Late quaternary vegetation history and paleoecology of Laguna Pedro Palo (subandean forest belt, Eastern Cordillera, Colombia)

235

Review of Palaeobotany and Palynology, 17 (1993): 235-262

Elsevier Science Publishers B.V., Amsterdam

Late Quaternary vegetation history and paleoecology of Laguna Pedro Palo (subandean forest belt, Eastern Cordillera, Colombia) Henry Hooghiemstra Hugo de Vries-Laboratory,

and Thomas

van der Hammen

Department of Palynology and PaleolActuo-Ecology, 318, 1098 SM Amsterdam, The Netherlanh

University of Amsterdam,

Kruislaan

(Received November 4, 1992;revised and accepted February 3, 1993)

ABSTRACT Hooghiemstra, H. and van der Hammen, T., 1993. Late Quatemary vegetation history and paleoecology of Laguna Pedro Palo (subandean forest belt, Eastern Cordillera, Colombia). Rev. Palaeobot. Palynol., 77: 235-262. Laguna Pedro Palo is situated on a saddle at ca. 2000 m altitude in the subandean forest belt on the western slope of the Eastern Cordillera of Colombia. Three pollen records from the site show the vegetation history and paleoclimatological sequence since ca. 13,000 BP. The pollen records were correlated on the basis of radiocarbon-dated horizons (11,950* 100, 11,380& 130, 10,380+90, and 10,280f90 yr BP) and palynostratigraphy. Almost no palynological data are available from the subandean forest belt during glacial conditions. Most important is the direct evidence that the upper Andean forest line shifted to below 2000 m alt. during glacial conditions, providing evidence of a glacial forest line depresssion of ca. 1400 m and an inferred temperature depression of ca. 8°C. During the early Late Glacial (or Pleniglacial-Late Glacial transition) under dry climatic conditions, (sub)andean (semi-)open dry vegetation was locally in contact with dry paramo vegetation and a proper Andean forest belt was apparently absent. Open shrubparamo and grassparamo vegetation surrounded the lake at 2000 m alt. Subsequently patches of Andean forest reached closer to the area, indicating gradually less dry conditions shortly before 12,000 yr BP. During the Guantiva interstadial (ca. 12,000-l 1,000 yr BP) a well-developed Andean forest belt occurred from ca. 1800 to 2800 m alt. consisting of mainly Quercus forest and Weinmannia forest. Climatic conditions were cool temperate and more humid than before the Guantiva interstadial. Alnus carr vegetation occurred around the lake. During the next El Abra stadial (ca. 11,OOO-10,150yr BP) a cooler climate ( - 3°C) is indicated by a lowering of the upper Andean forest line by ca. 500 m from its position during the Guantiva interstadial. Carr vegetation of Alnus diminished and disappeared at the end of the El Abra stadial, an approximate time-equivalent of the Younger Dryas event. Subsequently the upper Andean and subandean forest lines reached Holocene elevations, ca. 3300 and 2400 m, respectively. First a massive increase of Cecropia, a pioneer tree, and shortly later Acalypha and Alchornea dominated the pollen spectra of the subandean forest belt. In the Andean forest belt Quercus and Weinmannia forest types dominated with important contributions of Hedyosmum, Ilex, Juglam, Miconia, Urticaceae and Brunellia. A Late Holocene pollen record shows a phase showing human disturbance of the forest belt. Agricultural activity is indicated by relative high percentages of Grarnineae and Chenopodiaceae followed by a possible forest succession phase in which Cecropia played a major part. Finally a “post-conquest” phase (after ca. 1500 AD) is registered, shown by relative high percentages of Gramineae and Rumex acetosella.

Introduction In the Eastern Cordillera of Colombia many pollen records have been studied from elevations over 2500 m. At this elevation several intermontane basins are present that support lakes or a desiccated high plain. The long sediment sequences 0034-6667/93/!$06.00

0 1993 -

from these intermontane basins have provided excellent pollen records that document climatic change with high precision, e.g. Laguna de Fuquene at 2580 m alt. (Van Gee1 and Van der Hammen, 1974) and from the high plain of Bogota at 2550 m alt. (Van der Hammen and Gonzalez, 1960a; Schreve-Brinkman, 1978; Hooghiemstra,

Elsevier Science Publishers B.V. All rights reserved.

236

1984, 1989) (Fig. I). At higher elevations up to about 3900 m glacial lakes, dried lake depressions and peat bogs have been studied palynologically and provide a record of environmental change in response to climatic change (e.g. Van der Hammen and Gonzalez, 1960b; Van der Hammen, 1974; Van der Hammen et al., 1981; Melief, 1985; Kuhry, 1988; Helmens and Kuhry, 1986). Strong evidence from these sites has indicated that the present-day upper forest line (situated at ca. 3200-3400 m alt.) shifted as far down as below the elevation of the intermontane basins at ca. 2500 m alt. during the last glacial: these lakes recorded the presence of paramo vegetation during the last glacial. For estimating the altitudinal position of the upper forest line, numbers of arboreal percentages have been used as given by Van der Hammen and Gonzalez (1960a) and Hooghiemstra (1984). Because of the presence of steep slopes below 2500 m elevation, there are hardly any favourable locations for palynological studies at 1000-2500 m elevation. Laguna Pedro Palo (Fig. 2) at ca. 2000 m elevation represents one of the few known sites in Central Colombia. Laguna Pedro Palo is situated on the western slope of the Eastern Cordillera, towards the interandean (tropical) valley of the Rio Magdalena (Depto. de Cundinamarca, lat. 4”30’N, long. 74”23’W). It has a surface area of ca. 50,000 m2 and a maximum depth of ca. 25 m (Diaz Daza, 1983). Laguna Pedro Palo lies within the upper part of the subandean forest belt. The average annual temperature at the site is 17-18°C and because of the tropical climatic conditions differences in mean monthly temperature are small. Van der Hammen and Gonzalez (1960a) calculated a lapse rate of 0.66”C per 100 m altitude for the Eastern Cordillera. a value that will be used here. The study area has two dry seasons, from December to February and from July to August, and two wet seasons from March to June and from September to November; the latter ones are approximately coeval with the passing of the Intertropical Convergence Zone. Average annual precipitation in the study area is ca. 2000-4000 mm. The pollen diagram of Laguna Pedro Palo showed for the first time that paramo vegetation

H HOOC;HltMSIKAAND

ILVANDERHAMMLN

was present at 2000 m alt. during the last glacial. demonstrating that the upper forest line lay at 1800 m alt. during the last glacial (Van der Hammen, 1974). Unfortunately, the possible time control of the pollen records is limited to the presented 4 radiocarbon dates. A summary pollen diagram of section Pedro Palo III was used in earlier reviews (e.g. Van der Hammen, 1974: Flenley, 1979: Rivera. 1983: Kuhry, 1988). Recently, additional pollen records have become available from low elevation in the Colombian Western Cordillera (Monsalve, 1985) in the Pitalito Basin of the Eastern Cordillera at 1300 m alt. (Bakker, 1990). and in the Costa Rican Cordillera de Talamanca at 2300.--2400 m alt. (Hooghiemstra et al., 1992). The network of sites permits a modest comparison of the last glacial and the Holocene vegetation in the present-day subandean forest. The objective of this paper is to present the full data of the three pollen records from Laguna de Pedro Palo. The vegetational history and climatic sequence of this 2000 m elevation site are discussed and compared with other records. The altitudinal range of the modern transition from Andean forest to subandean forest is especially emphasized, as no other pollen record documenting this interval is (up to now) available.

