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Quaternary Research 61 (2004) 215–230

Fifty-thousand-year vegetation and climate history of Noel Kempff

Mercado National Park, Bolivian Amazon

Rachel E. Burbridge,a Francis E. Mayle,a,* and Timothy J. Killeenb,c

aDepartment of Geography, University of Leicester, Leicester LE1 7RH, UKbMuseo de Historia Natural ‘‘Noel Kempff Mercado,’’ Avenida Irala 565, Casilla 2489, Santa Cruz de la Sierra, Santa Cruz, BoliviacCenter for Applied Biodiversity Science, Conservation International, 2501 M Street, NW, Suite 200, Washington DC 20037, USA

Received 14 October 2002

Abstract

Pollen and charcoal records from two large, shallow lakes reveal that throughout most of the past 50,000 yr Noel Kempff Mercado

National Park, in northeastern lowland Bolivia (southwestern Amazon Basin), was predominantly covered by savannas and seasonally dry

semideciduous forests. Lowered atmospheric CO2 concentrations, in combination with a longer dry season, caused expansion of dry forests

and savannas during the last glacial period, especially at the last glacial maximum. These ecosystems persisted until the mid-Holocene,

although they underwent significant species reassortment. Forest communities containing a mixture of evergreen and semideciduous species

began to expand between 6000 and 3000 14C yr B.P. Humid evergreen rain forests expanded to cover most of the area within the past 200014C yr B.P., coincident with a reduction in fire frequencies. Comparisons between modern pollen spectra and vegetation reveal that the

Moraceae-dominated rain forest pollen spectra likely have a regional source area at least 2–3 km beyond the lake shore, whereas the grass-

and sedge-dominated savanna pollen spectra likely have a predominantly local source area. The Holocene vegetation changes are consistent

with independent paleoprecipitation records from the Bolivian Altiplano and paleovegetation records from other parts of southwestern

Amazonia. The progressive expansion in rain forests through the Holocene can be largely attributed to enhanced convective activity over

Amazonia, due to greater seasonality of insolation in the Southern Hemisphere tropics driven by the precession cycle according to the

Milankovitch Astronomical Theory.

D 2004 University of Washington. All rights reserved.

Keywords: Amazon; Bolivia; Pollen; Last glaciation; Quaternary; Holocene; Rain forest; Dry forest; Savanna

Introduction previously published summary paper (Mayle et al., 2000) to

The Quaternary environmental history of Amazonia is of

great scientific interest, but remains poorly understood due

to the paucity of data from this vast area. The aim of this

paper is to present a 50,000-yr paleoenvironmental recon-

struction of the ecosystems of Noel Kempff Mercado

National Park (NKMNP), northeastern Bolivia. The study

area (Fig. 1), located near the southwestern limit of Ama-

zonia, was chosen because of its climatically sensitive

ecotonal position between humid evergreen rain forests to

the north, semideciduous dry forests to the south, and

upland cerrado savannas to the east. We expand upon our

0033-5894/$ - see front matter D 2004 University of Washington. All rights rese

doi:10.1016/j.yqres.2003.12.004

* Corresponding author. Address beginning July 2004: Institute of

Geography, School of GeoSciences, The University of Edinburgh,

Drummond Street, Edinburgh EH8 9XP, UK. Fax: +44-116-252-3854.

E-mail address: [email protected] (F.E. Mayle).

show detailed pollen- and charcoal-based vegetation histo-

ries from two lacustrine sediment sequences and to discuss

the implications of these findings for the late Quaternary

paleoclimatology of southwestern Amazonia.

Study area

NKMNP is a 15,230-km2 protected area in northeastern

Bolivia near the southern margin of Amazonia, adjacent to

the Brazilian states of Rondonia and Mato Grosso (Fig. 1).

The Huanchaca Plateau, 600 to 900 m above sea level (asl),

dominates the eastern half of the park and is composed of

Pre-Cambrian sandstone and quartzite of the Brazilian

Shield (Litherland and Power, 1989). This table mountain

is dominated by edaphically derived upland savannas but

also contains patches of evergreen and deciduous forests

where soils are sufficiently deep and nutrient rich. The

rved.

