Mid- to Late-Holocene pollen-based biome reconstructions for Colombia

Quaternary Science Reviews 20 (2001) 1289}1308

Mid- to Late-Holocene pollen-based biome reconstructions for Colombia Robert Marchant *, Hermann Behling
, Juan Carlos Berrio , Antoine Cleef , Joost Duivenvoorden , Henry Hooghiemstra , Peter Kuhry, Bert Melief , Bas Van Geel , Thomas Van der Hammen , Guido Van Reenen , Michael Wille Hugo de Vries-Laboratory, Faculty of Biology, Institute of Biodiversity and Ecosystem Dynamics, University of Amsterdam, Postbus 94062, 1090 GB Amsterdam, Netherlands
Centre for Tropical Marine Ecology, Fahrenheitstrasse. 1, D-28359 Bremen, Germany Arctic Centre, University of Lapland, P.O. Box 122, FIN-96101 Rovaniemi, Finland Fundacion Tropenbos Colombia, Carrera 21 C 39-35, Santafe de Bogota& , Colombia

Abstract The assignment of Colombian pollen data to biomes allows the data to be synthesised at 10 &time windows' from the present-day to 6000 radiocarbon years before present (BP). The modern reconstructed biomes are compared to a map of modern potential vegetation to check the applicability of the method and the a priori assignment of pollen taxa to plant functional types and ultimately biomes. The reconstructed modern biomes are successful in describing the composition and distribution of modern vegetation. In particular, altitudinal variations in vegetation within the northern Andean Cordilleras are well described. At 6000 BP the biomes are mainly characteristic of warmer environmental conditions relative to those of the present-day. This trend continues until between 4000 and 3000 BP when there is a shift to more mesic vegetation that is thought to equate to an increase in precipitation levels. The period between 2500 and 1000 BP represents little or no change in biome assignment and is interpreted as a period of environmental stability. The in#uence attributed to human-induced impact on the vegetation is recorded from 5000 BP, but is particularly important from 2000 BP. The extent of this impact increases over the Late-Holocene period, and is recorded at increasingly high altitudes. Despite these changes, a number of sites do not change their biome assignment throughout the analysis. This asynchronous vegetation response is discussed within the context of site location, non-linear response of vegetation to Late-Holocene environmental change, regionally di!erential signals, localised human impact and methodological artefacts.  2001 Elsevier Science Ltd. All rights reserved.

1. Introduction Biome reconstructions from pollen data have been produced for most regions of the world at a continental (Prentice et al., 1996; Jolly et al., 1998b) or sub-continental scale (Williams et al., 1998). These spatially extensive reconstructions have demonstrated the ability of the biomisation technique to provide an objective reconstruction of vegetation using pollen data derived from multiple sites. In addition to these large-scale reconstructions, a focus on the Arctic region (Edwards et al., 2000), where the pollen data are of su$cient quality and quantity, has enabled a regional reconstruction that of-

* Corresponding author. E-mail address: [email protected] (R. Marchant).

fers greater ecological de"nition to the reconstructed vegetation than available at the scale of eastern North America (Williams et al., 1998). The biome reconstruction for Latin America (Marchant et al., in review) demonstrated the variable spatial distribution of pollenbased records in this region, with sites particularly being concentrated along the Andean spine. One country within Latin America that has an extensive coverage of fossil pollen data is Colombia (Fig. 1), and much of these pollen data are of a high quality with a number of localities having multiple radiocarbon-dated cores. However, in keeping with much of tropical palaeoecology, the initial research was focused on the abundant sedimentary basins located at relatively high altitudes (Table 1) where the sites are relatively accessible and the climate temperate. Furthermore, the sedimentary basins within lowland situations are mostly associated with riverine migration.

0277-3791/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 1 8 2 - 7

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Fig. 1. Map of potential Colombian vegetation today (after IGAC, 1998). The main vegetation units are depicted as are the location of sites with pollen data used in our analysis. Each site is assigned a number that relates to the information on location, altitude and age range of the sediments presented in Table 1. Vegetation changes over altitude are also indicated (insert).

Due to this association, the basins can be relatively transient with resultant sedimentary records having complex age}depth relationships. This concentration has resulted in relatively few records existing from lowland

Colombia (Fig. 1). As lowland vegetation dynamics remain relatively poorly resolved (Behling et al., 1999), there has recently been a focus on producing relatively lowland palaeoecological records (Behling and

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Table 1 Characteristics of the Colombia sites used in our analysis detailing location, altitude and the time windows encompassed by each of the sites. The table is arranged in order of ascending altitude. The numerical code assigned to each site relates to locators in Fig. 1 Site

Code Latitude Longitude Alt. (m)

6000

5000

4000

3000

2500

2000

1500

1000

500

Modern

Boca De Lopez Piusb El Camito Sardinas Monica-3 Marin ame-II Carimagua El Pinal Angel Agua Sucia Loma Linda Pitalito Piagua Genagra Timbio Libano Pedro Palo-III Ubaque La Herrera CUX El Abra-II FuH quene-II Alsacia Cienaga del Visitador Agua Blanca La Guitarra Ciega-I La Primavera La AmeH rica Guasca PalaH cio-PT1 Andabobos Laguna Verde Pen a Negra Los Bobos El Gobernador San Carlos Corralitos Valle de Lagunillas-VL-V La Rabona CorazoH n Partido El Trinagulo

37 1 2 14 7 15 12 11 13 9 8 3 5 4 6 10 16 17 39 18 19 20 22 23 21 24 25 26 27 41 28 29 36 30 35 31 42 38 32 33 34 40

# ! ! # ! ! # # # # # # ! ! ! ! # ! ! # # # # # # # # # # # # # ! # ! # # ! # # ! !

# # ! # ! ! # # # ! # # ! ! ! # # ! # # # # # # # # # # # ! # # ! # # # ! ! # # ! !

# # ! # ! ! # # ! ! # # ! ! ! # # ! # # ! # ! # # # # # # ! # # ! # # # ! ! # # ! !

# # # # # ! # # # # # # ! ! # # # ! # # # # # # # # # # # ! # # # # ! # ! ! # # ! !

# # # # # ! # # # # # # ! # # # # ! # # # # ! # # # # # # # ! # ! # # # ! ! # # ! !

# # # # # ! # # ! # # # # # # ! # ! # # # # ! # # # # # # # # # # # # # ! ! ! # ! !

# # # # # ! ! # ! # # # # # # ! # ! # # # # ! # # # # # # # # # ! # ! ! ! ! ! # ! !

# # # # # ! ! # ! # # # # # # ! # ! # # # # ! # # # ! # # # # # ! # # # ! ! # # ! !

# # # # # ! ! ! # # # # # # # # # ! # # # # ! # # # ! # # # # ! # # # # ! ! ! # ! !

# # # # # # ! ! # # # # # # # # # # # # # # # # # # # # # ! # # # # # # ! # # # # #

!75.44 10.80 !77.95 1.80 !76.60 2.53 !69.45 4.95 !72.50 !0.60 !72.05 0.75 !74.14 4.04 !70.40 4.09 !70.58 4.45 !73.52 3.66 !73.45 3.34 !76.50 2.75 !76.50 2.30 !76.35 2.50 !76.50 2.50 !75.50 4.50 !74.42 4.33 !73.95 4.50 !74.00 4.65 !74.18 4.70 !73.96 5.00 !73.87 5.67 !74.25 4.00 !72.89 6.18 !74.04 5.00 !74.67 4.00 !72.33 6.67 !74.25 4.00 !74.00 4.33 !74.15 3.95 !73.66 4.57 !74.16 4.88 !74.00 5.25 !74.00 5.03 !72.85 6.15 !74.23 4.00 !75.30 4.50 !72.30 6.50 !72.34 6.43 !74.25 4.00 !74.25 4.00 !74.25 4.00

0 10 50 80 160 160 180 185 205 300 310 1300 1700 1750 1750 1820 2000 2000 2500 2560 2570 2580 3100 3100 3250 3400 3510 3525 3550 3550 3550 3570 3625 3625 3800 3815 3850 3860 3880 4000 4100 4100

Hooghiemstra, 1999; Behling et al., 1999; Wille et al., 2000). Notwithstanding these caveats, the Colombian palaeoecological archive of vegetation change represents a remarkable record for Latin America, and indeed the wider tropics. The Colombian pollen data allow an understanding to be developed of regional environmental change, and its impact on tropical vegetation composition and distribution. This reconstruction rivals the resolution commonly available from more temperate latitudes where Quaternary science research has a longer ancestry and wider research base. The technique of interpreting pollen data in terms of biomes was initially developed to test reconstructed bi-

omes that are based on output from climate models, results being portrayed via a vegetation model (Prentice et al., 1992; Haxeltine and Prentice, 1996). However, the technique has a much wider utility, particularly where the pollen data are of su$cient quantity and quality. A particular use is to determine vegetation dynamics that combine a spatial and temporal perspective. The standard treatment of primary pollen data from Colombia presented here allows for an objective investigation of vegetation response to climatic forcing and past human}vegetation interactions from 6000 BP to present. The 6000 BP starting point was chosen as it was the focus of the Latin American reconstruction (Marchant et al., in