Vegetational

and climatic setting

We first summarize the present-day altitudinal vegetation zonation of the Eastern Cordillera of Colombia. The altitudinal vegetation zones of the Eastern Cordillera have been described by Cuatrecasas (1958, 1989) Espinal and Montenegro (1963) Van der Hammen and Gonzalez (1960a,b. 1963) Cleef (1981) and Cleef and Hooghiemstra (1984). In this study, the altitudinal zonation model for tropical mountains provided by Grubb (1974) has been used, but here adapted to the Colombian situation as proposed by Cuatrecasas (1958) and Van der Hammen and Gonzalez (1960a,b). The altitudinal distribution of the vegetation belts is largely determined by the mean annual temperature and, to a lesser extent, by the mean annual precipitation. From the tropical lowlands to the

H HOOGHIEMSTRAANU7

VAN DER HAMMEN

:;;.::;.: >.:: .yy :::.> :.g .:.:. .:.:. lil z_: : ::::: ::::. ..Y

5:;

TV

LATE QUATERNARY

VEGETATION

HISTORY

AND

PALEOECOLOGY

nival zone the following vegetation recognized (Fig. 3). Lowland tropical vegetation belt (0-ea. alt.)

OF LAGUNA

belts can be

1000 m

The lower tropical vegetation belt in the lower Magdalena Valley receives 1500-4000 mm precipitation and mean annual temperature is 21-30°C. Important arboreal taxa in this humid lower tropical forest include Apocynaceae, Bombacaceae, Euphorbiaceae (e.g. Acalypha, Amanoa, Croton), Humiriaceae (e.g. Humiriastrum, Saccoglottis, Vantanea), Leguminosae (e.g. Bauhinia, Znga, Macrolobium), Meliaceae (Cedrela), Moraceae (e.g. Cecropia, Ficus), Myristicaceae (e.g. Virola), Palmae (e.g. Mauritiu, Zriartea), Rubiaceae, Urticaceae (Cecropia), and Guttiferae (Clusia). The species diversity is high and the internal structure of the wet forests is complex, consisting of several strata, and is rich in epiphytes (orchids, bromeliades and pteridophytes). The lower tropical vegetation zone, along the eastern slopes of the Magdalena valley west to southwest of the high plain of Bogota contains dry tropical forests and woodlands. The average annual precipitation is 500- 1500 mm. Prominent taxa in this more open type of vegetation include Acalypha, Anthurium, Astronium, Bursera, Clusia, Croton, Eugenia, Erythrina, Fagara, Guazuma, Hirtella, Miconia, Mimosa, Pithecellobium, Randia, Scheelea, Solanum, Spondias, and representatives

of the Acanthaceae,

Cactaceae and Moraceae.

Subandean forest belt (ca. IOOO-23?0/2.500 m alt.)

Annual precipitation in the humid subandean forest belt is 1500-2700 mm and the mean annual temperature ranges between 19 and 23°C for the lower zones, and between 14 and 19°C for the higher ones. Taxa common in the subandean forest belt are Billia, Cecropia, Croton, Ficus, Znga, Turpinia, Malpighiaceae, and Meliaceae (Cleef and Hooghiemstra, 1984). Other common arboreal Clusia, taxa include Acalypha, Alchornea, Hedyosmum, Vismia, Hieronima, Morus, Cassia, Cleome, Miconia,

Ladenbergia, Tournefortia,

PEDRO

239

PALO

Sapotaceae, Bombacaceae, Weinmannia, Melastomataceae, Palmae, and Rubiaceae. At the upper limit (ca. 2000-2300 m alt.) a Clusia-Hieronima- Weinmannia forest may be present. Floristically, two types of subandean forests can be distinguished: an altitudinal lower forest with Znga, Cecropia, Vismia, and Hieronima, and an upper Quercus forest (2050-2300 m) with Znga and Cecropia. Around the Pedro Palo lake at ca. 2000 m alt., remnants of two types of forest exist, one with dominant Quercus, and the other a mixed Lauraceae-forest. The first type seems to prefer the higher sites on sandstone subsoil. Andean forest belt (2300/2500-3200/3.500

m alt.)

The Andean forest belt extends to the upper forest line. The average temperature ranges from 16 to 9°C the annual precipitation is 700-1400 mm in the dry interandean high plains and adjacent valleys, and 1000-3000 mm on the outer slopes of the Eastern Cordillera. Common arboreal taxa of the Andean forest belt are Alnus, Araliaceae (e.g. Hedyosmum, Clethra, Clusia, Oreopanax), Melastomataceae (e.g. Hesperomeles, Zlex, Miconia, Tibouchina), Myrica, Myrtaceae (e.g. Ugni, Eugenia), Loranthaceae (Gaiudendron), Symplocos, Rapanea, Podocarpus, Quercus, Viburnum, Vallea, Weinmannia, Xylosma, Saurauia, Duranta, Smilax, Dodonaea, Cordia, Phyllanthus, and Drimys. On the western slopes of the Eastern

Cordillera, west of the high plain of Bogota, oakforests with dominant Quercus humboldtii may be present from ca. 2000 to 3000 m alt. Although the altitudinal forest limit is mainly determined by temperature, it often lies at a slightly higher elevations on exposed wetter, outer slopes of the mountains than on the drier inner slopes, although under extremely wet conditions the reverse may occur. Subparamo vegetation zone (3200/3400-3500/3700 m alt.)

The average temperature of the subparamo zone ranges from 6 to 9°C. The annual precipitation is 700-2500 mm. On the basis of physiognomy and floristics a lower subparamo, dominated by dwarf trees and shrubs, and an upper subparamo, domi-

240

H HOOtiHIEMSTRA

nated by dwarf shrubs, may be recognized (Cleef, 1981). Common taxa in the lower subparamo include Escallonia, Hypericum, Myrica, Symplocos, and representatives of the Compositae (e.g. Ageratina, Diplostephium, Gynoxys. Senecio), Ericaceae (e.g. Gaultheria, Macleania, Vaccinium), Melastomataceae (e.g. Bucquetia, Miconia), and Rosaceae (e.g. Acaena, Hesperomeles, Polylepis). Several of these taxa, including Compositae, Ericaceae, Melastomataceae, Hypericum, Myrica, and Symplocos, may also be abundant in the uppermost zone of the Andean forest belt. Woody taxa from the upper dwarf shrub subparamo include Aragoa, Arcytophyllum, Diplostephium. and Hylpericum. Grassparamo vegetation zone (3500/3700-4200/4300 m alt.) The average annual temperature of the grassparamo zone ranges from 6 to 3°C and the annual precipitation is 700-2500 mm. The grassparamo vegetation zone contains graminoid communities associated with stemrosettes of the genera Espeletia and Espeletiopsis. Calamagrostis efiisa is the most prominent paramo bunchgrass. In the climatologitally humid paramos this species may be partly or entirely replaced by Chusquea tessellata bamboo. Other characteristic herbaceous paramo genera include Gentiana, Geranium, Plantago, Valeriana, of the Caryophyllaceae and representatives Cruciferae Arenaria, Stellaria), (Cerastium, and Ranunculaceae (Ranunculus), (Draba), Scrophulariaceae (Castilleja). The azonal vegetation with special reference to lake zonation was presented in Cleef and Hooghiemstra (1984). Lakes in the Andean forest and especially in the paramo belt may often support IsoPtes, Azolla, Ludwigia, Potamogeton, and Mvriophyllum, whereas Polygonum, Typha and Carex reedswamp (e.g. C. pichinchensis in the paramo belt, C. jamesonii near the upper forest line and C. acutata in the Andean forest belt) may occur in the marshy shore zones. Material and methods Laguna maximum