Fig. 1. Map of NKMNP showing locations of the two study sites. Modified from Mayle et al. (2000), reproduced by permission of the publisher.

R.E. Burbridge et al. / Quaternary Research 61 (2004) 215–230216

adjacent lowland peneplain to the west (200 to 250 m asl) is

blanketed by Cenozoic alluvial sediments and is dominated

by humid evergreen Amazonian rain forests, which give

way to semideciduous dry forests at the southern border of

NKMNP. Two black-water rivers originating on the Brazil-

ian Shield define the boundaries of the park: the Rıo Itenez

to the north and east (Brazilian border) and the Rıo Paragua

to the west (Killeen and Schulenberg, 1998). These are

bordered by seasonally flooded evergreen forests and sav-

annas, the spatial arrangement of which is controlled by

local edaphic, topographic, and hydrological conditions.

The climate is distinctly seasonal, characterized by f1400

to 1500 mm of mean annual precipitation and 25–26jCmean annual temperature, with a dry season lasting for f6

months. Precipitation falls predominantly in the austral

summer, originating from deep-cell convective activity over

the Amazon Basin and southerly extension of the Intertrop-

ical Convergence Zone (ITCZ).

Both study sites are large (4–6 km diameter), shallow

(2.0–2.5 m in the dry season), flat-bottomed lakes, formed

by subsidence of the underlying rocks along fault lines of

the Pre-Cambrian Shield. Small quartzite ridges of this

Shield protrude f15 m above the surrounding Tertiary

and Quaternary alluvium and border the northeastern shore

of Laguna Chaplin and the southwestern shore of Laguna

Bella Vista. Both sites are surrounded by humid evergreen

rain forest and have small, generally ephemeral, inflowing

and outflowing streams that drain into the neighboring

rivers. Laguna Chaplin (14j28VS, 61j04VW) and Laguna

Bella Vista (13j37VS, 61j33VW) are located f30 and 120

km, respectively, north of the southern limit of Amazon

humid evergreen rain forest.

R.E. Burbridge et al. / Quaternary Research 61 (2004) 215–230 217

Methodology

Laguna Bella Vista (LBV) was cored in August, 1995,

with a modified square-rod Livingstone piston corer

(Wright, 1967). The overlapping cores were taken near the

center of the lake. The inorganic clay sediments at the base

of these cores proved too stiff to penetrate further. Logistical

constraints prevented exploration of the sediment stratigra-

phy in other parts of the lake basin. Laguna Chaplin (LCH)

was cored in August, 1998, using a Geocore ‘‘drop-ham-

mer’’ piston corer (Colinvaux et al., 1999), f80 m from the

southern shore. This shoreline position was chosen because

reconnaissance cores with an auger revealed that the thick-

ness of the upper organic unit increased toward the south

shore. At both sites, uppermost surface sediments were

recovered with a clear Perspex plastic tube and piston. At

LCH, another two surface cores were taken farther from the

shore to allow determination of the extent of spatial varia-

tion in the surface pollen spectra across the basin.

Percentage loss-on-ignition (LOI) was undertaken to

determine organic content and facilitate correlation of over-

lapping cores. The scarcity of terrestrial plant macrofossils

(even after systematic sieving with a 250-Am mesh sieve)

necessitated accelerator mass spectrometer 14C (AMS) dat-

ing of bulk sediment samples. The absence of carbonate

deposits in the area meant that any dating anomalies due to a

‘‘bedrock hard-water effect’’ were likely to be negligible.

One-cubic-centimeter samples were processed for pollen

using standard techniques (Faegri and Iverson, 1989).

Pollen types were identified using published pollen floras

(Roubik and Moreno, 1991; Colinvaux et al., 1999) and

the modern pollen reference collection (f1000 taxa) at the

Fig. 2. Surface pollen spectra (sediment/water interface) of selected ta

University of Leicester. The latter was compiled from

herbarium material housed at the Noel Kempff Mercado

Natural History Museum, Santa Cruz, Bolivia, collected

from NKMNP. Pollen percentage, concentration, and

accumulation rate (PAR) diagrams were prepared using

the Psimpoll program (Bennett, 1997). Percentage calcu-

lations were based on the sum of total land pollen (TLP),

which included terrestrial Pteridophyte spores, Cyperaceae

pollen, and unidentified pollen types. Aquatic taxa and

indeterminable pollen types were excluded from the

terrestrial pollen sum. Pollen zones were produced by

comparing different statistical outputs (using binary and

optimal splitting and cluster analysis) as determined by

Psimpoll.

