Wednesday, August 5, 2009
Exhibit Hall NE & SE, Albuquerque Convention Center
Background/Question/Methods
In Gran Sabana the high incidence of fires has been considered the main factor causing forest loss. However, this hypothesis is supported by few empirical data. Furthermore, only two paleoecological studies considering the role of fire on late Holocene vegetation have been conducted in such a large area (18.000 Km2); both of them restricted to southern Gran Sabana. Here we present a paleoecological record of northern Gran Sabana, sampled from the Parupa riversides (5°43´N, 61°35´W, 1,226 m a.s.l.) with a Dachnovsky corer. An analysis of pollen and particulate charcoal of the200 cm peat was done with standard methods. Radiocarbon ages are not available yet, but sediments dated previously have shown that most peat deposits from upland and highland Guayana come from middle to late Holocene. Current vegetation is treeless savanna, with fragmented gallery forests and Pteridium aquilinum fernland patches.
Results/Conclusions
Palynological analysis showed the replacement of a pre-existing gallery forest by a treeless savanna, mediated by local fire events. The sequence of vegetation change was as follows: 1) between 200-150 cm trees and shrubs reached 30-40% of pollen sum, while savanna elements (mainly Poaceae and Cyperaceae) were about 60-70%. Arboreal pollen were dominated by Dimorphandra and Protium, but Coussapoa, Pourouma and Pouteria were also present, together with Miconia, Schefflera, Ilex and other pioneer shrubs and trees. The pollen spectra suggest the development of a thin and disturbed gallery forest belt, next to treeless savannas. Currently, these savanna-forest boundaries are highly exposed to fires settled in savanna areas. The past occurrence of local fire events at this depth is supported by the high concentration of particulate charcoal (>250 um size). 2) Between 150-140 cm Dimorphandra pollen decreased and Miconia increased, suggesting the development of a pioneer shrubby assemblage. Particulate charcoal is found everywhere in the record, but mainly at this depth suggesting that local fire encouraged the increase of Miconia. 3) At 140 cm Dimorphandra pollen and other gallery forest elements disappeared and savanna pollen increased to >90%. This assemblage prevailed until 40 cm, where pollen of Ilex increased slightly, and bushy vegetation was developed. 4) Between 40-0 cm shrubs decreased again while particulate carbon, together with Poaceae increased, showing the establishment of the current non-forested riverside. Our results support the idea that fire is a triggering factor in gallery forest degradation; however radiocarbon ages are necessary to set up a chronology and to achieve further conclusions.
In Gran Sabana the high incidence of fires has been considered the main factor causing forest loss. However, this hypothesis is supported by few empirical data. Furthermore, only two paleoecological studies considering the role of fire on late Holocene vegetation have been conducted in such a large area (18.000 Km2); both of them restricted to southern Gran Sabana. Here we present a paleoecological record of northern Gran Sabana, sampled from the Parupa riversides (5°43´N, 61°35´W, 1,226 m a.s.l.) with a Dachnovsky corer. An analysis of pollen and particulate charcoal of the
Results/Conclusions
Palynological analysis showed the replacement of a pre-existing gallery forest by a treeless savanna, mediated by local fire events. The sequence of vegetation change was as follows: 1) between 200-150 cm trees and shrubs reached 30-40% of pollen sum, while savanna elements (mainly Poaceae and Cyperaceae) were about 60-70%. Arboreal pollen were dominated by Dimorphandra and Protium, but Coussapoa, Pourouma and Pouteria were also present, together with Miconia, Schefflera, Ilex and other pioneer shrubs and trees. The pollen spectra suggest the development of a thin and disturbed gallery forest belt, next to treeless savannas. Currently, these savanna-forest boundaries are highly exposed to fires settled in savanna areas. The past occurrence of local fire events at this depth is supported by the high concentration of particulate charcoal (>250 um size). 2) Between 150-140 cm Dimorphandra pollen decreased and Miconia increased, suggesting the development of a pioneer shrubby assemblage. Particulate charcoal is found everywhere in the record, but mainly at this depth suggesting that local fire encouraged the increase of Miconia. 3) At 140 cm Dimorphandra pollen and other gallery forest elements disappeared and savanna pollen increased to >90%. This assemblage prevailed until 40 cm, where pollen of Ilex increased slightly, and bushy vegetation was developed. 4) Between 40-0 cm shrubs decreased again while particulate carbon, together with Poaceae increased, showing the establishment of the current non-forested riverside. Our results support the idea that fire is a triggering factor in gallery forest degradation; however radiocarbon ages are necessary to set up a chronology and to achieve further conclusions.