Pedro Palo has an elongated shape with and minimum diameters of ea. 250 and

AND

f

VAN DER HAMMEN

100 m. Cores Pedro Palo III (525 cm core length) and Pedro Palo V (575 cm core length) were collected at the northern rim of the lake where the bottom has a low gradient (Fig. 2). Core Pedro Palo II (185 cm core length) was collected at ca. 180 m distance from core Pedro Palo V in the northeast corner of the lake. The cores were handdrilled with a Dachnowsky sampler. Radiocarbon dates from organic-rich horizons are listed in Table I. All cores were sampled for pollen analysis at 12.5 cm intervals. Absolute pollen counts are not available. The ljthological sequences of the cores are provided on the main pollen diagrams. Sediment samples of ca. 1.5 cm3 were prepared by the standard acetolysis method.The identified pollen taxa have been grouped according to their altitudinal and ecological affinity (Table II), representing the following main vegetation belts:

(1) Trees

and shrubs of the subandean and Andean forest belts. (2) Compositae, that are abundant in the subparamo scrub vegetation zone. that dominate the grassparamo (3) Gramineae, vegetation zone. group: Polylepis forest (4) PolylepislAcaena occurs from near the upper forest line at 2900.-3300 m up to the lower limit of the superparamo at 4200.--4300 m alt. as an element of the highest Andean (dwarf) forest and as isolated patches in the grassparamo. The pollen analysis of the samples was carried out by Ries Sieswerda. Pollen grains of elements belonging to the subandean forest belt, Andean forest belt, subparamo belt and grassparamo belt were included in the pollen sum. A pollen sum of 200.--300 grains was aimed at, but for several samples this pollen sum could not be reached (see diagrams). Pollen zone boundaries were placed at horizons characterized by a maximum change in the composition and proportions of the pollen spectra. Correlation of the three pollen records has been based on the 4 radiocarbon-dated core intervals and palynostratigraphy. Equal numbered pollen zones are considered to be more or less time equivalent.

LATE QUATERNARY

VEGETATION

HISTORY

AND

PALEOECOLOGY

OF LAGUNA

PEDRO

241

PALO

TABLE I Radiocarbon dates from organic-rich horizons Core Pedro Palo III 180-200 cm core interval 380-400 cm core interval

(lake sediment) (lake sediment)

GrN-6758 GrN-6759

(Co1 193a) (Co1 194c)

11,380+ 130 yr BP 11,950~100 yr BP

Core Pedro Palo V 120- 135 cm core interval 270-285 cm core interval

(dark peaty lake sediment) (dark peaty lake sediment)

GrN-6733 GrN-6734

(Co1 190) (Co1 191)

10,280 f 90 yr BP 10,380 + 90 yr BP

Results

Hedyosmum (12-15%), Weinmannia Myrica (l-2%), Gramineae (52-42%), Compositae (12-9%), Hypericum (3-5%), Pediastrum (52-70%) and Botryococcus (112-300%), and relative low percentages of Alnus (4-6%), Juglans (448%) and Eugenia (O-2%).

(g-14%), (3-6%),

Concise description of pollen record Pedro Palo III (Fig. 4)

In the pollen record of the 525 cm long core Pedro Palo III five pollen zones (la, lb, lc, 2, and 4-5, respectively; the zonation scheme is explained below) can be recognized: Pollen zone la (525-495

cm core interval)

Pollen zone la is characterized by relatively high percentages of Gramineae (60-65%), Compositae (12%), Hypericum (3-4%) and Alnus (9-13%) and relatively low percentages of Quercus (4-6%), Hedyosmum (5-6%) and Miconia (2%). Weinmannia, Alchornea and Acalypha are virtually absent. Algae and fungal remains are present but with relatively low percentages. Pollen zone lb (495-445

cm core interval)

Pollen zone 1b is characterized by relatively high percentages of Hedyosmum (8-13%), Weinmannia (2-4%),

Zlex

(l-2%),

Miconia

(7-lo%),

Gramineae (44-60%), Compositae (9-12%), Ericaceae (2-4%) and Eugenia (l-2%) and relatively low percentages of Alnus (5-9’/), Quercus (4-12%) and Pilea (O-l %). Compared to the previous pollen zone, pollen zone lb is characterized by an increase of Quercus, Hedyosmum, Weinmannia,

Miconia,

Alchornea,

Weinmannia, Hypericum, Botryococcus,

Myrica,

Gaiadendron,

Alchornea,

Urticaceae, Pediastrum and and a decrease of Alnus, Miconia, Ericaceae and Eugenia. Gramineae first increase and than show a final decrease. Pollen zone 2 (395-l 70 cm core interval)

Pollen zone 2 is characterized by relatively high percentages of Alnus (lo-28%), Quercus (13-23%), Hedyosmum (15-28%) Weinmannia (3-7%) Rapanea (1.5-3%), Gaiadendron (l-4%), Miconia (3-9%), Compositae (5-lo%), Hypericum (2.5-g%), Myriophyllum (l-8%), and algae, and relatively low percentages of Alchornea (l-3%), Acalypha (l-3%), Cecropia (l-9%), Gramineae (30-8%), Urticaceae (l-5%), Pilea (l-5%) and

fungal remains. Compared to the previous pollen zone, pollen zone 2 is characterized by an increase of Alnus, Quercus, Hedyosmum, Rapanea, Hypericum, Urticaceae, Pilea, Myriophyllum and Potamogeton, and a decrease of Gramineae and Compositae.

Acalypha,

Urticaceae, Eugenia, Botryococcus and fungal spores, and a decrease of Alnus and Gramineae. Pollen zone Ic (445-395

Compared to the previous pollen zone, pollen zone lc is characterized by an increase of Quercus,

cm core interval)

Pollen zone lc is characterized by relatively high percentages of Podocarpus (2-3%) Quercus

Pollen zone 4-5 (170-O cm core interval)

Pollen zone 4-5 is characterized by relatively high percentages of Quercus (lo-42%), Weinmannia (4-14%), Juglans (l-10%), Alchornea (4-20%), Acalypha ([l] 5-l 1%) Cecropia (lo-58%), Urticaceae (6-18%), Pilea (4-28%),

242 TABLE

H. HOOGHIEMSTRA

AND

T VAN DER HAMMEN

II

Identified

pollen taxa in Laguna

SUBANDEAN FOREST *Acalyphu *Alchornea Anacardiaceae Billia colombiana *Cecropia Clusia-type Croton Hieronima Leguminosae Malpighiaceae Pilea Urticaceae Solanaceae Ericaceae

Pedro Palo sections

grouped

ELEMENTS

ANDEAN FOREST ELEMENTS +AlnUS Amaranthaceae *Bocconia Brunellio Borreria Chenopodiaceae Coriaria Dodonaea Drimys Euphorbiu Fuchsia ‘Gaiadendron Galium * Hedyosmum Heliocarpus *II&X * Juglans Labiatae Lythraceae *Myricu * Miconia *Melastomataceae s.1. *Malvaceae *Podocarpus Panopsis Polygalaceae Portulacaceae *QuercuS * Rapunea Relbunium * = included

according

to altitudinal

and ecological

ANDEAN FOREST Ranunculaceae Sapium *Symploco.~ *Sfyloceras Scrophulariaceae Thalictrum * Viburnum * Weinmannia

affinity

ELEMENTS

(continued)

TIMBERLINE FOREST *Polvlepis/Acaena SUBPARAMO *Compositae Hypericum

SCRUB

GRASSPARAMO Caryophyhaceae Cruciferae *Gramineae Geranium Pluntugo AQUATICS Cyperaceae Hydrocotyle IsoPte.7 Myriophyilum Polygonurn Potumogeion ALGAE Botryococcu.t Pediastrum PTERIDOPHYTES Monoletes verrucate Monoletes psilate Triletes verrucate Triletes divers Lycopodium reticulate Lycopodium foveolate Cyatheaceae FUNGAL REMAINS Fungal spores _

in the pollen sum.