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review) and is a period of much study (Prentice and Webb, 1998). We aim to determine the causal factors in driving vegetation change since 6000 yr ago be they climatic, edaphic or human-induced and, ultimately to place the present composition and distribution of Colombian vegetation within a relatively long timeframe. 1.1. Modern climate and vegetation Climate in Andean Colombia can be classi"ed as tropical diurnal (Kuhry et al., 1983). At a given location, di!erences in monthly temperature are small ((33C) although daily #uctuations may be large (203C), especially in dry seasons. Precipitation is highest ('10,000 mm yr\) in the ChocoH Paci"c region due to the proximal location of the Paci"c-based moisture source and the steeply rising ground of the Western Cordillera. Within Andean Colombia, precipitation is highest on the eastern, Llanos Orientales-facing slopes of the Eastern Cordillera. The concave nature of the Eastern Cordillera is thought to act as a natural receptacle for moisture with origin from the Atlantic Ocean. Low rainfall is recorded within rain shadow areas, such as on the lower slopes of the Magdalena Valley and the interandean plains (Kuhry et al., 1983). Three Cordilleras of the northern Andes, that are described by approximately 7000 m of altitudinal change (Fig. 1), dominate Colombia. Applying a lapse rate of 6.63C 1000 m\ (Van der Hammen and GonzaH lez, 1965), this altitudinal rise equates to a temperature change of nearly 303C, and as such results in a signi"cant change in vegetation recorded over a relatively small area. For example, there are transitions from cool high-altitude grasslands to &temperate' forests at mid-altitudes and some of the most diverse tropical rainforests in the world present within the ChocoH Paci"c region. Humbolt carried out the "rst description of the vegetation belts of the Eastern Cordillera of Colombia during 1845 and 1847. Cuatrecasas (1958) provided a more systematic survey that determined vegetation zones based mainly on altitude and aspect (Fig. 1) with the distribution and composition of the zonal vegetation belts re#ecting the prevailing climatic conditions. Van der Hammen and GonzaH lez (1960), and Cleef and Hooghiemstra (1984) provide reviews of the vegetation characteristics of Andean Colombia with lowland vegetation reviewed by Cavelier et al. (1998), Duivenvoorden (1995) and Gentry (1986); these reviews form the basis for the Colombian biomes presented here (Fig. 1). As such, these reviews are the basis for interpreting the relative position of these biomes in response to environmental change, although with some caveats, particularly with respect to regional variation in their altitudinal positions (Marchant et al., in press). Fig. 1 presents diagrammatically the distribution of vegetation in Colombia and the location of sites (Table 1) used for this analysis.

2. Method Prentice et al. (1996) and Marchant et al. (in press) have described the biomisation technique. The steps involved in the technique are depicted diagrammatically in Fig. 2. At the centre of the biomisation technique is the allocation of pollen taxa to a range of plant functional types (PFTs), and how these combine to form biomes. As is the case with biomes we can identify within the biosphere today, the constituent vegetation is comprised of a series of identi"able component parts * in our case a suite of PFTs. The intermediate step of allocating the pollen to PFTs, rather than directly to biomes, allows the internal composition of the biomes to be determined. Before turning the Colombian pollen data into biomes, it was necessary to de"ne a conceptual framework that represents Colombian vegetation; this framework is designed to be compatible with that identi"ed for the broader scale Latin American biome reconstruction (Marchant et al., in review). The framework originally developed for Latin America is based on gradients of moisture (Priestly}Taylor co-e$cient of plant-available moisture), temperature (mean temperature of the coldest month) and seasonality (growing degree-days) derived from the global climate data set of Leemans and Cramer (1991). This framework was tailored to "t the Colombian vegetation after investigating the ecology, growth form and environmental constraints for the taxa represented by their pollen. For example, the tropical rain forest biome includes non-frost-tolerant vegetation growing in humid regions, where the mean coldest-month temperature is '15.53C, and annual average temperature is in the range of 25}273C. This biome occurs only in wettest regions of tropical Latin America where rainfall meets '95% of annual evaporative demand (Prentice et al., 1992). This biome is characterised by structurally tall closed forest, 10 to '30 m, usually mixed tree species with dense understorey and low trees, plus liana thickets and emergent canopy. For Colombia this biome includes Amazonian, ChocoH and gallery rain forest. In addition to biomes determined by environmental constraints to growth, an additional category of °raded vegetation' was included. The rationale behind this was that within the time frame under investigation it is likely that human impact on the vegetation would be an important factor. By including a degraded vegetation category, it is possible to separate likely cultural impacts on the biota from other forcing mechanisms such as climatic change. The PFTs occurring in Colombia (Table 2) can be placed into "ve main groups; these comprise tropical (non-frost-tolerant), coniferous (needle-leafed), temperate (frost-tolerant), xerophytic (drought-tolerant), and frost- and droughttolerant taxa. The relationship between the altitudinal vegetation zonation and the Colombian biomes is portrayed in Fig. 1.

R. Marchant et al. / Quaternary Science Reviews 20 (2001) 1289}1308

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Fig. 2. Schematic representation of the biomisation technique. The process moves from raw pollen data depicted on the left of the "gure to biome output on the right.

Depending on the modern ecological range of the parent taxa responsible for producing the pollen identi"able within our data sets, a total of 296 Colombian pollen taxa were assigned to one or more PFTs. These assignments were made following reference to the known biology of plants from several #oras (Kahn and de Granville, 1992; Gentry, 1993), botanical and palynological studies (Beard, 1955; Van der Hammen, 1963, 1972; Wijmstra and Van der Hammen, 1966; Cleef and Hooghiemstra, 1984; Hooghiemstra and Cleef, 1984; Prance, 1985; Duivenvoorden and Cleef, 1994; Witte, 1994; Hooghiemstra and Van der Hammen, 1998) and personal communication with modern ecologists and palaeoecologists working in Colombia. The resultant pollen taxa vs. PFT assignments are presented in Table 3. One of the di$culties encountered in constructing the PFT}pollen matrix for Latin America was that many taxa were multiply assigned to a number of PFTs as a result of the high intra-generic diversity and the wide ecological ranges of the parent taxa present within some genera (the level of identi"cation commonly achieved during pollen analysis). Within the Colombian zoom, it is possible to make the PFTs more re"ned; however, a number of pollen taxa are still assigned to a number of PFTs due to the relatively cosmopolitan growth forms of the parent taxa involved (Table 3). The result of these assignments is that each pollen taxon will contribute numerically, via the PFTs, to a number of biomes. The overall contribution is quanti"ed in terms of a$nity scores to a biome. A problem can arise, as was the case in Latin America, where multiple

samples encompass the temporal range of time windows ($100 and $250 radiocarbon years) used here. As the temporal boundaries are relatively narrow, only two or three samples encompass a single time window. Nevertheless, multiple samples from a single site may have maximum a$nity to di!erent biomes. In previous applications when this was the case, the most common biome is used for mapping (Marchant et al., in review). Unfortunately, this approach results in some palaeoecological information being lost within the presentation of results. For example, of two sites recording the Tropical Rain Forest biome as dominant at two time intervals, one site demonstrates a large increase in the Tropical Dry Forest biome; although it is still sub-dominant, using the current protocols this shift is not portrayed. We know that Colombian vegetation communities often constitute a complex that incorporates components from a number of di!erent biomes. This is also likely to have been so during the past. This situation is particularly likely to arise for sites located in ecotonal position, such as near the boundary between di!erent biomes (Fig. 1). Therefore, when the highest a$nity scores are within a value of &5' of each other, then the two highest scoring biomes are used to describe the reconstructed vegetation. Regrettably this still results in lost information. This can only be fully preserved when the number of sites under analysis is relatively restricted or a speci"c hypothesis is under investigation (Marchant et al., in press). Within our analysis, this is only possible when we test the relationship between modern vegetation (Fig. 1) and the modern pollen-based reconstruction (Table 4).

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Table 2 The range of plant functional types and biomes identi"ed for Colombia encompassed by the pollen sites under investigation. The assignment of PFTs to biomes is also detailed Codes

Plant functional types

g man tx Tr  Tr  Te  Te  ctc  txts tf tef sf af cp wte wte  wte  ts aa Rud

Grass Mangrove Tree ferns Wet tropical raingreen trees Dry tropical raingreen trees Wet tropical evergreen trees Dry tropical evergreen trees Cool temperate conifer Tropical xerophytic trees and shrubs Tropical forbs/herbs Temperate forbs/herbs Steppe forbs/herbs Alpine forbs/herbs Cushion plants Warm temperate broadleafed evergreen trees and shrubs Cool temperate broadleafed evergreen trees and shrubs Cool temperate sclerophyll shrubs Temperate summergreen trees Alpine dwarf shrubs Cultivars/ruderal taxa

Codes

Biomes

Plant functional types

TRFO TSFO TDFO WEFO CEFO WAMF COMI STEP CGSH CGSS DEGE

Tropical Rain Forest Tropical Seasonal Forest Tropical Dry Forest Warm Evergreen Forest Cool Evergreen Forest Warm Mixed Forest Cool Mixed Forest Steppe Cool Grasslands Cool Grassland shrub Degraded vegetation

man, Tr , Te , tf   Tr , Te , tf, wte   Tr , Te , txts, sf, tef   Te , tf, wte  tx, tef, wte , ts  Tx, ctc , tef, wte, ts  Tx, ctc , tef, wte , wte    g, Tr , sf  g, cp, af g, af, aa, wte  Rud

Table 3 The assignment of Colombian pollen taxa to the plant functional types used for the biomisation process

Table 3 (Continued) PFT codes

Pollen taxa

Tr 

Acalypha, Alibertia, Andira-type, Annona, Aspidosperma, Astronium, Bignoniaceae, Byrsonima, Casearia-type, Convolvulaceae, Copaifera, Coprosoma, Cordia, Coriaria, Cuphea, Curatella, Erythrina, Hieronima, Hymenophylleace, Ipomoea, Leguminosae, Malvaceae, Mauritia, Melastomataceae, Meliaceae, Meliosma, Mimosa, Palicourea, Panopsis, Piper, Rosaceae, Schinus, Trema, Vallea, Warswiczia, Xylosma

