JOURNAL OF QUATERNARY SCIENCE (2000) 15 (7) 687–695 Copyright 2000 John Wiley & Sons, Ltd. Holocene Amazon rainforest–savanna dynamics and climatic implications: high-resolution pollen record from Laguna Loma Linda in eastern Colombia HERMANN BEHLING1,2,* and HENRY HOOGHIEMSTRA2 1Centre for Tropical Marine Ecology, Fahrenheitstrasse 1, 28359 Bremen, Germany 2Hugo de Vries-Laboratory, Department of Palynology and Paleo/Actuo-ecology,† University of Amsterdam, Kruislaan 318, 1098 SM Amsterdam, The Netherlands Behling, H. and Hooghiemstra, H. 2000. Holocene Amazon rainforest–savanna dynamics and climatic implications: high-resolution pollen record from Laguna Loma Linda in eastern Colombia. J. Quaternary Sci., Vol. 15, pp. 687–695. ISSN 0267-8179. Received 2 September 1999; Revised 15 February 2000; Accepted 23 February 2000 ABSTRACT: We present a high-resolution pollen record of a 695-cm-long sediment core from Laguna Loma Linda, located at an altitude of 310 m in the transitional zone between the savannas of the Llanos Orientales and the Amazonian rainforest, about 100 km from the Eastern Cordillera. Based on eight AMS 14C ages, the record represents the last 8700 14C yr BP. During the period from 8700 to 6000 14C yr BP the vegetation was dominated by grass savanna with only a few woody taxa, such as Curatella and Byrsonima, present in low abundance. Gallery forest along the drainage system apparently was poorly developed. Compared with today, precipitation must have been significantly lower and seasonality stronger. During the period from 6000 to 3600 14C yr BP, rainforest taxa increased markedly, reflecting an increase in precipitation. Rainforest and gallery forest taxa such as Moraceae/Urticaceae, Melastomataceae, Alchornea, Cecropia and Acalypha, were abundant, whereas Poaceae were reduced in frequency. From 3600 to 2300 14C yr BP rainforest taxa continued to increase; Moraceae/Urticaceae became very frequent, and Myrtaceae and Myrsine became common. Savanna vegetation decreased continuously. We infer that precipitation was still increasing, and that the length of the annual dry period possibly shortened. From 2300 14C yr BP onwards, grass savanna (mainly represented by Poaceae) expanded and Mauritia palms became frequent. This reflects increased human impact on the vegetation. Copyright 2000 John Wiley & Sons, Ltd. KEYWORDS: Amazonian rainforest; savanna; vegetation dynamics; climate dynamics; Holocene; human impact; Colombia. Introduction in precipitation and seasonality, which is reflected in the vegetation zonation: the savannas to the north gradually change into a large area of rainforest to the south. The late The Late Quaternary environmental history of the tropical Quaternary history of both ecosystems is still poorly known. lowland ecosystems in the neotropics is poorly known This explains the ongoing discussion in the literature about despite the importance of understanding the natural ampli- the degree to which rainforest and savanna replace each tude of tropical ecosystem dynamics, which would help to other when climatic conditions change (e.g. Van der Ham- verify global climate models and to better understand global men and Absy, 1994; Colinvaux et al., 1996; Hooghiemstra biodiversity in relation to ecosystem dynamics. The lowlands and Van der Hammen, 1998; Van der Hammen and Hooghi- of eastern Colombia are characterised by a strong gradient emstra, 2000). The Late Quaternary history of the savannas north of the Amazon basin is known from a number of sites. The first * Correspondence to: H. Behling, Centre for Tropical Marine Ecology, Fahrenheit- strasse 1, 28359 Bremen, Germany. Email: hbehlingȰzmt.uni-bremen.de palynological information on the Holocene vegetation history † Participating in The Netherlands Centre for Geo-ecological Research (ICG), and climatic change of the Llanos Orientales came from and the Institute of Biodiversity and Ecosystem Dynamics, IBED. Laguna de Agua Sucia (Wijmstra and Van der Hammen, 1966). Other palynological studies from savanna ecosystems Contract grant sponsor: The Netherlands Foundation for Scientific Research (NWO-GOA) of northern South America are available from the Gran Contract grant sponsor: Deutsche Forschungsgemeinschaft Sabana in Venezuela (Rull, 1992), the Rupununi savanna in Contract grant number: 750.195.10 Surinam (Wijmstra and Van der Hammen, 1966) and the 688 JOURNAL OF QUATERNARY SCIENCE belt of coastal Guianan savannas in Surinam (Wijmstra, (seasonality); it also depends on other environmental factors, 1971), British Guyana (Van der Hammen, 1963) and French such as biotic, pedological and physical factors (Cole, 1982, Guiana (Tissot and Marius, 1992). Recently, the savannas of 1986; Medina and Silva, 1990). the Colombian Llanos Orientales were explored by the Hugo From the sites mentioned above, the following Late Quat- de Vries Laboratory of Amsterdam University. Starting from ernary vegetation history has been inferred. During the last the city of Villavicencio, which is located in the foothills of 18000 14C yr BP the savanna ecosystem was relatively the