Pollen Records And Past Neotropical Savanna Environments

Table 1 and Fig. 4 show the relevant pollen sites available for this study and their site characteristics, such as location, 14C age range, number of radiocarbon dates, and references. Today, a few sites are within contact zones or within the forested areas of transition zones. The 32 pollen records listed are located between 7°N and 21°S, extending over a distance of more than 4000 km.

18.3.1. Northern Neotropics Llanos Orientates of Colombia

Six radiocarbon-dated pollen records from six lake cores are available for the Llanos Orientales and are located along a ca. 500-km-long transect extending northeast from the Andes to the Venezuelan border (Table 1, (Wijmstra and Van der Hammen, 1966;

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FIGURE 4 Locations of the sites of pollen records discussed in the text. The numbers (1-32) refer to Table 1.

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FIGURE 4 Locations of the sites of pollen records discussed in the text. The numbers (1-32) refer to Table 1.

Behling and Hooghiemstra, 1998, 1999, submitted.). No pollen records are available for the Orinoco Llanos in Venezuela, which belongs to the same savanna ecosystem. The Laguna El Pinal record for the central part of the Llanos Orientales extends back to the full glacial period at ca. 18,000 14C B.P. Sites Laguna Sardinas and Laguna Angel contain late glacial and Holocene deposits.

During full glacial time, savanna dominated by grasses (Poaceae) and a very few woody taxa, such as Byrsonima and Curatella, characterized the landscape in the Laguna El Pinal region. The lake was shallow and ephemeral. Gallery forests along the river system must have been rare. Such vegetation reflects the driest climatic conditions of the last 18,000 years; the annual precipitation must have been markedly lower and the annual dry season longer than they are today. During the late glacial period, at ca. 10,690 14C B.P., Laguna El Pinal developed into a permanent shallow lake. Permanent lakes also came into existence elsewhere in the Llanos Orientales (Laguna Angel ca. 10,030 14C B.P. and Laguna Sardinas ca. 11,570 14C B.P.), indicating a change to wetter climatic conditions than before. Increased humidity also is suggested in the region of Laguna El Pinal by greater forest representation, probably reflecting only gallery forests, and by the increased presence of Mauritia and Alchornea in the region of Laguna Sardinas.

Higher lake levels of Laguna El Pinal and Laguna Carimagua (ca. 8270 14C B.P.), point to wetter conditions during the Holocene than during full glacial times, but the late glacial period was apparently wetter than the early Holocene. The increase of the forest ed area in the records of Laguna Angel at 5260 14C B.P., Laguna Sardinas at 6390 14C B.P., and Laguna Cari-magua at 5570 14C B.P. reflect a change from dry early Holocene conditions to wetter conditions during the mid-Holocene. The period after ca. 3800 14C B.P. is characterized by a marked increase in the palm Mauri-tia and/or Mauritiella in the Llanos Orientales, which is interpreted as reflecting increased human impact on the savanna vegetation under the wettest climatic conditions of the Holocene. The detailed Holocene pollen record from Laguna Loma Linda (Behling and Hoog-hiemstra, submitted) comes from the contact zone between the savanna of the Llanos Orientales and the Amazon rain forest (Fig. 5). The grass-dominated savanna in the early Holocene changed at ca. 6060 14C B.P. into a grass/ shrub savanna with a marked expansion of the Amazon forest/ gallery forest. At ca. 3600 14C B.P., forests started to dominate the landscape, reflecting a further continuing change to wetter climatic conditions with a shorter annual dry season. During the last 2300 years, the proportion of forest has continuously decreased and stands of Mauritia have increased, pointing to an increasing human influence in this region. Gran Sabana of Venezuela