Te 

Alchornea, Amanoa, Anacardiaceae, Anemia, Apeiba, Apocynaceae, Arecaceae, Astronium, Bombacaceae, Brunellia, Bromeliaceae, Cardiospermum, Cecropia, Celastraceae, Celtis, Clethera, Combretaceae, Didymopanax, Euphorbiaceae, Euterpe, Fabaceae, Ficus, Guettarda, Hedyosmum, Humulus, Ilex, Iriartea, Lecythidaceae, Leguminosae, Mabea, Machaerium, Macrolobium, Macrocarpea, Malpighiaceae, Mauritia, Marcgraviaceae, Maripa, Maytenus, Meliaceae, Menispermaceae, Moraceae, Myrsine, Myrtaceae, Ocotea-type, Oreopanax, Palmae, Pseudobombax, Pseudopanax laetevirens, Psychotria, Rauvolxa, Rubiaceae, Salix, Sapium, Sapotaceae, Schizolobium, Siparuna, Sophora, Spirotheca, Strutanthus, Taperira, Tetrochidium, Tournefortia, Urticaceae, Vismia

Te 

Bauhinia, Bignoniaceae, Boraginaceae, Bougainvillea, Bromeliaceae, Brosimum, Brunellia, Bulnesia, Bursera, Caryophyllaceae, Casearia-type, Cecropia, Celtis, Crotolaria, Didymopanax, Hippocrateaceae, Humiria, Hymenophyllaceae, Inga, Leguminosae, Macrocarpea, Menispermaceae, Metopium, Sapium, Sapotaceae, Schinus, Strutanthus, Taperira, Ternstroemia, Vitis, Warszewiczia, Xylosma

ctc 

Podocarpus

txts

Acalypha, Alternanthera, Anacardiaceae, Bauhinia, Byrsonima, Byttneria, Caryocar, Casearia, Clusia, Colignonia, Copaifera, Cuphea, Curatella, Dodonaea, Erythrina, Evolvus, Ipomoea, Larrea, Pepermonia, Protium, Schinus, Siparuna, Stryphnodendron

tf

Acanthaceae, Apiaceae, Apium, Asteraceae, Begonia, Bromeliaceae, Calyceraceae, Caryophyllaceae, Cruciferae, Eriogonum, Geraniaceae, Genipa, Gomphorena, Gunnera, Hebenaria, Hippeastrum, Hydrocotyle, Jungia, Justicia, Malvaceae, Moraceae, Muehlenbeckia, Ranunculaceae, Rubiaceae, Scrophulariaceae, Siparuna, Thalictrum, Umbelliferae, Urticaceae, Verbenaceae, Viburnum, Xyris

PFT codes

Pollen taxa

g

Poaceae

man

Acrostichum, Avicennia, Rhizophora

tx

Alsophila, Cnemidaria, Cyathea, Dicksonia

tef

Tr 

Acalypha, Aegiphila, Alchornea, Anthodiscus, Anthostomella fuegiana, Apocynaceae, Araliaceae, Arecaceae, Arcella, Bombacaceae, Croton, Crotalaria, Hieronima, Heliocarpus, Humiria, Inga, Lamanonia, Lecythidaceae, Leguminosae, Loranthaceae, Macrolobium, Malpighiaceae, Malvaceae, Mauritia, Moraceae, Myrtaceae, Ocotea-type, Oreopanax, Piper, Pisonia, Polygonaceae, Rhipsalis, Rubiaceae, Rutaceae, Tiliaceae, Urticaceae, Vismia

Acanthaceae, Apiaceae, Apium, Asteraceae, Borreria, Bravaisia, Bromeliaceae, Euphorbia, Galium, Genipa, Gomphrena, Gunnera, Halenia, Hebenaria, Iresine, Iridaceae, Jungia, Justicia, Malvaceae, Moraceae, Nertea, Stevia, Umbelliferae, Urticaceae, Xyris, Zornia

sf

Amaranthaceae/Chenopodiaceae, Anarcardiaceae, Apiaceae, Borreria, Caryophyllaceae, Ephedra, Eriocaulon, Eriogonum, Euphorbiaceae, Gaimardia, Gilia, Gleichenia, Gomphrena, Halenia, Hebenaria, Lamiaceae, Mutisia, Polygala, Rhamanaceae, Xyris

R. Marchant et al. / Quaternary Science Reviews 20 (2001) 1289}1308 Table 3 (Continued) PFT codes

Pollen taxa

af

Arenaria, Asteraceae, Astragalus, Azorella, Bartsia-type, Borreria, Calceolaria, Caryophyllaceae, Cruciferae, Draba, Epilobium, Eriocaulon, Eriogonum, Galium, Geranium, Gompherena, Hebenaria, Heliocarpus, Hippeastrum, Hydrocotyle, Iridaceae, Lupinus, Lysipomia, Montia, Moritzia-type, Muehlenbeckia, Nertea, Oxalis, Polygonum, Portulaccaceae, Puya, Ranunculaceae, Relbunium, Rumex, Scrophulariaceae, Scutellaria-type, Selaginella, Senecio, Thalictrum, Umbelliferae, Valeriana, Verbenaceae, Viola, Xyris

cp

Azorella, Caryophyllaceae, Plantago

wte

Acalypha, Aegiphila, Alchornea, Alibertia, Allophylus, Araliaceae, Brunellia, Calliandra, Clusia, Croton, Crotolaria, Cydista, Daphnopsis, Embothrium, Euterpe, Genipa, Gentiana, Griselinia, Hedyosmum, Heliocarpus, Hydrangea, Ilex, Labiatae, Lamiaceae, Ludwigia, Luehea, Melastomatacae, Metopium, Mimosa, Myrica, Myrtaceae, Panopsis, Passiyora, Proteaceae, Schinus, Sebastiana, Solanaceae, Trema, Vernonia, Weinmannia

wte 

Abiata, Acaena-Polylepis, Aegiphila, Alnus, Asplenium, Bocconia, Brunellia, Clethra, Croton, Crotolaria, Dodonaea, Drimys, Ericaceae, Fuchsia, Hedyosmum, Hydrangea, Juglans, Labiatae, Lamanonia, Loranthaceae, Melastomatacae, Miconia, Mimosa, Mutsia, Myrtaceae, Myrica, Myrsine, Passiyora, Quercus, Proteaceae, Pseudopanax, Psychotria, Solanaceae, Viburnum, Vicia, Weinmannia

wte 

Abatia, Acaena/Polylepis, Asteraceae, Bocconia, Daphanopsis, Dodoneae, Drimys, Ericaceae, Escallonia, Eugenia, Halenia, Hesperomeles, Hypericum, Jamesonia, Mutisia, Myrica, Myrtaceae, Pepermonia

ts

Alnus, Cayaponia, Juglans, Loranthaceae, Palicourea, Styloceras, Thymelaeaceae, Ulmaceae, Vallea

aa

Amaryllidaceae, Aragoa, Arcytophyllum, Asteraceae, Baccharis, Cassia, Clethera, Cruciferae, Draba, Ephedra, Ericaceae, Escallonia, Gaiadendron, Gentianaceae, Guttiferae, Hypericum, Orycthantus, Puya, Ribes, Rosaceae, Rumex, Satureja, Sisyrinchium, Solanaceae, Tetrochidium, Valeriana

Rud.

Alium, Amaranthaceae/Chenopodiaceae, Ambrosia, Eucalyptus, Mauritia, Phaseolus, Phylanthus, Pinus, Rumex, Schinus, Zea mays

2.1. Data sources The pollen data are derived from sites that extend over a 4000 m altitudinal range. Locational information regarding the speci"c sites and age ranges encompassed by the sediments is provided within Table 1. All data used for this reconstruction are raw pollen counts; this allows for the allocation of all pollen taxa determined by the original analyst to PFTs, rather than would be the case if

1295

digitised data sets were used. For sites where multiple cores are available, the single core with the best age range for our analysis is used; for these sites the original core notation has been used to specify which core is used. One hundred and eighteen samples derived from core tops ((250 BP) taken from 40 sites comprise the modern data set (Table 1, Fig. 1). Only 37 of these sites were used for the Mid- to Late-Holocene analysis (Table 5) because only sites with secure radiocarbon-based chronologies can be used. For each site, the time window approach used involves analysing fossil samples within a narrow range of radiocarbon years either side of a central date. To determine which samples should undergo biomisation, on a site-by-site basis, a linear age}depth model was applied to the strati"ed pollen data. Samples were identi"ed as being temporally secure after checking the available dating control for any anomalies. Particularly we checked which sites had sedimentary hiati, rapid changes in sediment type and dating problems (such as age reversals and large standard errors attached to the uncallibrated radiocarbon data). The resolution required to select samples at 500 yr intervals was only common towards the tops of the sedimentary records where reduced sediment compression has allowed for a higher temporal resolution to be achieved. Therefore, the period from the present to 3000 BP is represented by time windows every 500 yr, each with a temporal range of $100 yr. Between 6000 and 3000 BP, 1000 yr intervals between each time windows are employed with a temporal range of $250 BP. All samples that were within the age range for each time window were compiled to produce a combined "le that was then checked to standardise nomenclature (Fig. 2). For example, the combined "le contains many taxa that contain synonyms such as Gramineae and Poaceae, Myrsine and Rapanea; these were combined to form a single taxon using the nomenclature after Kewensis (1997). All aquatic taxa were removed from the combined "le as these commonly respond to more local environmental conditions, particularly hydrology, rather than being re#ective of the wider environment. This removal includes Cyperaceae; although this pollen taxon is an important component of dryland savannahs, unfortunately it is also associated with the local swamp vegetation. Inclusion of this taxon with the pollen vs. PFT matrix would lead to many sites, which can be dominated by Cyperaceae, recording the tropical dry forest biome erroneously. It has been suggested that Poaceae pollen could similarly be excluded from our analysis (Mark Bush, pers. comm.). However, as the distribution of this taxon is more widespread throughout our range of vegetation types, and as such would not add bias to a particular biome, it is retained for the analysis. In total, 37 sites had samples that could be included in our analysis at past time windows although not all the sites have samples present at every time window (Table 1).