Concentrations of pollen-slide charcoal particles >12 Amwere determined. Since particle fragmentation occurs during

pollen processing, a sieving-based methodology, modified

from Rhodes (1998), was adopted to minimize fragmenta-

tion and determine the size-class distribution of charcoal

particles to obtain insights into differing charcoal source

areas (e.g., local vs regional). Particles were sorted into

categories of 54–180, 181–250, 251–500, and >500 Am.

Modern pollen and charcoal

Modern pollen percentages, both between the three

LCH surface samples and between the surface samples

of the two sites, show similarities (Fig. 2). All four modern

pollen spectra are dominated by 40–55% Moraceae/Urti-

caceae pollen, providing a signature indicative of the

surrounding seasonally flooded, riparian, evergreen rain

xa from Laguna Chaplin (LCH) and Laguna Bella Vista (LBV).


forests and terra firme, humid, evergreen rain forests on

higher ground (Fig. 1). The dominance of the Moraceae

family (in both abundance and species richness) in these

forests (Killeen, 1998; Killeen et al., 2001) suggests that

the majority of grains identified as Moraceae/Urticaceae

type can be attributed to the Moraceae rather than the

Urticaceae family. Although the study area is predominantly

covered by rain forest, LBV lies within 5–10 km of several

areas of seasonally flooded savanna. LCH is bordered to the

south by a patch of savanna marsh and is only 20 km away

from extensive cerrado savannas on the Huanchaca Plateau

to the east and seasonally flooded savannas to the south.

However, pollen from these Poaceae- and Cyperaceae-

dominated ecosystems appears to be poorly dispersed and

significantly underrepresented, since the surface pollen

spectra of both sites contain <10% Poaceae and Cyper-

Fig. 3. Age–depth plot for (A) Laguna C

aceae pollen. These findings are supported by preliminary

percentage and PAR data (W.D. Gosling, personal commu-

nication, 2002) from artificial funnel pollen traps (Gosling

et al., 2003) from NKMNP, which show that seasonally

flooded savannas surprisingly contain more Moraceae than

Poaceae pollen (32% vs 18%, respectively) even though

the nearest Moraceae populations were f3 km away.

Clearly, the high Moraceae pollen percentages in the

surface sediment samples can be attributed to both local

populations growing around the lake shores and regional

populations several kilometers away. In contrast, the Poa-

ceae and Cyperaceae pollen have a predominantly local

source area, possibly no farther than 2–3 km from the lake

shore.

The complete absence of charcoal in the LBV surface

sediments (Fig. 9) attests to the rarity of fires in rain forest

haplin and (B) Laguna Bella Vista.

Table 2

AMS radiocarbon dates for Laguna Bella Vista

Laboratory

code

Sample depth

(cm)

AMS 14C yr B.P.

(F 1 j)Calibrated yr B.P.

(1 j)

AA39713 28–32 1,650 F 40 1,510 (1,530) 1,550

AA33345 40.5–41.5 2,740 F 50 2,860 (2,790) 2,770

AA39714 50.5–51.5 3,220 F40 3,460 (3,400) 3,370


ecosystems. It further demonstrates that the local rain forests

surrounding this lake serve as an effective barrier to char-

coal generated by the frequent fires we observed in the patch

of seasonally flooded savanna 5 km away. The presence of

charcoal (albeit negligible) in the surface sediments of LCH

(Fig. 6) is likely due to its closer proximity to the upland

savannas on the Huanchaca Plateau.
AA33344 60.5–61.5 7,980 F 70 9,000 (8,940, 8,930,
8,860, 8,840, 8,780)