Cyperaceae (7-55 [97]%) and fungal spores ([45] 200- 1500 [2900]%), and relatively low percentages of Podocarpus (2-O%), Alnus (5-O%), Hedyosmum Gramineae (20-4%), Compositae (1 l-2%),

(2 l- 1%), Botryococcus ([500] 50-2%) and pteridophyte spores. Compared to the previous pollen zone, pollen zone 4-5 is characterized by an increase of Quercus. Weinmannia, Juglans,

pp.243-245

LATE QUATERNARY VEGETATION HISTORY AND PALEOECOLOGY OF LAGUNA PEDRO PALO

LAGUNA

DE

PEDRO PALO III

, . - - - - - - - - - M A I N ANDEAN FOREST ELEMENTS

ALT. ca. 2000 m ABOVE SEALEVEL ANAL: Ries Sieswerda

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Fig. 4. Pollen percentage diagram Laguna de Pedro Palo III (525 cm core length) at 2000 malt. in the Eastern Cordillera of Colombia. At ca. 170 cm core depth a possible disconformity is indicated.

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pp.246-248

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Fig. 5. Pollen percentage diagram Laguna de Pedro Palo V (575 cm core length) at 2000 m all. in the Eastern Cordillera of Colombia. At ca. 20 cm core depth a possible disconformity is indicated.

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pp. 252-254

LAGUNA

Fig, 7. Correlation (see explanation

PEDRO

of the pollen in the text).

PALO

III

records

Laguna

de Pedro

LAGUNA

PEDRO

PALO

V

'OL ZOI

15

-

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-

?lb

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Palo V, III and II (Eastern

Cordillera,

Colombia,

2000 m ait.)

H. HOOGHIEMSTRA

LAGUNA EASTERN

AND T. VAN DER HAMMEN

PEDRO

CORDILLERA,

PALO

II

COLOMBIA

(2000 M ALT.)

POLLEN ~,RAT,ZONES GR*‘PHY

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based

on 4 radiocarbon

dates

and palynostratigraphy

LATE QUATERNARY

VEGETATION

HISTORY

AND PALEOECOLOGY

OF LACUNA

Alchornea, Acalypha, Cecropia, Urticaceae, Pilea, Sapium, Cyperaceae, and fungal spores, and a decrease of Podocarpus, Alnus, Hedyosmum, Miconia, Gramineae, Compositae, Hypericum, Ericaceae, Botryococcus and pteridophyte spores. Concise description of pollen record Pedro Palo V (Fig. 5)

PEDRO

255

PALO

Quercus (1 l-16%) and of Compared to the previous pollen zone, pollen zone 3b is characterized by an increase of Gaiadendron, Miconia, Alchornea, Compositae, Ericaceae, Solanaceae, Clusia-type, Cyperaceae, Hydrocotyle, and fungal remains, and a decrease of Alnus, Quercus and Botryococcus.

percentages

Botryococcus.

Pollen zone 4 (195-95

In the pollen record of the 575 cm long core Pedro Palo V five pollen zones (2, 3a, 3b, 4 and 5, respectively) can be recognized. Pollen zone 2 (575-480

cm core interval)

Pollen zone 2 is characterized by relatively high of Alnus (35-52%), Quercus percentages (12-30%),

Hedyosmum (7-13%),

Miconia (2-7%),

Gramineae (7-14%), Compositae (3-8%), Cyperaceae (lo-27%), and algae, and relatively low percentages of Alchornea (O-2%), Acalypha (0.5-2%), Cecropia (0- 1%), Urticaceae (0- 1 %), Solanaceae (2-3%) Clusia-type (O-2%) and fungal remains. Pollen zone 3a (480-29.5 cm core interval)

Pollen zone 3a is characterized by relatively high percentages of Alnus (29-63%), Quercus (lo-22%),

Hedyosmum (3-13%),

Miconia (2-7%)

Gramineae

(5-17%), Compositae (2-7%), Hypercium (l-7%), Cyperaceae (6-24%), and relatively low percentages of algae, and Weinmannia (l-5%), Alchornea (O-2%), Acalypha (O-6%), Cecropia (O-l %), Urticaceae (O-3%), Solanaceae (0-2[6] %), Clusia-type (O-3%), and fungal remains, and the absence of Brunellia. Compared to the previous pollen zone, pollen zone 3a is characterized by an increase of Alnus and algae and a decrease of Quercus and Solanaceae. Pollen zone 3b (295-195

cm core interval)

Pollen zone 3b is characterized by relatively high percentages of Alnus (20-49%), Hedyosmum (2-16%), (O-3%),

Weinmannia (l-12%), Gaiadendron Miconia (5- 13%) Viburnum (O-4%),

Gramineae (4-19%), Compositae (5-16%) Ericaceae (l-6%), Solanaceae (3-9%), Clusia-type (l-l I%), Cyperaceae (5-55%), Hydrocotyle (1- 15%), and fungal remains, and relatively low

cm core interval)

Pollen zone 4 is characterized by relatively high percentages of Quercus (5-19%), Weinmannia (2-12%),

Juglans

(l-8%),

Cecropia

(30-70%),

and Urticaceae (2-13%), and relatively low percentages of Alnus (3- 14%) Hedyosmum (I- 11%) Miconia (l-8%), Alchornea (2-20%) Gramineae (l-7%), and Compositae (3-7%). Compared to the previous pollen zone, pollen zone 4 is characterized by an increase of Juglans, Cecropia and Urticaceae, and a decrease of Alnus, Hedyosmum, Miconia, Gramineae, Compositae, Ericaceae, Clusia-type and Cyperaceae. Pollen zone 5 (95-O cm core interval)

Pollen zone 5 is characterized by relatively high percentages of Zlex (l[O]-18%) Miconia (l-12%), Alchornea (15-92%), Acalypha (1- 17%) Brunellia (2-g%), and fungal remains, and relatively low percentages of Alnus (O-2%), Hedyosmum (l-4%), Cecropia (2-16%), and Gramineae (l-3%). Compared to the previous pollen zone, pollen zone 5 is characterized by an increase of ZZex, Miconia, Alchornea, Acalypha, Brunellia, and fungal remains, and a decrease of Alnus, Cecropia, Gramineae and Solanaceae. Concise description of pollen recordPedro (Fig. 6)

Palo ZZ

In the pollen record of the 185 cm long core Pedro Palo II three pollen zones (6a, 6b and 6c, respectively) can be recognized. Pollen zone 6a (185-I 70 cm core interval)

Pollen zone 6a is characterized by relatively high of Juglans (2-7%), Alchornea percentages (lo-17%) Gramineae (25-46%), Compositae (20-30%), Chenopodiaceae (1O-O%), and Cyperaceae (9-30%), and low percentages of

256

Miconiu

(O-S%),

H. HOOGHIEMSTRA

(2-8’~) Urticaceae

Acalypha

(O-3%)

Pollen zone 6h ( 170-55

(l-2%), Cecropia and Pilea (I %).

cm core inter&)

Pollen zone 6b is characterized by relatively high percentages of Quercus (7- 18%), Hedyosmum (2-7%),