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Table 4 Biome a$nity scores to the range of biomes identi"ed by our analysis for all Colombia sites with modern core top samples. The sites are arranged in order of ascending altitude Site

TRFO

TDFO

STEP

TSFO

WEFO

CEFO

WAMF COMI

CGSS

CGSH

DEGR

BEST

Boca De Lopez Jotaordo Piusb El Camito Sardinas Monica Marin ame-II Angel Agua Sucia Loma Linda Pitalito Piagua Genagra Rio Timbio Libano Pedro Palo-III Ubaque Herrera CUX El Abra-II FuH quene-II Alsacia Cienaga del Visitador Agua Blanca La Guitarra Ciega-I La Primavera AmeH rica PalaH cio-PT1 Andabobos Laguna Verde Pen a Negra Los Bobos El Gobernador Corralitos Valle de Lagunillas-VL-V La Rabona CorazoH n Partido El Trinagulo

29.3 33.1 27.4 28.1 33.5 23.8 29.4 18.4 2.3 21.6 2.0 5.7 9.5 4.1 5.6 18.4 3.1 5.1 2.2 0.0 1.8 5.3 3.0 0.7 4.6 3.4 4.0 2.2 2.7 3.8 2.3 4.0 1.0 5.3 0.0 0.0 0.6 0.5 0.6

7.2 3.5 10.9 12.5 13.4 16.2 4.4 12.8 15.2 7.6 9.4 8.3 6.6 5.1 4.1 5.2 3.2 3.0 1.0 0.0 3.4 4.8 2.8 0.5 2.8 5.5 2.0 0.3 0.0 2.1 2.6 2.8 1.8 2.4 0.0 0.0 0.0 2.4 3.6

8.3 6.9 3.8 9.8 13.1 2.0 2.2 13.2 21.5 12.3 18.1 7.8 10.4 8.6 5.1 2.2 8.6 15.6 5.5 5.9 10.1 7.0 5.4 14.7 9.4 8.2 9.7 7.0 6.8 15.7 10.9 12.6 9.3 6.3 12.2 10.8 18.3 14.3 5.8

9.8 18.2 22.1 15.4 12.3 16.5 19.6 15.7 10.5 13.1 8.0 10.0 19.3 4.2 9.9 16.4 5.9 2.6 1.8 4.1 1.4 4.4 0.7 0.7 7.5 8.7 7.6 6.0 5.1 8.2 4.9 5.5 2.6 1.4 0.9 0.7 3.3 2.1 3.3

4.3 15.2 9.9 8.4 7.1 8.3 18.4 5.0 5.2 6.3 5.0 21.4 9.6 10.4 15.4 12.5 4.7 2.6 0.8 4.5 1.4 4.1 0.7 0.2 5.8 7.1 7.6 6.0 5.1 6.1 4.1 4.1 2.6 8.3 0.9 1.7 2.3 11.4 9.4

6.7 5.1 4.1 7.5 2.6 6.0 4.5 2.7 2.8 4.8 9.2 6.1 9.2 15.4 15.2 11.1 19.1 12.1 16.4 25.3 15.8 8.6 19.2 9.3 9.9 11.0 9.8 14.9 21.0 10.5 11.9 11.5 18.2 16.4 14.5 15.4 7.6 11.8 15.9

13.6 9.8 13.9 5.4 4.1 9.5 13.4 5.0 3.5 6.1 5.3 8.4 7.0 8.7 19.4 20.0 18.2 8.9 8.5 15.5 5.4 16.2 16.1 6.7 10.4 12.8 9.1 10.6 10.6 10.9 6.0 7.1 10.5 12.8 5.2 6.2 7.1 5.4 2.2

3.1 1.2 1.3 2.1 4.4 8.9 0.4 5.8 11.2 7.7 10.2 6.9 7.8 9.4 4.0 2.2 8.3 14.5 14.6 13.5 10.0 9.7 7.7 12.4 10.6 7.4 14.5 7.3 6.8 15.0 13.8 14.3 10.6 7.3 18.3 24.0 18.3 15.8 18.7

5.4 1.2 2.2 2.5 5.4 7.2 2.1 7.5 15.4 8.7 22.5 9.7 8.2 8.0 6.7 3.9 13.0 24.3 12.3 12.7 21.0 17.5 20.2 23.8 20.0 14.0 19.4 17.0 13.7 20.3 20.4 20.1 14.7 12.4 25.7 25.9 25.9 20.4 22.6

5.4 0.0 0.0 0.0 1.5 1.1 0.0 5.4 5.0 6.8 0.0 0.0 2.4 6.1 0.0 0.0 0.9 8.4 12.0 0.0 17.8 0.6 3.2 16.5 1.6 0.0 6.2 0.0 0.3 0.0 8.8 8.6 4.0 8.1 4.1 3.1 2.1 0.0 1.1

TRFO TRFO TRFO TRFO TRFO TRFO TRFO TRFO STEP TRFO CGSH WEFO TSFO COMI WAMF WAMF CEFO CGSH COMI CEFO CGSH COMI COMI CGSH CGSH COMI CGSH COMI COMI CGSH CGSH CGSH COMI COMI CGSH CGSH CGSH CGSH CGSH

3. Results Results are presented in Tables 4 and 5 organized according to site altitude * from low to high. The modern reconstruction lists the a$nity scores to all the biomes with the highest scoring biomes being used to compare the pollen-based reconstruction (Table 4) with the map of potential vegetation (Fig. 1). Table 5 details all the sites with the biomes for the reconstructions at past time windows and how they change through the Mid- to Late-Holocene time series. When there is a change to a biome characteristic of a colder and/or drier environment, these are highlighted in blue. Conversely, when there is a change to a biome characteristic of a warmer and/or wetter environment, this is highlighted in red. Speci"c sites that show a high a$nity

5.4 5.1 4.1 7.5 2.6 2.1 4.5 2.7 5.1 4.8 9.6 15.7 7.2 20.0 14.6 7.9 15.1 12.0 24.6 18.5 11.8 21.2 21.7 13.3 15.6 22.1 9.8 28.8 26.1 10.4 14.0 11.5 24.9 18.8 18.2 12.4 10.7 16.0 17.0

score to the degraded vegetation category are highlighted in green. As can be seen from the modern reconstruction (Table 4), the a$nity scores to the degraded vegetation category are relatively low; therefore, a cuto! of &5' is used to determine when a site can be designated as recording degraded vegetation in conjunction with the extant vegetation. Sites that do not change in their biome assignment throughout the analysis are highlighted in light brown. The shifts in vegetation, and hence the reconstructed biomes presented here, may result from one or more of a suite of factors that includes changes in temperature, precipitation, seasonality, ecological dynamics and human impact; the causal mechanisms thought to be responsible for the changes will be emphasised in the discussion.

Table 5 Biome reconstruction at past time windows of 6000, 5000, 4000, 3000, 2500, 2000, 1500, 1000, 500 BP and present-day. Where the biomes change from one time window to the next, this is classi"ed into two di!erent responses: when there is a shift to a biome indicative of a wetter and/or warmer environment this is indicated by red; when there is a shift to a biome indicative of a drier and/or cooler environment this is indicated by blue. When sites demonstrate an a$nity to the degraded vegetation category (a$nity score '5) this is highlighted in green. Sites that do not change their biome assignment throughout our analysis are highlighted in light brown.

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3.1. Modern biome reconstruction A visual comparison of biomes reconstructed from core tops (Table 4) with a map of potential modern vegetation (Fig. 1) shows that for the majority of the sites, the biomes predicted from the modern core-top pollen data accurately re#ect the broad features seen in the vegetation map derived from the Atlas of Colombia (IGAC, 1998). Sites at low altitudes ((300 m) are mainly assigned to the Tropical Rain Forest biome (e.g. Boca De Lopez, El Caimito, JotaordoH , Angel, Loma Linda, Piusbi, Sardinas, Marin ame-II and Monica-3). However, within these broad classi"cations there is signi"cant variation; Boca De Lopez, JotaordoH and Marin ame-II are the only &true' Tropical Rain Forest sites as demonstrated by the a$nity scores to the other biomes being very low. Interestingly, JotaordoH , from the Paci"c ChocoH , has the lowest a$nity to biomes other than the Tropical Rain Forest assignment. These true Tropical Rain Forest sites are in marked comparison to El Caimito, Angel, Loma Linda and Piusbi; although registering Tropical Rain Forest as the most dominant biome, they also have a high a$nity to the Tropical Dry Forest, Tropical Seasonal Forest and Steppe biomes, these assignments being indicative of more open vegetation. The other assignment recorded at low altitudes is Steppe (e.g. Agua Sucia); this site also has a high a$nity to the Cool Grassland Shrub biome. Sites located at mid-altitudes (1700}2600 m) are described by a number of di!erent biomes including Tropical Seasonal Forest (Genagra), Warm Mixed Forest (Pedro Palo-III, Libano and Piagua), Cool Mixed Forest (CUX and Rio Timbio), Steppe (Genagra) and Cool Evergreen Forest (El Abra II) biomes. Within this wide range of biomes, Warm Evergreen Forest and Tropical Seasonal Forest are more commonly assigned at the more mesic lower elevations (Table 4). It is noteworthy that between Loma Linda (310 m) and Pitalito (1300 m) the a$nity scores to the Tropical Rain Forest biome decrease signi"cantly. Sites within the altitudinal range of 2600}3800 m are most commonly characterised by the Cool Mixed Forest biome (e.g. Cienaga del Vistador) and the Cool Grassland Shrub biome (e.g. La Primevera), the latter being more common towards the top of this altitudinal range. A number of sites within this altitudinal range (e.g. Agua Blanca, Andabobos and La Guitarra) demonstrate Cool Grassland Shrub as being the dominant biome although these sites also have high a$nity scores to the Cool Evergreen Forest and Cool Mixed Forest biomes. Many of the sites with a high a$nity to the Cool Grassland Shrub biome also comprise degraded vegetation. Above 3800 m the dominant biome is Cool Grassland Shrub with the a$nity to arboreal biomes being quite low, as recorded at CorazoH n Partido, La Rabona and Lagunillas. The latter site also comprises a high a$nity to the Cool Grassland biome. Approximately 25% of all the sites exhibit a high a$nity ('5) to