Five Holocene records are available for the Gran Sabana in southeastern Venezuela at elevations between 800 and 950 m (Rinaldi et al., 1990; Rull, 1991 [Table 1]). The longest record is from Santa Cruz de Mapauri, with an extrapolated basal age of ca. 9800 14C years. Grass savanna predominates, with changes in the floristic composition that are interpreted as oscillations between drier and wetter periods. The Quebrada Arapan record indicates a predominance of grassland without marked changes during the last ca. 3100 14C years. At Laguna Divina Pastora, a swamp surrounded by treeless savanna was present between ca. 5300 and 2700 14C B.P., but forests must have been nearby. From ca. 27001500 14C B.P., the swamp changed into a lake, which was larger than it is today and was surrounded by abundant stands of Mauritia. Laguna Santa Teresa was a lake larger than it is now from ca. 5100-3900 14C B.P. and was surrounded by forest. After 3900 14C B.P., the lake became smaller and the forest retreated, but Mauritia stands markedly expanded. Since ca. 1700 14C B.P., the modern situation has prevailed, i.e., with a smaller lake and denser stands of Mauritia. The valley Urue record indicates that between 1700 and 1050 14C B.P., dense stands of Mauritia existed, apparently surrounded by savanna. The modern environmental setting with stands of Mauritia and a treeless savanna developed after 1050 14C B.P.

FIGURE 5 Summary pollen diagrams of the sites Laguna Loma Linda (Colombia) and Lago do Pires (Brazil). Rupununi Savanna of Suriname

Of the three published records for the Rupununi savanna, only the pollen record for Lake Moriru has radiocarbon time control (Wijmstra and Van der Hammen 1966). Based on the two radiocarbon dates, the base of the core may extend into the late glacial or full glacial period. Prior to ca. 10,000 (or 9000 14C B.P.), the pollen record reflects a closed savanna woodland with abundant Byrsonima and some periods with more grass savanna. The authors suggest that a marked extension of savanna could be coeval with the Younger Dryas or Allerod intervals. From 10,000 (or 9000) to 5000 14C B.P., the proportions of woodland and grassland savanna changed repeatedly. After 5000 14C B.P., and especially during the last 3000 14C years, grassland savanna became dominant. Roraima-Rupununi Savanna of Northern Brazil

The savanna region around Boa Vista, in the Brazilian state of Roraima, belongs to the same savanna system of Rupununi in Suriname. Only one short record, for Lago Galheiro, has been published (Absy 1979). The 40- to 50-cm interval of this 260-cm-long core has been dated to 205 ± 75 14C B.P. There are two gaps in the sediment record, suggesting dry periods. A grassdominated savanna with little presence of arboreal taxa and Mauritia occurs throughout this late Holocene with no marked changes in the vegetation. Coastal Savanna of Guyana, Suriname, and French Guiana

In the coastal region, palynological studies were undertaken in Guyana (Van der Hammen, 1963), Suriname (Wijmstra, 1971), and French Guiana (Tissot and Marius, 1992). The 120-m-long record from the Alliance Borehole (T28) in Suriname is not dated. The pollen diagram apparently shows several glacial-interglacial cycles with mangrove vegetation (Avicennia, Rhizopho-ra) during interglacial periods and savanna vegetation (Poaceae, Curatella, Byrsonima) during glacial periods. This sequence of vegetational change is also found in the pollen record of Ogle Bridge in Suriname, with a basal age of >45,000 14C years. During the full glacial period, savanna vegetation was present. After this period, a sea-level rise changed the environment of these sites, and locally, a mangrove developed. Swamp savanna, with mainly Poaceae and Cyperaceae, has been present during the last 5000 years in the coastal regions of Guyana, Suriname, and French Guiana.

18.3.2. Southern Neotropics Coastal Savannas of Northern Brazil

The pollen record of Lago Arari, on Marajo Island in the mouth of the Amazon River in northeastern Para State, represents the last 7500 14C years (Absy, 1985). At ca. 6520 14C B.P., there is a marked change from the more or less closed to open swamp savanna and forest. Several smaller fluctuations between forest and savanna vegetation occur during the late Holocene.

Two cores from Lagoa da Curu^a (Behling, 1996), an isolated lake about 15 km from the Atlantic Ocean in northeastern Para State, reflect the last 11,700 14C years (Profile A) and 9440 14C years (Profile B). The pollen spectra are indicative of closed and dense Amazon rain forest. The rare occurrence of the conifer Podocarpus suggests a marked cooling at low latitudes during late glacial times. The higher representation of Poaceae pollen and charcoal particles during the Holocene suggests the presence of anthropogenic open vegetation.

