepted 6 September 2009.
- Geological history;
- Plant distribution;
- δ13C isotope
Question: How can the coexistence of savanna and forest in Amazonian areas with relatively uniform climates be explained?
Location: Eastern Marajó Island, northeast Amazonia, Brazil.
Methods: The study integrated floristic analysis, terrain morphology, sedimentology and δ13C of soil organic matter. Floristic analysis involved rapid ecological assessment of 33 sites, determination of occurrence, specific richness, hierarchical distribution and matrix of floristic similarity between paired vegetation types. Terrain characterization was based on analysis of Landsat images using 4(R), 5(G) and 7(B) composition and digital elevation model (DEM). Sedimentology involved field descriptions of surface and core sediments. Finally, radiocarbon dating and analysis of δ13C of soil profile organic matter and natural ecotone forest-savanna was undertaken.
Results: Slight tectonic subsidence in eastern Marajó Island favours seasonal flooding, making it unsuitable for forest growth. However, this area displays slightly convex-up, sinuous morphologies related to paleochannels, covered by forest. Terra-firme lowland forests are expanding from west to east, preferentially occupying paleochannels and replacing savanna. Slack, running water during channel abandonment leads to disappearance of varzea/gallery forest at channel margins. Long-abandoned channels sustain continuous terra-firme forests, because of longer times for more species to establish. Recently abandoned channels have had less time to become sites for widespread tree development, and are either not vegetated or covered by savanna.
Conclusion: Landforms in eastern Marajó Island reflect changes in the physical environment due to reactivation of tectonic faults during the latest Quaternary. This promoted a dynamic history of channel abandonment, which controlled a set of interrelated parameters (soil type, topography, hydrology) that determined species location. Inclusion of a geological perspective for paleoenvironmental reconstruction can increase understanding of plant distribution in Amazonia.