the degraded vegetation category, this being recorded throughout the altitudinal range. 3.2. Past biome reconstructions The biomes reconstructed for each time window are listed in Table 5; these will be described for each time window with an emphasis placed on how the given reconstruction has varied in relation to the previous reconstruction. 3.2.1. 6000$250 BP The 6000$250 BP reconstruction (Table 5) shows an increase in biomes characteristic of more xeric environments in comparison to the present-day (Fig. 1). For example, the Tropical Rain Forest (TRFO) biome that presently dominates Boca De Lopez is co-dominant with Tropical Seasonal Forest (TSFO) at this period. Similarly, Loma Linda has a more open reconstructed vegetation with the Tropical Rain Forest and Steppe (STEP) biomes co-dominant, relative to the TRFO biome reconstruction of the present-day. One of the main changes at mid-altitudes is an increase in the number of Cool Evergreen Forest (CEFO) biome assignments, particularly at the demise of the Cool Mixed Forest (COMI) biome. A common change at higher altitude sites is a shift from the Cool Grassland Shrub (CGSH) biome assignment recorded for the present-day to an increase in the Cool Evergreen Forest and Cool Mixed Forest biomes. Relative to the present-day, no site records a relatively high a$nity to the degraded vegetation (DEGR) category. 3.2.2. 5000$250 BP The 5000$250 BP reconstruction is characterised by some sites, particularly those at lower altitudes, shifting towards more xeric biomes whereas the reverse shift is recorded at higher altitudes (Table 5). For example, relatively low altitudinal sites indicate more open vegetation with El Pin al losing the co-dominance with the Tropical Dry Forest biome component; the site being characterised by the Steppe biome. Boca De Lopez and Larga both exclude a co-dominant biome (Steppe and Tropical Seasonal Forests, respectively), becoming dominated by the Tropical Rain Forest biome, the former site demonstrating co-dominance with the degraded vegetation category. An expansion in the number of the Cool Grassland Shrub biomes is recorded at mid-altitudes; for example, CUX exhibits a transition from Cool Evergreen Forest to co-dominance of Cool Evergreen Forest and Cool Grassland Shrub biomes (Table 5). This latter biome also expands its range at higher altitudes, Pedro Palo-III, La Primavera and Palacio all becoming dominated by this biome. The only site to exhibit a high a$nity to the degraded vegetation category (Boca De Lopez) is situated at a coastal location and quite northwards from the main concentration of sites (Fig. 1).

R. Marchant et al. / Quaternary Science Reviews 20 (2001) 1289}1308

3.2.3. 4000$250 BP The reconstruction at 4000$250 BP is largely similar to the broad trends recorded at 5000$250 BP. At relatively low altitudes the majority of sites do not change biome assignments although Carimague demonstrates an increase in the presence of the Steppe biome at the demise of the Tropical Dry Forest biome. At mid-altitudes, there is a coherent increase in the amount of Cool Evergreen Forest, as recorded at CUX, FuH quene-II and Herrera. At higher altitudes, as in the previous period, there is an increase in the Cool Grassland Shrub biome, now being recorded at Agua Blanca, La Guitarra and Ciega. The number of sites to exhibit a high a$nity to the degraded vegetation category remains unchanged relative to the previous time window although the a$nity to this category at Boca De Lopez is reduced. 3.2.4. 3000$250 BP Between the 4000$250 and 3000$250 BP time windows all the sites that change are characterised by a shift to biomes indicative of a warmer/wetter environment. Interestingly, the shifts are most commonly recorded at mid-altitude sites with the transition concentrated between Libano (1820 m) and Pena Negra (3625 m). Most of the changes are characterised by an increase in the Cool Evergreen Forest and Cool Mixed Forest biomes relative to the previous period. Fewer sites from low altitudes change assignment relative to the previous time window. Carimague exhibits an increased a$nity to the Tropical Rain Forest biome, Libano an increase in the Tropical Seasonal Forest biome and El Pin al exhibits a high a$nity to the degraded vegetation category relative to the previous time window. 3.2.5. 2500$100 BP This time window records relatively few sites changing their biome assignment relative to the previous period. However, a number of sites at mid-altitudes (Agua Blanca and Pedro Palo-III) shift toward more xeric biomes; for example, the former site excluding the grassland component. Sites at high altitudes (Ciega I and El Gobernador) all show an inverse response, with an increase in the Cool Grassland Shrub component. The number of sites with an a$nity to the degraded vegetation category increases three-fold relative to the previous time window; sites recording a relatively high a$nity to this category remain to be recorded at low altitudes. 3.2.6. 2000$100 BP Similar to the previous time window, the majority of the sites do not change in their biome assignments, indeed, only three low altitude sites change. El Caimito and Monica-3 demonstrate a transition from the Tropical Rain Forest to the Tropical Seasonal Forest biome. Carimague demonstrates a transition from the Tropical Seasonal Forest to the Tropical Rain Forest biome.

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Similar to the last time window, the number of sites exhibiting an a$nity to the degraded vegetation category increases. This increase remains to be recorded at the relatively low altitudes, now extending as high as Genagra (1750 m). 3.2.7. 1500$100 BP Similar to the previous time window, the 1500$100 yr BP reconstruction is characterised by relatively few sites ("ve) changing their assignments. Two of the sites (Agua Blanca and Palacio) are characterised by an increase in the Cool Mixed Forest biome: a transition to a more mesic vegetation type. The reverse situation is apparent from Pena Negra with an increase in the Cool Grassland Shrub biome. Pedro Palo-III is characterised by a change to the Warm Mixed Forest biome. Relative to the last time window, the total number of sites exhibiting an a$nity to the degraded vegetation category does not change, although the record from Herrera (2500 m) exhibits vegetation disturbance whereas the record from Carimagua is truncated at this point. 3.2.8. 1000$100 BP This time window is characterised by broadly similar biomes to the previous time window, although four high altitude sites (Ciengra del Vistador, Guasca, Palacio and Pena Negra) show an increase in the Cool Mixed Forest and Cool Evergreen Forest biomes with a concomitant reduced a$nity to the Cool Grassland Shrub biome. Two mid-altitude sites (Herrera and Piagua) demonstrate an increase in the Cool Grassland Shrub biome. The main factor to change at this time window is an increase in the number of sites recording a high a$nity to the degraded vegetation category with two new sites recording this signal. Indeed, the majority of the sites from 0 m (Boca De Lopez) to 2580 m (FuH quene-II) now record a high a$nity to this category. 3.2.9. 500$100 BP Relative to the previous time window, this period is dominated by an increase in mesic biomes, with 12 sites out of 17 that change their biome assignment conforming to this trend. The dominant change is for sites, particularly above 2500 m, to exhibit an increase in the Cool Grassland Shrub biome. Sites located at lower altitudes are characterised by an increase in the Tropical Seasonal Forest biome. Interestingly, three of the four sites that demonstrate a shift to more xeric biomes (Pedro Palo-III, Genagra and Piagua) are located within a narrow altitudinal range (1700}2000 m). As in the previous time window, one of the main factors to change at this time window is an increase in the number of sites recording a high a$nity to the degraded vegetation category. This signal is now recorded by sites throughout the majority of the altitudinal range considered here (0}3625 m).

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3.3. Transition to modern reconstruction The transition from 500 BP to the modern reconstruction is characterised by a relatively low amount of change in biome assignment in comparison to the previous period. The changes in biome assignments, unlike the previous time windows, are not coherent. The most notable change is the increase in the number of sites recording a high a$nity to the degraded vegetation category with the majority of sites recording such a transition. Unlike previous periods this is now recorded at all but the highest altitude sites. 3.4. Temporal biome development In addition to recording how sites change over the Mid- to Late-Holocene period, for each time window it is also possible to describe how the vegetation has changed at a single site. The vegetation change at a particular site can be classi"ed into three categories: (1) sites whose pollen data record change in a broadly coherent pattern throughout the past 6000 yr and therefore are re#ective of broad-scale environmental changes; (2) sites that record oscillations between di!erent biome assignments and may be located in an ecotonal position where the pollen-based reconstruction may &#icker' between two biomes and (3) sites whose pollen data do not record change in their biome assignment throughout our analysis (Table 5). The "rst two categories have been described within the individual time windows above; those sites whose pollen data do not record change are the focus here. The sites to maintain the same overall biome are: La Rabona and Lagunillas that support the Cool Grassland and Shrub biome, El Abra-II that continues to support the Cool Evergreen Forest biome, Angel, Puisbi and Sardinas, that continue to support the Tropical Rain Forest biome and Agua Sucia that continues to support the Steppe biome. The reasons for the constancy of these assignments will be discussed below.

