| dc.description.abstracteng | This thesis contributes to better understanding of long-term vegetation history in eastern Amazonia and the possible role that climate, fire, soil and human may play to change the landscape. To achieve this aim, three new sediment records form eastern and southeastern Amazonia have been studied with a multi-proxy approach (pollen, charcoal, lithology, extractable Fe, remote sensing) and seven already published palynological records from other parts of tropical South America have been reviewed.
The first record named Curiaú (CUR) retrieved from a small swamp near the mouth of Amazon River. This 750 cm sediment core presents the first continuous record of vegetation change from the coastal savanna belt since the last 11600 cal yr BP. Occurrence of such savanna belt that cover the coastal area from British Guiana to northeastern Brazil cannot be explained by present-day climate. Therefore investigating the long-term environmental change in the area helps to find the origin of this unexpected savanna. Results indicate that the savanna has been always present since the beginning of the Holocene. However, the size and composition of the forest surrounding the study site experienced strong changes. The onset of Holocene was accompanied by a forest type composed primarily of the genus Micropholis (Sapotaceae) that has no modern analog and was more likely a relict of previously more extensive forest developed under cooler and moister Late Glacial conditions. This forest became locally extinct after 11,200 cal yr BP when an already existing grassy savanna with few scattered shrubs and trees replaced the forest. The savanna expansion occurred under a drier phase probably due to displacement of Intertropical Convergence Zone (ITCZ) to its northernmost position. Presence of pollen from mangrove trees of Rhizophora together with signals of swamp forest formation since the early Holocene and later (at ca. 10,000 cal yr BP), the Mauritia swamp development in the coastal depressions imply early Holocene sea level rise that caused rise of the ground water table. During the mid-Holocene between 8500 and 5600 cal yr BP gallery forest (composed mainly of Euphorbiaceae) and swamp forest succeeded the treeless savanna. This implies a wetter climatic condition probably due to collision of ITCZ and SACZ. The modern vegetation with continuous changes in the area of forest prevailed at ca. 5600 cal yr BP. This unstable condition might occur due to the intensification of ENSO that hampered the stability of the ecosystem during the last 5 millennia. Frequent charcoal particles especially during the early Holocene may imply that natural and/or anthropogenic fires might be the reason for occurrence of the savanna. However the synchrony of vegetation changes with the climate dynamics propose that climate is the main factor that maintain the coastal savanna belt.
A review on 8 palynological records (including CUR) from 5 different lowland ecosystems located in Amazonia and surroundings indicates that the predominance of forests with specific taxa composition as was recorded in CUR, might occur synchronously in the whole lowland tropical South America during the Late Glacial-Holocene Transition (LG-HT ≈ 13,000-10,000 cal yr BP). Because the palaeo-precipitation proxies from the Andes and the Amazon River Fan proposed that the LG-HT,
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especially the Younger Dryas chronozone (YD, ca. 12,800-11500 cal yr BP) has been the driest period in the Amazon Basin, prevalence of such forest during the LG-HT is questionable. To explain this inconsistency between vegetation and climate, three different scenarios are propose based on the specific taxa composition and climatic condition of LG-HT.
The second sediment core called Vereda (VRD) was taken from a small swamp surrounded with fringe of Mauritia. This record reveals constant dominance of Mauritia around the swamp and occurrence of open savanna in surrounding area since 2500 cal yr BP. Because there is no significant correlation between fire frequency and the percentage of Mauritia neither in VRD nor in some other records from eastern Amazonia, the previous belief about the role of manmade fire in the late Holocene expansion of Mauritia is dubious. In order to further examine the relation between dynamics of Mauritia, fire frequency, ENSO intensity, history of human settlement and sea level changes a review on different records from tropical South America is required.
The third sediment core (Buriti (BRT)) was recovered from a forest hollow on the slopes around the plateau of Serra Sul dos Carajás located on southeastern Amazonia. This record shows constant dominance of forest elements on the slopes of the plateau since 6600 cal yr BP whereas the records from savanna on top of the plateau demonstrate the maximum signal of arboreal vegetation since 2500 cal yr BP. Because the site BRT is located in a short distance from modern forest borders and almost at the same elevation, it can be concluded that through a slow process lasted for ca. 4000 years forest borders shifted toward savanna horizontally. Therefore it can be proposed that some limiting factors might delay the forest expansion. Among different possible factors, natural or anthropogenic fires in the savanna and human hunting/gathering activities may play an important role by pushing back the forest borders. However the evidence suggests that this long period was more likely spent on the formation of sufficient soil layer that could support forest establishment.
In general the results of our studies clearly suggest that although the role of fire and human cannot be excluded, vegetation dynamics in eastern part of tropical South America is mainly derived by climatic and geological factors such as LG-HT specific climatic condition, sea level changes, shift in position of ITCZ and intensification of ENSO. Where the climatic conditions are equal, the different vegetation histories in adjacent ecosystems might be due to difference in soil properties.
In addition it has been realized that there are synchronous vegetation signals in the records from different location of tropical South America since the Late Glacial. The LG-HT expansion of a specific forest type, the early to mid-Holocene savanna expansion and the late Holocene forest expansion are the most important synchronous changes. The latter two changes are attributed to the shift in position of ITCZ due to the change in insolation according to Milankovitch cycles. Increase in population of Mauritia also shows some degrees of synchrony among different records that must be more investigated. | de |