Eastern Cordillera, we collected sediment cores in 13 stable, except for minor changes in floral composition, and lakes and swamps along a 500 km long transect, running in the proportion of savanna to forest. During the Last from west to east (Fig. 1). The sites are located between Glacial Maximum (LGM) the reduced area of gallery forest ° Ј ° Ј ° ° 3 30 to 4 30 N latitude and 69 to 74 W longitude, at and the non-permanent lake conditions (Laguna El Pin˜al) elevations between 80 and 450 m in the provinces of Meta reflect the driest period of the whole record, probably due and Vichada. Results from the pollen analysis of cores from to low rainfall rates and a long annual dry season. In this Lagunas Angel and Sardinas (Behling and Hooghiemstra, region gallery forest started to expand during the Late-glacial, 1998), and Lagunas El Pin˜al and Carimagua (Behling and at around 10700 14C yr BP (Laguna El Pin˜al and Laguna Hooghiemstra, 1999) have been published. An overview and Sardinas), indicating wetter conditions than during the LGM. comparison of the savanna ecosystems north and south of During the early Holocene, gallery forest diminished com- the Amazonian rainforest has been prepared by Behling and pared with the Late-glacial, reflecting markedly drier con- Hooghiemstra (in press). ditions again, but not as dry as the LGM. Starting at 6400 The Late Quaternary history of the northwestern part of 14C yr BP (Laguna Sardinas), and at 5300 14C yr BP (Laguna the Amazonian rainforest area is known from only a few Angel), gallery forests expanded, suggesting a change to sites. In the Colombian part, Van der Hammen et al. (1992a, wetter conditions. The marked increase of palms (Mauritia b) studied the middle Caqueta´ River area, Urrego (1991, and Mauritiella) during the last ca. 3500 14C yr BP in most 1997) studied pollen records from sites at Marin˜ame and Quinche´ representing the complete Holocene, also located of the sites of the Llanos Orientales, points to increased along the middle Caqueta´ River. Behling et al. (1999) studied human impact on the vegetation under the wettest climate cores from Pantano de Monica, located in the middle regime registered in the Holocene. Caqueta´ River area, also representing the complete Holo- The pollen records from the middle Caqueta´ River area cene. From the Brazilian part of northwest Amazonia, Colin- show that during the Late-glacial and early Holocene the vaux et al. (1996) published a Late Quaternary pollen record lower terraces were better drained than during the last 4000 from Laguna La Pata. yr. Observed changes are the result of changes in the drain- Except for Laguna de Agua Sucia, all sites previously age system of the Caqueta´ River basin. These changes might studied are located far from the savanna–rainforest boundary. be either the consequence of river dynamics in the valley The site presented in this paper, Laguna Loma Linda, is of the Rio Caqueta´, or of climate change, i.e. less precipi- located in the present-day transition between savanna and tation and reduced river discharge. rainforest and, as a consequence, is sensitive for registering The objective of this paper is to document in detail the changes between both types. The geographical location of dynamics between savanna and rainforest during the Holo- the savanna–rainforest boundary depends mainly on climato- cene from the Amazon lowlands and to infer a record of logical factors, i.e. differences in the total annual precipi- climatic change. The effects of possible human impact on tation and the distribution of rain throughout the year both ecosystems are evaluated. Figure 1 Map showing the geographical location of the site studied, Laguna Loma Linda, and the sites mentioned in the text, Laguna Angel and Laguna Sardinas, Laguna El Pinal, Laguna Carimagua and Laguna Agua Sucia. Copyright 2000 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 15(7) 687–695 (2000) HOLOCENE AMAZON RAINFOREST–SAVANNA DYNAMICS 689 Environmental setting of the study area including lake borders and swampy areas, often dense stands of the palm Mauritia are found. Mauritia swamp forest is called ‘morichal’. The smaller palm Mauritiella grows along Laguna Loma Linda (3°18ЈN, 73°23ЈW) is located in the lake shores in shallow water and surrounds several lakes in province of Meta, about 120 km southeast of the city Villa- the Llanos Orientales. vicencio. The distance to the Eastern Cordillera is about 100 The forest of Colombian Amazonia is a dense and tall km. The lake is situated within the borders of a military and highly diverse rainforest. A detailed vegetation survey area, some 10 km north of the town of Puerto Lleras, in a and mapping of the middle Caqueta´ region has been pub- soft rolling landscape at 310 m elevation (Fig. 2). The lake lished by Duivenvoorden and Lips (1993, 1995), Urrego is about 2.5 km long and 1 km wide, and was part of a (1991, 1997) and Botero (1999), who has published veg- former small river which became dammed probably at the etation and geomorphological maps of the total lowlands of beginning of the Holocene.