The pollen record from Lago Aquiri in northern Maranhao State (Behling and Costa, 1997) represents the period from 6700-7450 14C B.P. The presence at that time of mangrove vegetation indicates that the Atlantic coast migrated deeply inland within the drainage system of the Rio Mearim. Amazon rain forest occurred behind the mangrove zone, while cerrado vegetation played only a minor role in this area. The uppermost part of the core, probably representing the last 150 years, shows very different environmental conditions, with seasonally inundated swamp savannas and secondary forest. This change reflects disturbance related to human settlement. Cerrado of Northern Brazil

The core from a swamp on the Carajas table mountain, at 700-800 m elevation in southeastern Para State, has a basal date of >50,000 14C B.P. (Absy et al., 1991). The pollen record shows several alternations between arboreal and herbaceous savanna taxa (Poaceae, Aster-aceae, Borreria, Cuphea), interpreted as alternations between forest and edaphic savanna in the surrounding lowland and on the hilltops. Savanna extension, reflecting dry episodes, occurred at ca. 60,000 and ca.

40,000 14C B.P., between ca. 22,900 and 12,500 14C B.P., and between ca. 7500 and 3000 14C B.P.

The pollen record from the Katira Creek valley in the state of Rondonia, western Brazil, represents the period from 49,000 (41,300) to ca. 18,000 (?) 14C B.P., but includes only five pollen samples (Absy and Van der Hammen, 1976; Van der Hammen and Absy, 1994). The pollen diagram shows a predominance of Mauritia at the base of the core, followed by an interval in which Poaceae pollens dominate. This result suggests that the present-day rain forest at this site was replaced by savanna vegetation during glacial times. Cerrado of Central Brazil

Two pollen records from palm swamps are available: the record from Agua Emendadas represents the period of 26,000-3500 14C B.P. (Barberi, 1994; Salgado-Labouriau et al., 1998), and the record from Crominia reaches back to 32,000 14C B.P. (Ferraz-Vicentini and Salgado-Labouriau, 1996). In the Agua Emendadas record, the period from 26,000-21,500 14C B.P. is marked by abundant grassy vegetation and the presence of marsh, cerrado, and gallery forest taxa. Pollen grains of the palm Mauritia were not found. This period has been interpreted as moist and cold. The core interval representing the period from 21,500-7200 14C B.P. contains a thin sand layer without palynomorphs, suggesting that a climate with a long dry season prevailed. This period is followed by a transitional phase until ca. 5000 14C B.P.; this transitional phase is characterized by the development of a Mauritia palm swamp, suggesting an increase in moisture. During the late Holocene, vegetation and climate were similar to present-day conditions.

The swamp near Crominia is located about 320 km southwest of Agua Emendadas. In the lower part of the core, three samples from the interval of 281-154 cm depth have a radiocarbon age of ca. 32,000 14C years. Vegetational and climatic conditions inferred from this core interval are similar to modern conditions: i.e., the presence of cerrado, gallery forest, and Mauritia palm swamp under semihumid seasonal climatic conditions. From 32,000 to ca. 20,000 14C B.P., the vegetation was treeless grassland with gallery forest, without the presence of a palm swamp. This period is interpreted as having wetter climatic conditions and lower temperatures than today. The period from ca. 18,500 to ca. 10,500 14C B.P. apparently had a landscape with reduced vegetation cover under very dry climatic conditions. During the following period, until ca. 6500 14C B.P., conditions were dry, and since ca. 5000 14C B.P., the modern vegetational composition is found in this region. Cerrado of Southeastern Brazil

Several pollen records are available from sites in the southeastern Brazilian highland between ca. 700 and 1200 m elevation. The pollen records of Salitre and Serra Negra are from inland sites about 750 km from the Atlantic Ocean. The vegetation of this mountain region is complex, including montane vegetation, semidecid-uous forest, and cerrado. The base of the pollen record from Lagoa Campestre of Salitre has a radiocarbon age of >32,000 14C years (Ledru, 1993; Ledru et al., 1994, 1996) and is located only a few kilometers from the site Lagoa da Serra Negra with a pollen record dating back to >42,000 14C B.P. (De Oliveira, 1992).