Weinmannia

Acal_ypha

(2- 12%),

(l-8%),

Miconia

Cecropia

(12-27%), (12-41%),

Urticaceae (3-9%), and Pilea (l-7%), and relatively low percentages of Gramineae (2-l 5%, with a peak of 47%), Compositae (4-17%). Rumex (O-4%), and Cyperaceae (l-17%). Compared to the previous pollen zone, pollen zone 6b is characterized by an increase of Quercus, Miconia, Acalypha, Cecropiu, Urticaceae, and Pileu, and a decrease of Alchornea, Gramineae, Compositae, and Cyperaceae. Pollen zone 6c (55-O cm core inter&)

Pollen zone 6c is characterized by relatively high percentages of Gramineae (I 5-7 1%), Compositae (IO-22%), Rumex (R. acetocella-type) (8&20%), Cruciferae (2--8%) Cyperaceae (lo-61%) and H_vdrocotyle (l-6%), and relatively low percentages of Miconiu (2.558%), Acalypha (1.5-S%), Cecropia (1.5S25%), and Urticaceae (l-5%). Compared to the previous pollen zone, pollen zone 6c is characterized by an increase of Myricu, Gramineae, Compositae, Rumes, Rapanea, Cruciferae, Cyperaceae, and Hydrocotyle, a Juglans, Miconia, of Hedyosmum, decrease Cecropiu. Urticaceae, and Pilea, and the total disappearance of Alnus. Correlation

qf’the pollen records

On the basis of 4 radiocarbon datings of sections Pedro Palo III and Pedro Palo V and the recognized pollen zones a correlation of the three pollen records is presented in Fig. 7. Core Pedro Palo III shows in the interval of 5255495 cm core depth (pollen zone 1) cold conditions which date from before ca. 11,950 i: 100 yr BP (base of pollen zone 2; see Figs. 4, 7). Apparently this interval is of early Late Glacial age and represents zone I of the general Colombian Cordilleran pollen zonation (Van der Hammen and Gonzalez, 1965). A transitional phase to

AND

T VAN

DER HAMMtN

warming climatic conditions at 495-l 70 cm core depth (pollen zone 3) is radiocarbon-dated ca. 11,950-l 1,300 yr BP. It corresponds in time (and in the warmer conditions) to the Guantiva interstadial (zone II of the general Colombian Cordilleran pollen zonation). The upper 170 cm of core Pedro Palo III shows the presence of subandean forest and represents Holocene conditions. The 175-l 81 cm level shows a sharp lithological discontinuity and apparently represents a hiatus: the presence of wood fragments in volcanic ash and lowered water-level conditions (see below) point to a pos;ible phase of non-deposition or erosion. Core Pedro Palo V starts its record later: Andean pollen zone I is not represented. The pollen spectra of the core interval 575 480 cm resemble very much the top of pollen zone 2 of core Pedro Palo III. Apparently, a low water level in Laguna Pedro Palo gave rise to erosion at the site of section V. resulting in a loss of the oldest part of the pollen record. The interval of 480- 195 cm is interpreted as representing the colder El Abra stadial (zone III of the general Colombian Cordilleran pollen zonation). The radiocarbon date at ca. 275 cm core depth of 10,380+90 yr BP fits well into the established climate chronology for this area (e.g. Van der Hammen and Gonzalez, 1965; Van Gee1 and Van der Hammen, 1973; Van der Hammen. 1974; Schreve-Brinkman, 1978; Kuhry. 1988). Pollen zone 4 shows a rapid increase of subandean Cecropiu forest which is followed by a replacement by subandean Alchornea forest in pollen zone 5. Both pollen zones clearly reflect Holocene conditions which are, however. difficult to correlate with certainty with the Holocene part of core Pedro Palo III. The radiocarbon date of 10,280 +90 yr BP in pollen zone 4 seems to be slightly too old (100-300 yr: possibly due to carbon contamination) as compared to the date at 275 cm and as compared to the general climate chronology of this area. Taking into account the statistical errors. it almost fits at the base of the Holocene. Core Pedro Palo II shows the effects of anthropogenic disturbance of the local forests and should represent Late Holocene conditions, as explained in the next section. Based on the above mentioned correlations of

LATE QUATERNARY

VEGETATION

HISTORY

AND

PALEOECOLOGY

OF LACUNA

dated pollen records we arrive at the following ages for the recognized pollen zones: Pollen zone 1 (a-b-c) : early Late Glacial (zone I) interstadial : Guantiva Pollen zone 2 (zone II) ca. 12,000- 11,000 yr BP : El Abra stadial (zone III) Pollen zone 3 (a-b) ca. ll,OOO-ca. 10,200 (or ca. 10,000) yr BP : Earliest Holocene Pollen zone 4 : Early (?) Holocene Pollen zone 5 Pollen zone 6 (a-b-c) : Late Holocene. Upper Quaternary vegetation and climatic sequence Pollen zone 1 (Pedro Palo III) - ca. 13,000-12,000 yr BP

About 50-70% of the pollen of open paramo vegetation types indicates the presence of shrubparamo (with Compositae, Hypericum and Ericaceae) and grassparamo (ca. 40-60% Gramineae pollen) in the vicinity of Laguna Pedro Palo. Anus, a tree that never occurs above the forest line is represented by ca. lo-12% only, a value that is attributed to a background extraregional effect and indicative of an altitudinal position of the upper Andean forest line below the elevation of Laguna Pedro Palo. Quercus, another wind-pollinator, is represented initially by very low values, and Weinmannia is, initially, virtually absent. Later do the percentages of these and other elements, such as Hedyosmum, Myrica and Viburnum, increase somewhat, as do several elements of the Xylosma-Durantha- Vallea forest, such as Ilex, Rapanea, Solanum, Miconia and Viburnum. The lake is situated on a saddle of a west-dipping slope to the lower tropical vegetation belt. Only some 25% of tree pollen that belong almost exclusively to wind-pollinated taxa (long distance transport) and Gramineae and Compositae accounting for 75% of the pollen are found, suggesting that no or almost no Andean forest (or subandean forest) was present on these slopes. At present dry forest and open xerophytic vegetation are found on these lower slopes and in the Magdalena Valley. It seems likely therefore

PEDRO

PALO

257

that the early pollen zone 1 data have to be interpreted as indicating the local absence of a proper Andean forest belt and that the tropical to subandean (semi-)open dry vegetation was locally in contact with dry paramo vegetation. Such a situation exists today under dry climatic conditions, e.g. in certain parts of the Chicamocha valley (Van der Hammen, 1981). Farther north along wetter western slopes there may have been a zone of forest between the two vegetation belts, with forest all the way down to the elevation of the Magdalena river. In that case, the early Late Glacial upper forest limit may have been at ca. 1800 m alt. (Kuhry, 1988). In Laguna Pedro Palo open water conditions with Pediastrum and Botryococcus, submerged Zsot;tes, and swamp vegetation that included Cyperaceae and Hydrocotyle were present. Pollen zone la reflects the maximum expansion of open scrub vegetation and grassparamo. Pollen zone 1b shows a greater variety of paramo herbs, such as Caryophyllaceae and Geranium and an increasing amount of Andean forest taxa, that indicate the formation of montane forest on the slopes below the lake and patches of Andean forest possibly reached closer to Laguna Pedro Palo. An increase in aquatic elements (Zsot;tes) and algae seems to point to a rising water level and to less dry climatic conditions. During pollen zone lc, we note an increase in the contribution of several Andean forest taxa and at the transition of zone lc to zone 2 open vegetation was replaced by Andean forest, reflecting the beginning of an amelioration of the climate. Pollen zone 2 (cores Pedro Palo III and V) - ca. 12,000-11,300 yr BP