4. Discussion The discussion will focus on a number of synchronous responses of the Colombian pollen data, as portrayed via the biome reconstructions, to Mid- to Late-Holocene environmental change (Table 5). Initially we focus on the modern reconstruction, particularly on how the pollenbased vegetation reconstruction concurs with the potential vegetation (Fig. 1). We will then discuss how the Colombia vegetation has responded to the Mid-Holocene period, a transition to mesic environmental conditions between 4000 and 3000 BP. This is followed by a period of relatively little change in overall biome response. We interpret the latter as a period of environmental stability prior to between 1000 and 500 BP when

there is a transition to more xeric biomes recorded at a range of altitudes. A particular focus will be on the spread of a$nity to the degraded vegetation category. This will be discussed in the light of archaeological evidence of cultural development and how this may have impacted on the vegetation and the resultant pollen records. Throughout the discussion of these thematic areas, a particular emphasis will be placed on the location and altitudinal position of vegetation response to environmental change. 4.1. Modern reconstruction Two reassuring factors are noticeable from the modern reconstruction. Firstly, vegetation, via its pollen proxy, is highly sensitive to the changing environmental gradients characteristic of the Andean Cordilleras and lowland transitions from Tropical Rain Forest to savannah ecosystems (Fig. 1). This is most apparent in the reconstructions from high-altitude sites situated close to the forest limit where the vegetation change is most sensitive to environmental thresholds (Marchant et al., in press). Secondly, catchments that are known to exert an edaphic in#uence on the extant vegetation, e.g. the catchment of Agua Sucia supporting Steppe, surrounded by tropical rain forest, are assigned to the biomes correctly, even though our conceptual framework for the biomes is based on the climatological tolerances of vegetation. Our biomes reconstructed from the pollen data are unable to di!erentiate between savannas that are a function of edaphic, rather than climatic in#uences. Basing our interpretations within an understanding of site-speci"c environmental conditions we can indicate the likely source or bu!er to paleoenvironmental change. For example, although there has been signi"cant climate change over the Late Quaternary, El Pin al re#ects virtually unchanged pollen spectra due to strong edaphic in#uence (Behling and Hooghiemstra, 1999). Furthermore, the framework is able to di!erentiate intra-biome di!erences when the range of a$nity scores is investigated. For example, El Caimito, Angel, Piusbi and JotaordoH all record the Tropical Rain Forest biome as dominant. JotaordoH is located in the ChocoH Paci"c region, El Caimito and Piusbi are located farther to the south where the climate is drier (although still hyper-humid), whereas Angel is located within a gallery forest situation and is associated with the highest a$nity scores to the Tropical Dry Forests and Steppe biomes. Due to the #oristic and structural similarities between warm and cool grasslands (Tarasov et al., 1998), grassdominated biomes can be di$cult to distinguish from one another. By careful assignment of the pollen taxa to PFTs (Table 3), di!erentiation is possible, although there remains a high a$nity score for the Cool Grassland Shrub biome at low altitudes with the reverse for the Steppe biome at high altitudes. For example, Timbio and

R. Marchant et al. / Quaternary Science Reviews 20 (2001) 1289}1308

Herrera, both mid-altitude sites, record Cool Grassland Shrub although it is likely this was a composite vegetation association between our warm and cool grassland biomes. Nevertheless, using the biomisation method the Colombian pollen data are shown to resemble large-scale vegetation patterns despite many pollen taxa having a di!erent ecological interpretation under di!erent environmental settings (Grabandt, 1980). Representation of parent vegetation by pollen is likely to be subjected to inter-annual variability (Behling et al., 1997), and tropical vegetation is di$cult to reconstruct through pollen assemblages (Bush, 1991; Mancini, 1993; Bush and Rivera, 1998; Marchant and Taylor, 2000). These factors demonstrate the importance of careful construction of the initial input matrices for the biomisation process, and allowing the multiple assignment of the pollen taxa to the PFTs. One of the possible problems in providing a reliable modern calibration is the signal imparted by considerable extent of human impact on the modern vegetation. However, within our analysis the &modern' samples are derived from sedimentary columns and hence they may stem from the last century and be re#ective of a period prior to this intensive human-induced change. The large increase in the number of sites recording the Cool Grassland Shrub biome, particularly at mid-altitude sites that should support Cool Mixed Forest and/or Cool Evergreen Forest, is thought to exhibit human impact with pollen spectra increasingly being dominated by Poaceae and hence recording a shift to a more open vegetation. Within our analysis, Poaceae cannot be used as an indicator of vegetation disturbance because it is an important contributor to high- and low-altitudinal grass-dominated biomes, and this allocation would result in many sites erroneously recording anthropogenic impact. The number of sites recording a$nity to the degraded vegetation category are quite numerous beyond the recent historical period, thus vegetation disturbance in Colombia has a greater longevity than the colonial period; this will be discussed further. The lack of a coherent transition between our 500 BP and modern reconstructions may re#ect a period of "ne adjustment of the vegetation to the pervasive climatic regime, with sites &#ickering' between the most dominant biomes. Notwithstanding these reservations, the generally correct biome assignments for the speci"c sites, when compared to a map of potential vegetation (Fig. 1), con"rm the robustness of our application of the biomisation method to the Colombian pollen data, and validate the design of our input matrices. Thus, our reconstructions at past time windows can be carried out with con"dence. 4.2. Mid-Holocene warm period and biome response The records of dry environmental conditions vary considerably between sites * occurring broadly between 6500 and 4500 BP (Behling et al., 1999). Our data indi-

1301

cate that the cooling is recorded by higher altitude sites while there was continued warming at lower altitudes. Although, Bosman et al. (1994) suggested that relatively warm conditions persisted within Colombia until 6500 BP, the results presented here indicate that the trend towards relatively drier climatic conditions continued until at least 5000 BP, and commonly is recorded at high altitudes, where the forest limit lowered. Indeed, tree line appears to have been at its lowest between 7000 and 5000 BP (Kuhry, 1988) reaching 3100 m between 6650 and 5200 BP (Bosman et al., 1994). For example, within the FuH quene-II sediments, the Mid-Holocene hypsithermal is recorded by maximum levels of Quercus (Van Geel and Van der Hammen, 1973). Within our analysis, this maximum increases the a$nity to the Cool Evergreen Forest biome, whereas at lowland situations, for example Carimague, change is characterised by an increase in the Steppe/Tropical Dry Forest complex. Overall, our reconstruction starts with a range of biomes re#ective of a warmer environment relative to the present-day. This environmental signal is recorded throughout the majority of our sites and therefore is likely to represent a regional signal of environmental change. Indeed, a dry phase is recorded at many sites in northwestern South America where savanna woodland elements were replaced by deciduous forest elements (Markgraf, 1989). Farther south, the Holocene hypsithermal period appears to have begun about 8900 BP in northern Argentina with a particularly dry period around 6400 BP (Alcalde and Kulemeyer, 1999). A similar onset is also recorded in northern Chile, where desiccation of the Puna ecosystem is recorded between 8000 and 6500 BP (Baied and Wheeler, 1993). On the central Peruvian Andes a dry warm climate is experienced between 7000 and 4000 BP (Hansen et al., 1994). As within our reconstruction, this warming is not just restricted to high-altitude sites; in lowland Chile, drier than present conditions continued from the early Holocene until approximately 5000 BP (Heusser, 1982). In contrast to the relatively equatorial sites of Colombia, the circum-Carribean area records a shift to dry conditions much later, where it occurs at approximately 2500 BP (Bradbury et al., 1981; Bush et al., 1992; Curtis et al., 1999). This relative warm period is also recorded within a range of di!erent environmental systems other than a purely vegetation response. For example, between 6000 and 3800 BP the level of Lake Titicaca was approximately 100 m below the present-day level with modern water levels not being reached until about 2100 BP (Cross et al., 2000). A similar response is recorded by Lake Valencia (Bradbury et al., 1981). This regional picture of a warmer climate is also apparent from the
O record of an ice core taken from highland Peru that indicates Mid-Holocene climatic warming between 8200 and 5200 BP, with maximum aridity between 6500 and 5200 BP (Thompson et al., 1995).

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Unlike the large temperature changes associated with full glacial conditions, the temperature changes associated with the Holocene hypsithermal are thought to be relatively small, possibly about 13C warmer relative to today. This shift is consistent with the Holocene reconstruction by COHMAP (1988). Although Servant et al. (1993) suggested that dry periods were more frequent during the Mid-Holocene, the moisture #ux was likely also changed with the seasonally dry interval being more prominent (Markgraf, 1989). One of the main mechanisms used to determine moisture shifts is #uctuations in the Southern Oscillation and the migration of the InterTropical Convergence Zone (ITCZ) (Martin et al., 1997). However, due to the topographical in#uence of the Andes and the convergence of westerly and easterly winds, the ITCZ has a sinusoidal pro"le over northern South America. Therefore, to explain moisture changes in Colombia, we need to demonstrate the importance of convective moisture sources. According to Martin et al. (1993), El Nin o conditions were numerous prior to our 4000 BP reconstruction with polar frontal systems being blocked due to an enhancement of the subtropical jet stream. This climatic scenario would lead to an aridity response of vegetation in Colombia due to decreased southerly and westerly advective moisture following reduced in#uence of polar advection within the tropical zone. The winter period in particular would be drier (Servant et al., 1993). Alternatively, the asynchronous timing of environmental changes across tropical America may suggest orbital control over climatic changes that are analogous to the mechanism that drives the AfroAsian monsoon (Cross et al., 2000). To determine the validity of this mechanism modelling studies are needed, such as those focused at a similar period on Saharan Africa (Kutzbach et al., 1996). These link the observed environmental changes to increased monsoon activity due to orbitally driven increase in northern hemisphere insolation. Whatever the mechanism used to explain the vegetation changes, tropical palaeoecology is highly responsive and able to describe global environmental changes. For example, the Holocene record of decreased methane, detected within the GRIP ice cores, may be re#ective of Mid-Holocene drying of southern hemisphere wetlands. The tropical records seem to be more in phase with the observed changes than those from more temperate latitudes (Cross et al., 2000). 4.3. Relatively rapid environmental change about 3500 BP From our analysis, a Mid-Holocene period of vegetation change in response to a wetter environment occurs between our 4000 and 3000 BP time windows. This synchronous climatic signal to wetter climatic conditions is thought to be centred between 3860 and 3590 BP (Behling et al., 1999). At many lowland sites this transition is characterised by a marked increase in palms

(Mauritia and Mauritella) that began approximately 3800 BP in response to a wetter climate, probably with a short dry season and/or human in#uence (Behling and Hooghiemstra, 1999). It appears that this period is characterised by a shift that ended the preceding dry period with some systems, such as rivers, lagging in response to a changed hydrological regime. For example, Van der Hammen et al. (1992) suggested that reduced discharge of the CaquetaH River in Colombian Amazonia re#ects drier climatic conditions for the period 4000}3000 yr BP. A dry period is suggested from lowland Ecuador between 4200 and 3150 BP and Amazonia (Liu and Colinvaux, 1985; Bush and Colinvaux, 1988) and the Chaco of northern Argentina between 3500 and 1000 BP (Iriondo and Garcia, 1993). Our analysis, however, shows an increase in moisture levels with a transition towards more mesic vegetation recorded throughout the altitudinal range considered here. Our interpretation is supported by an increase in the amount of hydrophytic vegetation within the Lake Valencia catchment (Markgraf, 1989) and an increase in the level of lake Titicaca between 3600 and 3200 BP (Cross et al., 2000). More widely, this signal is recorded throughout tropical South America (Martin et al., 1993). This would result in the northward expansion of polar frontal systems and consequent increase in wetter climates in northwestern South America, alternatively, greater southward migration of the ITCZ during the southern hemisphere summers (Francis Mayle, pers. comm.). Given the widespread occurrence of this signal, this time period is highly interesting and its characterisation should be a target of future investigation as it appears to be a period of rapid climatic change, to which some archives responded faster than others.