In the pollen record of Salitre, the period from ca. 50,000-40,000 14C B.P. is marked by low arboreal pollen percentages reflecting arid climatic conditions. The following period until 27,000 14C B.P. shows high percentages of tree pollen (mainly Myrtaceae), which are indicative of moist climatic conditions. After a gap in the sedimentary sequence, the period from 16,00011,000 14C B.P. reflects a succession of different forest types, and moisture must have increased. After 12,000 14C B.P., the presence of characteristic Araucaria forest taxa indicates colder and moister climatic conditions. An absence of these taxa during a following short period possibly reflects an equivalent of the Younger Dryas interval. At the beginning of the Holocene, from 10,000-8500 14C B.P., the Araucaria forest increased again, indicating a cool and moist climate. After 8500 14C B.P., the Araucaria forest was replaced by a meso-phytic semideciduous forest, suggesting a warmer and drier climate. At 5000 14C B.P., a short interval with high percentages of Poaceae reflects aridity. After 4000 14C B.P., the climate was moister and comparable to present-day conditions. Higher percentages of Poaceae and Cyperaceae indicate a short, dry period at ca. 1000 14C B.P.

The pollen record of Serra Negra shows several changes in vegetation and climate before 40,000 14C B.P. (zones SN 1 to SN 6 in Ledru et al., 1998). Zone SN 7, representing the period of ca. 40,000-14,340 14C B.P., shows alternating cold and warm conditions, suggesting a time transgressive mosaic of different forest types reflecting a very cold climate with alternating moister and drier phases. A possible hiatus in the record during this period, including the full glacial period, cannot be excluded (Ledru et al., 1998). During most of the Holocene, the regional vegetation was dominated by a mosaic of cerrado and semideciduous forest, indicative of a seasonal climate. Comparisons and correlations between the Serra Negra and the Salitre records have already been presented by Ledru et al. (1996).

The pollen record from Catas Altas reflects the period of >48,000 to ca. 18,000 14C B.P. (Behling and Lichte, 1997). The landscape was characterized by grasslands with small areas of subtropical gallery forests with Araucaria. Today, this region is mainly covered by semi-deciduous forest with some cerrado vegetation in drier areas. Subtropical grasslands apparently migrated more than 750 km during glacial times, from southern to southeastern Brazil, reflecting a dry and cold climate with the occurrence of heavy frosts. The average temperature must have been 5°-7°C colder than today. The somewhat stronger presence of gallery forests suggests that the period from >48,000-26,500 14C B.P. was slightly wetter.

Lagoa dos Olhos (De Oliveira, 1992) and Lagoa Santa (Parizzi et al., 1998) are located 1 km from each other in karst terrain at 730 m elevation near the city of Belo Horizonte, ca. 400 km from the Atlantic Ocean. The Lagoa dos Olhos record is interpreted as an open canopy mosaic forest of Podocarpus, Rapanea, and Cary-ocar from 19,500-13,700 14C B.P., suggesting colder and more humid climatic conditions than exist now. During most of the Holocene, the region was apparently occupied by a mosaic of cerrado and semideciduous forest, indicative of strongly seasonal precipitation. The pollen record of Lagoa Santa starts at ca. 5400 14C ka during a period of increasing moisture. From ca. 5400 to ca. 4600 14C B.P., the presence of swamp vegetation reflects drier climatic conditions than are present now. After ca. 4600 14C B.P., a mosaic of cerrado, semidecid-uous forest, and gallery forest reflects climatic conditions similar to those of today, with a dry season of 34 months.

The lakes of Pires (Behling, 1995b), Nova (Behling, in preparation), and Silvana (Rodriges-Filho et al., submitted) are located ca. 250 km from the Atlantic Ocean in the semideciduous forest region of the Atlantic lowland. The best-studied record is the 16-m-long core from Lago do Pires, representing the Holocene (Fig. 5). During the early Holocene from 9720-5530 14C B.P., cerrado vegetation with some gallery forest indicates low precipitation and a dry season of ca. 6 months. Within this period, an interval from 8810-7500 14C B.P. with expanding gallery forests suggests greater precipitation and a shorter dry period (6-5 months). From 5530-2780 14C B.P., the vegetation was characterized by extensive forest in the valleys and cerrado on hills, indicating higher precipitation and a dry season of ca. 5 months. From 2780-970 14C B.P., denser cerrado vegetation on the hills and slopes suggests a dry season of 4-5 months. During the latest Holocene, the presence of a closed semideciduous forest is indicative of modern climatic conditions. Inferred environmental conditions for Lago Nova (located near Lago do Pires) and

Lagoa Silvana (located 180 km south of Lago do Pires) are similar. Both records indicate that during the end of the late glacial period, savanna was the main vegetation in this region.

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