During the period of pollen zone 2 the upper forest line probably shifted upslope to ca. 2800 m alt. or more and the Pedro Palo lake was situated in the Andean forest belt. This period is coeval with the Guantiva interstadial which represents the most distinct climatic warming of the Late Glacial (e.g. Van der Hammen and Gonzalez, 1965; Van Gee1 and Van der Hammen, 1973; Schreve-Brinkman, 1978; etc.). Alnus was locally present and may have formed carr vegetation on

258

H HOO(-;HIEMS'lKAAND I VAN DtKHAMMtN

poorly drained parts of the basin but may also have been abundant in the forest on the surrounding slopes. QUUCUS forest with stands of Hedyosmum and Viburnum was present. Weinmannia forest, including such elements as Miconia, Eugenia, Rapanea, Myrica, Gaiadendron. and Symplocos, was also present in the region. The pioneer shrub Dodonaea, which today is abundant on eroded soils, was present during the interval of rapid upward shift of the upper forest line. Polylepis apparently did not play an important part in these forests. Compositae and Hypericum still attain moderate percentages in pollen zone 2. Compositae in particular were an important constituent of the forest. Elements of the subandean forest belt (Alchornea, Acalypha and Cecropia) attain ca. 5-8 %, indicating that the uppermost limit of the subandean forest belt may have reached up to ca. 1800 m alt. Locally Afnus carr and open with Cyperaceae swamp vegetation and and M.vriophyllum, Hydrocotyle occurred, Potumogeton and Isoetes occurred in somewhat especially Botrvococcus, deeper water. Algae, reached high values. Pollen zone 3a (core Pedro Palo V) - cu. 11,000-10,400 vr BP About 11,000 to ca. 10,400 yr BP, Andean forest elements such as Quercus and Weinmanniu, but also subandean forest elements, such as Alchornea and Acalypha, diminished somewhat in representation suggesting a lowering of the upper forest line by ca. 300 m; in the upper part there is a increase in Gramineae. A cooling of the climate is inferred and this period coincides with the first part of the El Abra stadial. High representation of algae, such as Botryococcus and Pediastrum, absence of Zsoetes, which is indicative of relatively deep water conditions, and an increase of Alnus carr may be indicative of a low lake level during the first part of the El Abra stadial. Pollen zone 3b (core Pedro Palo V) - cu. 10,400~~10,150 yr BP About elements

10,380 yr BP, an increase of paramo including Gramineae, Compositae, and

Ericaceae, and a marked decrease of Quercus indicates a more markedly cooler period. The upper forest line possibly dropped an additional 200&300 m in elevation. Lower percentages of the subandean forest elements are hardly noticed (the total contribution stays at ca. 10%) and no conclusions concerning shifts of the upper limit of subandean forest are inferred. Changes in the Andean forest belt are reflected by the presence of the pioneer Dodonueu and by the increase of Gaiadendron and Viburnum, and especially the abundant representation of Clusia. The extension of Alnus swamp diminished and cyperaceous swamp with Hydrocotyle increased around Laguna Pedro Palo. Increased amounts of fungal remains indicate the local presence of decomposing peat: the lake was apparently filled with sediments up to the water level of that time. At the end of pollen zone 3b Alnus carr disappeared from Laguna Pedro Palo (percentages below 10%) and open water with a swamp zone at the shallow parts of the lake remained. Pollen zone 3b includes a radiocarbon date of 10,380+90 yr BP which is within the age range of the El Abra stadial. Pollen zone 3b, as a whole, corresponds to the El Abra stadial, reflecting a temperature depression that in time is approximately equivalent to the Younger Dryas event.

Pollen zone 4 (core Pedro Palo V and (.?I III) - cu. 10,150~Early Holocene Pollen zone 4 is characterized by a massive increase of subandean forest elements, especially Palo III, also Cecropiu and, in core Pedro Acul_vpha. Laguna Pedro Palo was now situated in the subandean forest belt: total arboreal percentages reached 90-95%. It is not possible to estimate the altitudinal position of the upper Andean forest line on the basis of the data from Pedro Palo only. Taxa of the subandean forest belt are represented by ca. 65%, suggesting that its upper limit lay at about 2400 m. Quercus and Weinmannia must have been prominent in the Andean forest belt, but they may also have occurred in the upper subandean belt. Open water with a zone of cyperaceous reed swamp occurred in the lake. Pollen zone 4 represents the beginning of the Holocene.

LATE QUATERNARY

VEGETATION

HISTORY

AND

PALEOECOLOGY

OF LAGUNA

Pollen zone 5 (core Pedro Palo V and [?I III) Early to Middle Holocene

A distinct change in the composition of the subandean forest elements took place at the transition of pollen zone 4 to 5: Cecropia-dominated forest (with important amounts of Urticaceae, Pilea, and Acalypha in core Pedro Palo III) was replaced by mainly Alchornea-dominated forest (core Pedro Palo V). Quercus and Weinmannia were prominent and Ilex, Miconia and Brunellia became increasingly common elements, probably especially in the Andean forest belt. Locally, cyperaceous reed swamp occurred. High amounts of fungal remains are indicative of decomposition of the (silty) peat bog. Pollen zone 5 probably represent Early to Middle Holocene conditions. Pollen zone 6a-c (core Pedro Palo II) - Late Holocene

PEDRO

PALO

259

aquatic and swamp elements, a low representation of algae, and a high representation of fungal remains point to low lake level conditions and a considerable degree of decomposition of peaty material in the lake basin. Pollen zone 6c shows a dramatic decrease in the contribution of arboreal elements, especially of Cecropia and Miconia. Oak forest is apparently present in the region, which is corroborated by the present-day natural situation around Laguna Pedro Palo (Rivera, 1983). High representation of Gramineae and Rumex (R. acetosella) is indicative of a post-conquest age for this interval. R. acetosella was introduced with the Spanish conquest around 1500 AD (Van der Hammen, 1962). The pollen diagram indicates the local presence of cyperaceous reed swamp (with aquatics and algae), a situation which closely resembles the presentday situation. Discussion

Pollen zone 6 of core Pedro Palo II should be of Late Holocene age, as shown by important human disturbance of the forest. According to Van der Hammen and Correal Urrego (1978) shortly after 2500 BP the first signs of agricultural activities are found in the area of the high plain of Bogota. Pollen zone 6a shows a fair representation of subandean forest elements (Alchornea) and many elements of the Andean (to subandean) forest are also registered, such as Quercus, Hedyosmum, Weinmannia, Juglans and Miconia. A high representation of Gramineae and Chenopodiaceae possibly indicates the presence of Indian agriculture close to the lake. The upper subandean forest limit possibly did not differ much from that of the present-day. Open water (algae) with cyperaceous reedswamp is present in the basin. Pollen zone 6b shows a dramatic increase of Cecropia but also of Acalypha, Miconia, Urticaceae, and Pilea. Such an important change in the composition of the upper subandean and Andean forest belt is not expected on the base of possible changes in climatic conditions. More probably we are dealing with a successional stage in the regeneration of disturbed forest, in which Cecropia, with its pioneer qualities, could have played an important part. A near-absence of

The three pollen records of Laguna Pedro Palo show important vegetational changes at a key-site in the paleoecological reconstruction of the Eastern Cordillera for the last ca. 13,000 years. Four radiocarbon-dated horizons provide time control but several hiatuses are apparently present (Fig. 7). The 170 cm level of core Pedro Palo III apparently represents such a hiatus-horizon. Lowering of the lake-level during drier conditions possibly gave rise to erosion; the upper 170 cm of core Pedro Palo III is, because of its high percentages of subandean forest elements, almost certainly of Holocene age. The lake was probably filled with sediments along the shores and changes in lakelevel conditions may have caused local erosion of sediments and redeposition. The base of core Pedro Palo V seems to correlate perfectly with the top of pollen zone 2 of core Pedro Palo III. This could be explained by a lowering of the lake level of some 4 m towards the end of the Guantiva interstadial and the early El Abra stadial, and by local erosion of the exposed lake sediments by the small stream that entered the lake near to site Pedro Palo V (see Fig. 1). Core Pedro Palo V lost the older part of the record, possibly due to erosion under low lake-level conditions, and sedimentation