4.4. Period of vegetation in pace with the climate For the period between 3000 and 1000 BP some sites #icker between biome assignments but few biome assignments change. A few sites do change within this period with these sites being situated at mid- to upper-altitudes. This may be indicative of relatively subtle environmental change that was speci"c to this altitudinal band. The vegetation at this altitude was more responsive to a phase of relatively low-magnitude environmental change and/or this is re#ective of a maturation of the vegetation composition. Similarly, the water levels of Lake Titicaca were quite stable during this period (Cross et al., 2000), as is the environment reconstructed from the HuascaraH n ice-cap, Peru (Thompson et al., 1995). Wider a"eld, this period is relatively stable in reconstructed temperature recorded as derived from the GISP 2 and GRIP ice cores from Greenland (Willemse and ToK rnqvist, 1999). This period of stability may be attributed to a relatively stable Southern Oscillation with La Nin a events being infrequent after 2800 BP (Martin et al., 1993).

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4.5. Environmental drying at 800 BP Between the 1000 and 500 yr BP reconstructions, there is a period of environmental change characterised by a transition to more xeric vegetation. We interpret this vegetation signal as a period of reduced plant-available moisture caused by a shift to drier climatic conditions. Crucially for this period, the observed changes are not all associated with sites with high a$nities to the degraded vegetation category and therefore cannot solely be attributed to human impact but must also re#ect a climatic signal. Farther to the south, palaeoclimatic records from the Quelccaya ice-cap in Peru indicate a dry period from 540 to 610 AD that was also associated with a temperature decrease of between 0.5 and 13C (Thompson et al., 1985). Reduced precipitation would have a!ected the entire Central Andean areas and western South America (Binford et al., 1997), for example a dry period is recorded by a low stand at lake Titicaca (Binford et al., 1997). Palynological and geological evidences from the Amazon Basin, and other areas of tropical South America indicate a Holocene dry period extended between 750 and 500 BP (Piperno and Becker, 1996). It may be that this dry phase coincided with markedly oscillating climates in the south and central Andes (Thompson et al., 1994). Farther north, this was one of the driest period recorded within the Yucatan Peninsula, coincident with the cultural collapse of the extensive Mayan civilisation (Hodell et al., 1995). Unfortunately, the resolution of many palaeoarchives is not su$cient to fully characterise this period, and therefore infer causal mechanisms such as longduration, low-phase Southern Oscillation. However, it does seem that coastal Peru was a!ected be a series of disastrous El Nin o years around 1050 AD (ChepstowLusty et al., 1996) which would have produced relatively dry environmental conditions in Colombia. Farther north in California, an intense and rapid phase of cooling is detected within the Santa Barbara Basin sedimentary record, possibly as a result of a period of volcanic activity and/or solar forcing (Schimmelmann et al., 1998), although the latter suggestion has been refuted (Van Geel et al., 1999). Whatever the forcing mechanism, it does seems that the last 1000 yr has witnessed particularly strong, brief climatic events that have been recorded, when the resolution has been su$cient, in Central and South America (Piperno and Becker, 1996). 4.6. Human impact on the Colombian biomes One of the main changes to be detected from our analysis is the stepwise increase during the Mid- to LateHolocene in the number of sites having an a$nity to degraded vegetation. This is particularly pertinent as although it is known that human in#uence on vegetation patterns and soil development began to be signi"cant from the Late Holocene onwards (Thouret et al., 1997),

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the timing and location of this is poorly understood. The earliest signal of degradation comes from Boca De Lopez in the northeast of Colombia, with early disturbance also recorded at other tropical rain forest localities (Table 5). In spite of the suggestion that lowland tropical forests were unable to support substantial human populations with humid climates, poor soils and profusion of pests (Pringle, 1998), Holden (1998) suggested that farming in the Americas may have begun within tropical forests. Within our analysis, a number of sites within the tropical rain forest demonstrate vegetation disturbance that may be attributed to early human activity. Archaeological investigations demonstrate that human occupation dates back to approximately 10,000 BP (Van der Hammen and Correal-Urrego, 1978; Gnecco and Mohammed, 1994). Palaeoindians probably lived in the ChocoH Rain forest since at least 3460 (Behling et al., 1998), while archaeological records from Colombian Amazonia date back to 9000 BP (Gnecco, 1999). Farther north, Zea mays and Squash, likely to be Curcurbita moschata, have been reported from Central Panama from the Mid-Holocene (Piperno et al., 1990). The a$nity scores to the degraded biomes are relatively low throughout our analysis. This can be partly explained because taxa that are indicative of human activity, such as Zea mays, although being present around sites, such as Piusbi during the last 1710 yr (Behling et al., 1998) and Pitalito since 4700 BP (Wille, pers. comm.), are not numerically important within the pollen count and hence make a relatively minimal contribution to the biome a$nity scores. Numerous records of pre-Hispanic Zea mays have been recorded from 5200 BP onwards; for example, Pena Negra and Agua Blanca both have a long record of this food crop that is most likely attributable to agricultural activity by preHispanic populations on the eastern slopes of the Magdalene Valley (Kuhry, 1988). Also, no anemophilous weedy indicators of agriculture are present in northern South America such as Ambrosia in North America (Webb-III, pers. comm.). This relatively lowland concentration of degraded vegetation does not appear to extend to the higher altitudes until approximately 2000 BP. Rather than an increase from an agricultural base in the lowlands, this expanse can be attributed to pre-Incan Andean cultures, particularly those based at Tiwanaku (Morris, 1999). However, due to the maintenance of this impact throughout our analysis (Table 5) we suggest that there was no signi"cant collapse in the well-developed agricultural systems that have been widely documented for the Peruvian cultures (Bray, 1990; Chepstow-Lusty et al., 1996) and the Maya (Hodell et al., 1995). Indeed, the substantial &decline' in regional agriculture after 2000 BP, that has been attributed to a loss of soil fertility and/or climate change (Chepstow-Lusty et al., 1996), is not apparent from our analysis. Thus, these well-documented changes may also have a strong cultural component that

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was only recorded when large, centralised populations develop. Alternatively, the impact of climate change on human and natural populations is most likely to impact in geographic locations where climate is a controlling factor on the range of agricultural activity (Seltzer and Hastorf, 1990). Charcoal dated before approximately 4000 BP within the Rio Buritaca watershed (Herrera, 1985) provides direct evidence to support the suggestion that the savanisation process in Colombia was initiated by Amerindians by means of the frequent use of "re or clearing lands for the cultivation of Zea mays. Cavelier et al. (1998) suggest that this impact was able to transform moist forest ecosystems to forest ecosystems of secondary growth and eventually savanna. A link between the importance of "re in delimiting the distribution of lowland tropical savannahs is well documented, the incidence of "re increasingly being able to penetrate into moist gallery forests (Biddulph and Kellman, 1998). Within our analysis, a notable lowland site (El Pin al) is situated within savannah setting where early vegetation disturbance is recorded. However, the impact is not so marked as to change the overall biome assignment; these impacts appear relatively small, with the main #oristic characteristics, form and functioning of the original ecosystem being maintained. The "rst Spanish conquests in Colombia date from 1528. By 1530 AD the Spanish began to colonise Colombia, taking advantage of agricultural lands (Cavelier et al., 1998). The in#ux resulted in a high mortality of Amerindians, with agricultural "elds being abandoned and a concomitant migration of populations to higher altitudes (Cavelier et al., 1998). The start of forest recovery at Timbio is thought to occur slightly before the arrival of the Spanish conquistadors (Wille, pers. comm.) when agricultural practice appears to have changed. Although Behling et al. (1998) suggested that some tropical landscapes revert to forest following the arrival of the Spanish, our analysis does not show this to be the case. Indeed, relatively few sites diminish their a$nity to the degraded vegetation following earlier human impact. What is apparent is that the rich and diversi"ed agriculture in much of Colombia today (Monasterio and Sarmiento, 1984) has a long history and originated in the lowlands. 4.7. Environmental stability at some sites within the Late Holocene Our analysis shows that the ability for a site's pollen spectra to be re#ective of climatic and anthropogenic impacts is very much determined by the location of the site. For example, relatively passive sites may be located within the middle of a biome's &bioclimatic' space. Conversely, those sites close to the transition between biomes will be sensitive to environmental change, in some cases being too responsive with the result that they &#icker'