260

stopped at site Pedro Palo III. Pollen zone 2 corresponds in time to the Guantiva interstadial [first reported by Van der Hammen and Gonzalez (1960a,b, 1965) and named by Van Gee1 and Van der Hammen (1973)], which is dated in the Pedro Palo records at ca. 12,000-l 1,000 yr BP. Pollen zone 3 shows colder conditions and includes a radiocarbon age of 10,380*90 yr BP. This pollen zone corresponds to the El Abra stadial (first reported by Van der Hammen and Gonzalez (1960a,b, 1965) and named by Van Gee1 and Van der Hammen, 1973). Comparing the Pedro Palo pollen record (2000 m alt.) with other important records from this area, such as El Abra (Sabana de Bogota) at 2570 m alt (Schreve-Brinkman, 1978) Laguna de Fuquene at 2580 m alt. (Van Gee1 and Van der Hammen, 1973) and Paramo de Palacio at 3550 m alt. (Van der Hammen and Gonzalez, 1960a.b) the temperature decrease suggested by the vegetational change in pollen zone 3 is not very marked. This, however, seems to be caused by the fact that the upper forest limit was, after 12,000 yr BP, well above 2000 m alt. for all the time, while the local alder carr vegetation may have partly veiled the effects of regional change in the pollen record. During the last part of the El Abra stadial the Andean forests became floristically more diversified. This may be due to the gradual development from pioneer forest to climax forest. We interpret the dramatic increase of Cecropiu at the 190 cm level in core Pedro Palo V as the beginning of the Holocene. The radiocarbon date of 10,280+_90 yr BP at ca. 125 cm core depth, therefore, seems slightly too old as most records indicate an age of ca. 10,150 yr BP for the start of the Holocene (or a transitionary phase until 9500 yr BP; Van der Hammen and Gonzalez, 1960a,b, 1965; Van der Hammen, 1974). The sequence of Cecropiu-dominated forest to Alchorna- (and Acalyphu-) dominated forest at the start of the Holocene is also recorded in Laguna de Fuquene (Van Gee1 and Van der Hammen, 1973). The local presence of alder (percentages over 15%) is registered in the basin of Pedro Palo during pollen zones 2 (core Pedro Palo III) and 3 (core Pedro Palo V) representing the interval of ca. 12,000-10,200 yr BP.

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Core Pedro Palo 11 starts erosional phase, and only Holocene age, with evidence been recorded in this core.

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I

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apparently after an sediments of Late of agriculture, have

Summary and conclusions

The vegetation on the western slopes of the central part of the Eastern Cordillera of Colombia has responded to Late Quaternary climatic changes mainly by altitudinal shifts of the vegetation belts. The shifts were caused by changes of temperature and also by changes in precipitation. The upper limits of the Andean forest belt (transition to open paramo vegetation) and subandean forest belt are well traceable features in the pollen record, and important changes in the floristic composition of these forest belts could also be recorded. On the basis of three cores from Laguna Pedro Palo at 2000 m alt.. two of them including four 14C-dated horizons, the following conclusions can be drawn: (1) Pollen zone 1 represents the interval of ca. 13,000-- 12,000 yr BP and possibly corresponds to the La Ciega stadial. The base might correspond to the Pleniglacial -Late Glacial transition. Shrubparamo and grassparamo were present in the vicinity of the lake. At that time the tropical (sub)andean (semi-)open dry vegetation seems locally to have been in contact with dry paramo vegetation and a zone of montane forest seems to have been absent. From pollen zone lb onward patches of proper Andean forest may have been present near the lake. At the end of pollen zone lc a proper Andean forest belt established. Regionally the upper Andean forest line is estimated at an elevation of ca. 1800 m in pollen zones 1a and 1b and ca. 2000.-2100 m in pollen zone lc. Quercus forest and Weinmanniu forest were the main constituents of the Andean forest belt. (2) Pollen zone 2 includes 2 radiocarbon-dated horizons and represents the interval of ca. 12,000& 11,000 yr BP. A distinct warming of climate by ca. 5°C is evidenced and the upper Andean forest line shifted from ca. 2000.-2100 m to ca. 2800 m alt. The upper subandean forest belt reached to ca. 1800 m. This interval correlates with the Guantiva interstadial which correlates in

LATE QUATERNARY

VEGETATION

HISTORY

AND

PALEOECOLOGY

OF LAGUNA

time with the Belling and Allerod interstadials of Europe. In the basin of Pedro Palo an Alnus carr vegetation developed. (3) Pollen zone 3 includes 1 radiocarbon-dated horizon and represents the interval of ca. 1l,OOO-ca. 10,000 yr BP. The upper Andean forest line probably shifted from ca. 2800 to 2600 m (pollen zone 3a) and subsequently to ca. 2300 m (pollen zone 3b), evidencing a temperature decrease of at least ca. 3°C. This interval represents the El Abra stadial which in time approximately corresponds with the Younger Dryas oscillation. Relatively dry climatic conditions seem to prevail. The lake-level lowered, possibly several metres, and the exposed shore sediments were apparently locally eroded by moving water, while the sedimentation stopped at the 175 cm level in core Pedro Palo III. At Pedro Palo V several metres of older sediments may have disappeared. Alnus carr, which is mainly limited to the Andean forest zone, disappeared from the basin at the end of the El Abra stadial. (4) Pollen zone 4 represents the beginning of the Holocene. It includes a radiocarbon-dated horizon of 10,280 f 90 yr BP which is thought to be slightly too old. The upper subandean forest line shifted to ca. 2400-2500 m alt. and Cecropiu-dominated pioneer forest grew on the slopes. Quercus and Weinmannia forest types were probably dominant in the Andean forest belt. (5) Pollen zone 5 is estimated to be of Early to Middle Holocene age and evidences a distinct change in the composition of the subandean forest belt: the pioneer forest of Cecropiu was replaced by a mixed forest with Acalypha and Alchornea as important elements. (6) Pollen zone 6 is of Late Holocene age and witnesses important human disturbance in the forest belt. Pollen zone 6a shows high representations of Gramineae and Chenopodiaceae, pointing to Indian agricultural activity in the surroundings of the lake. Pollen zone 6b seems to represent a successional stage in the regeneration of disturbed forest, The high representation of Gramineae and Rumex in pollen zone 6c indicates a post-conquest age (after ca. 1500 AD) for this interval. (7) The Laguna Pedro Palo pollen record evidences that the upper Andean forest line was

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lowered by ca. 1400 m in the early Late Glacial, corresponding to a temperature depression of maximally ca. 8-9°C. Very dry conditions during the late Pleniglacial to early Late Glacial caused an (almost) absence of a proper Andean forest belt and a dry subandean (semi-)open vegetation must have been locally in contact with dry paramo vegetation. This situation occurs today locally in the interandean Chicamocha valley. The conclusion of a very dry (late) Pleniglacial, a wetter Late Glacial Guantiva interstadial and a drier El Abra stadial is in general agreement with earlier results from the area (e.g. from lake Fuquene). Acknowledgements

The authors thank Ries Sieswerda for the pollen analysis and Prof. W.G. Mook (Groningen University) for the radiocarbon datings. Mrs. Elly Beglinger prepared the pollen samples and Hans Koerts-Meijer prepared the pollen diagrams. Mrs. E. IJssel is thanked for typing the manuscript. Dr. H.J.B. Birks and an anonymous reviewer are thanked for critical comments and improving the English text. References Bakker, J., 1990. Tectonic and Climatic Controls on Late Quatemary Sedimentary Processes in a Neotectonic Intramontane Basin (The Pitalito Basin, South Colombia). Ph. D. thesis. Wageningen Agric. Univ., Wageningen, 160 pp. [also in: The Quatemary of Colombia, Vol. 161. Cleef, A.M., 1981. The Vegetation of the Paramos of the Colombian Cordillera Oriental. Diss. Bot., 61. Cramer, Vaduz [also in: The Quatemary of Colombia, Vol. 91. Cleef, A.M. and Hooghiemstra, H., 1984. Present vegetation of the area of the high plain of Bogota, In: H. Hooghiemstra, Vegetational and Climatic History of the High Plain of Bogota, Colombia: a Continuous Record of the Last 3.5 Million Years. Diss. Bot., 79: 42-66. Cramer, Vaduz [also in: The Quatemary of Colombia, Vol. IO]. Cuatrecasas, J., 1958. Aspectos de la vegetation natural de Colombia. Rev. Acad. Colomb. Cienc. Exact. Fis. Nat. (Bogota), lO(40): 221-264. Cuatrecasas, J., 1989. Aspectos de la vegetacibn natural en Colombia. Perez-Arbelaezia, 2(8): 155-283. Diaz Daza, L., 1983. Apuntos geograficos e historicos de la region de la Laguna de Pedro Palo. Oikos (Bogota), l(1): 7-10. Espinal, L.S. and Montenegro, E., 1963. Formaciones vegetales de Colombia. Mem. Explic. Mapa Ecolbgico, Inst. Geogr. Agustin Codazzi, Bogota, 201 pp.

262 Flenley. J.R., 1979. The Equatorial Rain Forest, A Geological History. Butterworths, London, 162 pp. Grubb, P.J.. 1974. Factors controlling the distribution of forest types on tropical mountains: new facts and a new perspective. In: Altitudinal Zonation in Malesia. 3rd Aberdeen Hull Symp.. Hull. Dept. Geogr., Univ. Hull, Misc. Ser.. 16, pp. 13-45. Helmens, K.F., 1990. NeogeneeQuaternary Geology of the High Plain of Bogota, Eastern Cordillera, Colombia (Stratigraphy, Paleoenvironments and Landscape Evolution). Diss. Bot., 163: 202 pp. Cramer, Berlin [also in: The Quaternary of Colombia, Vol. 171. Helmens, K.F. and Kuhry, P., 1986. Middle and Late Quaternary vegetational and climatic history of the Paramo de Agua Blanca (Eastern Cordillera. Colombia). Palaeogeogr. Palaeoclimatol. Palaeoecol., 56: 291-335 [also in: The Quaternary of Colombia, Vol. 151. Hooghiemstra, H., 1984. Vegetational and Climatic History of the High Plain of Bogota, Colombia: A Continuous Record of the Last 3.5 Million Year. Diss. Bot., 79. 368 pp. Cramer, Vaduz. [also in: The Quaternary of Colombia, Vol. IO]. Hooghiemstra, H., 1989. Quaternary and Upper-Pliocene glaciations and forest development in the tropical Andes: evidence from a long high-resolution pollen record from the sedimentary basin of Bogota, Colombia. Palaeogeogr. Palaeoclimatol. Palaeoecol., 72: 1 l-26. Hooghiemstra, H. and Sarmiento, G.. 1991. Long continental pollen record from a tropical intermontane basin: Late Pliocene and Pleistocene history from a 540-meter core. Episodes, 14(2): lO7- I 15. Hooghiemstra, H., Cleef. A.M., Noldus, G. and Kappelle, M., 1992. Upper Quaternary vegetation dynamics and paleoclimatology of La Chonta bog area (Cordillerd de Talamanca. Costa Rica). J. Quat. Sci.. 7: 2055225. Kuhry, P., 1988. Palaeobotanical-Palaeoecological Studies of Tropical High Andean Peatbog Sections (Cordillera Oriental, Colombia). Diss. Bot., 116, 241 pp. Cramer, Berlin (also in: The Quaternary of Colombia, Vol. 141. Melief. A.B.M.. 1985. Late Quaternary Paleoecology of the Parque National Natural 10s Nevados (Cordillera Central), and Sumapaz (Cordillera Oriental) Areas, Colombia. Ph.D. Dissertation. Univ. Amsterdam, Amsterdam, 162 pp. (also in: The Quaternary of Colombia, Vol. 121. Monsalve. J.G.. 1985. A pollen core from the Hacienda Lusitania. Archaologisches Projekt im westlichen Kolumbien/Siidamerika. Pro Calima (Basel), 4: 40-44.

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Rivera. D., 1983. Algunos aspectos de la vegetation de la region de la Laguna de Pedro Palo. Oikos (Bogota), I( I ): 11-16. Schreve-Brinkman. E.J.. 197X. A palynological study of the Upper Quaternary sequence in the El Abra corridor and rock shelters (Colombia). Palaeogeogr. Palaeoclimatol. Palaeoecol.. 25: I-109 [also in: The Quaternary of Colombia, Vol. 61. Van der Hammen, T., 1962. Palinologia de la region de “Laguna de 10s Bobos”. Historia de su clima, vegetation y agricultura durante 10s ultimos 5000 aiios. Rev. Acad. Colomb. Cienc. Exact. Fis. Nat.. I l(44): 359-361. Van der Hammen, T.. 1974. The Pleistocene changes of vegetation and climate in tropical South America. J. Biogeogr.. 4: 3-26. Van der Hammen, T., 1981. Environmental changes in the Northern Andes and the extinction of Mastodon. Geol. Mijnbouw, 60: 369-372. Van der Hammen, T. and Correal Urrego. G., 1978. Prehistoric man on the Sabana de Bogota: data for an ecological prehistory. Palaeogeogr. Palaeoclimatol. Palaeoecol., 25: 1799190. Van der Hammen. T. and Gonzalez, E., l96Oa. Upper Pleistocene and Holocene climate and vegetation in the Sabana de Bogota (Colombia, South America). Leidse Geol. Meded.. 25: 261-315. Van der Hammen. T. and Gonzalez. I?.. 1960b. Holocene and Late Glacial climate and vegetation of paramo de Palacio (Eastern Cordillera, Colombia. South America). Geol. Mijnbouw. 39(12): 737-746. Van der Hammen, T. and Gonzalez, E.. 1963. Historia de clima y vegetation del Pleistocene Superior y del Holocene en la Sabana de Bogota. Bol. Geol., Il(l!3): 1899266. Van der Hammen. T. and Gonzalez, E.. 1965. A Late Glacial and Holocene pollen diagram from Cienaga del Visitador (Dept. Boyaca, Colombia). Leidse Geol. Meded., 32: 1933201. Van der Hammen. T., Barelds, J.. de Jong. H. and de Veer. A.A., 1981. Glacial sequence and environmental history in the Sierra Nevada del Cocuy (Colombia). Palaeogeogr. Palaeoclimatol. Palaeoecol.. 32: 247-340. (also in: The Quaternary of Colombia. Vol. 81. Van Geel, B. and Van der Hammen. T., 1973. Upper Quaternary vegetational and climatic sequence of the Fuquene area (Eastern Cordillera. Colombia). Palaeogeogr. Palaeoclimatol. Palaeoecol.. 14: 9-92.