between biome assignments. For those sites that exhibit resilience to changing their biome assignment, some sitespeci"c factor may be important in maintaining vegetation stability. A good example is Angel, the Steppe biome is reconstructed throughout due to the local importance of edaphic factors imposed by a sandstone substrate within the catchment (Behling and Hooghiemstra, 1999). The other sites that are resilient to change may also involve some site-speci"c phenomena, although for some sites it is a result of location, being positioned far from the boundaries of bioclimatic space. For example, La Rabona (4000 m) and Lagunillas (3880 m) are above the forest limit and are located some 300}400 m of vertical altitudinal change away from the in#uence of arboreal components to register in the pollen record. Given that temperature is the main control on the position of the Andean Colombia upper forest limit (Hooghiemstra, 1984), this equates to a temperature shift of approximately 33C if the environmental lapse rate was 63C 1000 m\ (Van der Hammen and GonzaH lez, 1960) required to change the biome assignment. This magnitude of change was only recorded during the late glacial period and not at any time during our analysis. 4.8. Areas for further study The Late Quaternary vegetation history of the Neotropical phytogeographical realm remains relatively poorly resolved despite the importance in model testing (Bush and Colinvaux, 1988), developing biogeographical theory (Tuomisto and Ruokolainen, 1997), and understanding issues concerned with biodiversity and human}environmental interactions. Understanding the modi"cation of biome distribution due to past environmental changes is increasingly important. It could be investigated by using vegetation models to depict output from climate modelling studies (Claussen and Esh, 1994; IndermuK hle et al., 1999). Where the data are of su$cient quality and quantity, the model-based reconstruction should be compared directly with the reconstructed biomes presented here. By applying one method to a selection of well-distributed pollen data with robust chronologies, it is possible to determine temporal vegetation dynamics at regional to continental scales. Environmental change is rarely spatially uniform and as such necessitates an even greater number of sites to determine more precisely the complexity of vegetation response and the environmental mechanisms driving this. For example, di!erent altitudinal bands appear to respond in a di!erent manner such that the blanket application of lapse rates to site-speci"c calculations of palaeoclimatic parameters must be treated with caution. Indeed, new sites, located in key areas, are required to re"ne our understanding of the Neotropical response to Holocene and millennial-scale climatic changes. The new data will also show how this signal can be interpreted in the light

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of comparable data from other areas and palaeoenvironmental archives (Jolly et al., 1998a). In addition to these direct applications, the biomes and PFTs derived from the pollen data could be used to translate the output to speci"c biogeochemical components, such as carbon budgets, and thus provide a direct test for modelling studies concerned with these components (Foley et al., 1996).

5. Conclusions 5.1. Late-Holocene vegetation dynamics The Prentice et al. (1996) method for assigning biomes to pollen data provides an objective basis for determining regional-scale vegetation dynamics and the environmental controls that a!ect them. Our analysis showed that a factor complex must be invoked to explain the range of vegetation changes over the Mid- to Late-Holocene period that encompasses changes in temperature, moisture and moisture availability, human impact and ecological dynamics. Although Behling et al. (1999) indicate that the present network of sites is insu$cient to evaluate whether climatic change in Colombia was a major factor in forcing Late-Holocene vegetation dynamics, our analysis of the data showed that there are some common forcing signals. Speci"cally, the vegetation appears to respond to (1) the Mid-Holocene hypsithermal period, (2) a period of increased moisture centred on approximately 3500 BP and (3) environmental aridity centred on approximately 800 BP. The data presented here represent a high-resolution palaeoarchive suitable for testing and re"nement of climate models. 5.2. Vegetation disturbance We showed that the tailoring of the biomisation technique to demonstrate disturbance of the vegetation, that is likely to have its origin in human impact, is a useful addition to understanding an important control on Late-Holocene vegetation dynamics in Colombia. After an initial focus within lowland ecosystems, human setting this disturbance expanded, in a stepwise manner, to high altitudes, becoming present at the range of altitudes analysed here. Unlike the close correlation between climate change, vegetation response and cultural development, the relationship appears to be weaker than is apparent for the large centralised cultures such as the Maya and those associated with Tiwanaku.

Acknowledgements The present research project is funded by the Netherlands Organisation for Scienti"c Research (NWO) under

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award 750:198:08 made to Henry Hooghiemstra. Numerous people have o!ered support and comment on the research at various stages of development of this research none more so than the people who have made their pollen data available for the analysis. Without such contributions, this work would not have been possible. In particular, we thank Sandy Harrison and Colin Prentice for discussions throughout the development of the broader-scale assignment of biomes to Latin American pollen data. Particular thanks must go to Vera Markgraf and Eric Grimm for their energies in establishing, and developing, the Latin American Pollen Database (LAPD). Francis Mayle and Tom Webb III are thanked for comment on an earlier draft of this paper.

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Tuomisto, H., Ruokolainen, K., 1997. The role of ecological knowledge in explaining biogeography, and biodiversity in Amazonia. Biodiversity and Conservation 6, 347}357. Van der Hammen, T., 1963. A palynological study on the Quaternary of British Guyana. Leidse Geologische Mededelingen 29, 126}168. Van der Hammen, T., 1972. Changes in vegetation and climate in the Amazon basin and surrounding areas during the Pleistocene. Geologie en Mijnbouw 51, 641}643. Van der Hammen, T., Urrego, L.E., Espejo, N., Duivenvoorden, J.F., Lips, J.M., 1992. Late-glacial and Holocene sedimentation and #uctuations of river water level in the CaquetaH area (Colombian Amazonia). Journal of Quaternary Science 7, 57}67. Van Geel, B., Van der Plicht, J., Renssen, H., 1999. Comment on `A Large Californian #ood and correlative global climatic events 400 years agoa (Schimmelmann et al., 1998).. Quaternary Research 51, 108}110. Willemse, N.W., Tornqvist, T.E., 1999. Holocene century-scale temperature variability from West Greenland lake records. Geology 27, 580}584. Williams, J.W., Summer, R.S., Webb III, T., 1998. Applying plant functional types to construct biome maps from eastern North American pollen data: comparisons with model results. Quaternary Science Reviews 17, 607}627. Witte, H.J.L., 1994. Present and past vegetation and climate in the North Andes (Cordillera Central, Colombia); a quantitative approach. Ph.D. Thesis, University of Amsterdam, The Netherlands.

Papers from the sites Behling, H., Berrio, J.C., Hooghiemstra, H., 1999. Late Quaternary pollen records from the middle CaquetaH river basin in central Colombian Amazon. Palaeogeography, Palaeoclimatology, Palaeoecology 145, 193}213. Behling, H., Hooghiemstra, H., 1998. Late Quaternary palaeoecology and palaeoclimatology from pollen records of the savannas of the Llano Orientales in Colombia. Palaeogeography, Palaeoclimatology, Palaeoecology 139, 251}267. Behling, H., Hooghiemstra, H., 1999. Environmental history of the Colombian savannas of the Llanos Orientales since the Last Glacial Maximum from lake records El Pin al and Carimagua. Journal of Palaeolimnology 21, 461}476. Bosman, A.F., Hooghiemstra, H., Cleef, A., 1994. Holocene mire development and climatic change from a high Andean Plantago rigida cushion mire. The Holocene 4, 233}243. Kuhry, P., 1988. Palaeobotanical}Palaeoecological Studies of the Tropical High Andean Peatbog Sections Cordillera Oriental, Colombia, Dissertationes Botanicae, Vol. 116. J. Cramer, Berlin, pp. 1}241. Kuhry, P., Salomons, J.B., Riezebos, P.A., Van der Hammen, T., 1983. PaleoecologiaH de los uH ltimos 6.000 anos en el area de la Laguna de Otun-El Bosque. In: Van der Hammen, T., Perez, P.A., Pinto, P. (Eds.), Studies on Tropical Andean Ecosystems, Vol. 1: La Cordillera Central Colombiana transecto Parque Los Nevados. Cramer, Vaduz. Van der Hammen, T., Correal Urrego, G., 1978. Prehistoric man on the sobono de BogotoH ; data for an ecological prehistory. Palaeogeography, Palaeoclimatology, Palaeoecology 25, 179}190. Van der Hammen, T., GonzaH lez, E., 1960. Upper Pleistocene and Holocene climate and vegetation of the Sabana de BogotaH . Leids Geologische Mededelingen 25, 261}315. Van der Hammen, T., GonzaH lez, E., 1965. A Late glacial and Holocene pollen diagram from Cienaga del Vistador Dep. Boyaca, Colombia. Leids Geologische Mededelingen 32, 193}201. Van Geel, B., Van der Hammen, T., 1973. Upper Quaternary vegetational and climatic sequence of the FuH quene area Eastern Cordillera, Colombia. Palaeogeography, Palaeoclimatology, Palaeoecology 14, 9}92.

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Wijmstra, T.A., Van der Hammen, T., 1966. Palynological data on the history of tropical savannas in northern South America. Leidse Geologische Mededelingen 38, 71}90. Wille, M., Negret, J.A., Hooghiemstra, H., 2000. Paleoenvironmental history of the Popayan are since 2700 yr BP at Timbio, Southern Colombia. Review of Paleobotany and Palynology 109, 45}63

Further reading . Van der Hammen, T., Absy, M., 1994. Amazonia during the last glacial. Palaeogeography Palaeoclimatology Palaeoecology 109, 247}261. Wijmstra, T.A., 1971. The Palynology of the Guiana Coastal Basin. University of Amsterdam, 72pp. Amsterdam, The Netherlands. Hooghiemstra, H., Van der Hammen, T., 1993. Late Quaternary vegetation history and paleoecology of the Laguna Pedro Palo (sub-

andean forest belt, Eastern Cordillera, Colombia). Review of Palaeobotany and Palynology 77, 235}262. Melief, A.B.M., 1985. Late Quaternary Paleoecology of the Parque National los Nevados Cordillera Central, and Sumapaz Cordillera Oriental areas, Colombia. Dissertation, University of Amsterdam, Amsterdam, The Netherlands. Van der Hammen, T., 1974. The Pleistocene changes of vegetation and climate in tropical South America. Journal of Biogeography 1, 3}26. Van der Hammen, T., 1962. PalinologmH a de la region de Laguna de los Bobos. Hitoria de su clima, vegetacioH n y agricultura durante los uH ltimos 5000 an os. Revista Academia Colombiana Ciencias Exactas Fisicas Naturales 11, 359}361. Van der Hammen, T., Barelds, T.J., de Jong, H., de Veer, A.A., 1980. Glacial sequence and environmental history in the Sierra Nevada del Cocuy Colombia. Palaeogeography, Palaeoclimatology, Palaeoecology 32, 247}340.