8,640

AA39715 80.5–81.5 9,380 F 70 10,650 (10,570) 10,490

AA39716 100.5–101.5 9,590 F 70 11,110 (11,060, 10,950,

10,770) 10,710

Beta-100829 121.5–122.5 9,720 F 60 11,180 (11,160) 11,100

AA39717 125.5–126.5 9,820 F 70 11,230 (11,200) 11,170

AA39718 133–133.5 11,030 F 80 13,130 (13,000) 12,930

AA33343 137.5–138.5 38,600 F 1000 —

AA39719 147.5–148.5 46,600 F 2900 —

AA39720 187.5–188.5 41,400 F 1500 —

AA33342 197.5–198.5 43,900 F 1900 —

AA33341 247.5–248.5 43,700 F 2000 —

Beta-100828 279–282 >50,850 —

All samples are carbonate-free bulk sediment, except for sample Beta-

100828, which consists of unidentified twig fragments. See Table 1 for

explanation.

Sediment chronology and stratigraphy

Sedimentation rates varied markedly, both within and

between LCH-3 and LBV (Fig. 3; Tables 1 and 2). At LCH

a sudden change occurred near 6000 14C yr B.P., with slow

sedimentation (0.02 mm yr�1) during the early Holocene

and much faster sedimentation (by a factor of 10) by the late

Holocene (0.2 mm yr�1). Deposition of Holocene sediments

at LBV was more complex, with a period of slow sedimen-

tation in the mid-Holocene (0.02 mm yr�1, 7980–3220 14C

yr B.P.) bracketed by more rapid deposition in the early and

late Holocene (1.0 and 0.2 mm yr�1, respectively). LBV

was completely dry between 11,030 and 38,600 14C yr B.P.,

but LCH apparently never dried up for any significant

period (i.e., millennia), although sedimentation was ex-

tremely slow throughout the Pleistocene. In contrast to

LBV and lakes in other parts of Amazonia (Ledru et al.,

1998), the date of 17,820 F 140 14C yr B.P. indicates

deposition during the last glacial maximum at LCH.

At both sites, the reliability of the chronology, and

therefore sedimentation rates, of the clays and sands older

Table 1

AMS radiocarbon dates for Laguna Chaplin

Laboratory

code

Sample depth

(cm)

AMS 14C yr B.P.

(F 1 j)Calibrated yr B.P.

(1 j)

Beta-137570 36.5–37.5 710 F 50 670 (660) 570

AA39700 51.5–52.5 2,240 F 40 2,320 (2,300, 2,240,

2,170) 2,150

AA39701 69.5–70.5 2,740 F 40 2,850 (2,790) 2,770

AA39702 85–86 3,870 F 50 4,350 (4,250) 4,160

AA39703 100–101 4,330 F 80 4,960 (4,850) 4,830

AA39704 125–126 6,040 F 50 6,900 (6,860, 6,830,

6,800) 6,750

AA39705 135–136 9,000 F 100 10,230 (10,180)

9,930

AA39706 155–156 17,820 F 140 21,470 (21,210)

20,960

AA39707 175–176 31,060 F 440 —

AA39708 195–196 34,820 F 700 —

AA39709 213–214 37,750 F 970 —

AA39710 250–251 43,400 F 1900 —

AA39711 285–286 41,200 F 1400 —

AA39712 296–297 38,100 F 1000 —

All samples are carbonate-free bulk sediment. Radiocarbon ages less than

21,000 14C yr B.P. have been calibrated into years before present (cal yr

B.P.) (Stuiver et al., 1998). The radiocarbon dates have been calibrated by a

simple intercept with a linear interpolation of the calibration data points.

The 1 j cal age ranges are shown, with the cal age intercepts in parentheses.

All ages have been rounded to the nearest 10 years.

than f38,000 14C yr B.P. is more difficult to ascertain.

Since the 14C dates all overlap at 2 standard deviations (2 j),these sediments might be the same age, having been depos-

ited extremely rapidly, perhaps as a series of discrete facies.

However, the marked pollen changes through these sedi-

ments, many of which do not coincide with lithological

boundaries, argue against such an interpretation. More likely

these dates overlap because they are close to, or at, the

effective limit of radiocarbon dating. Some of these dates

appear anomalously young. Although there was no indica-

tion of contamination, during either field or laboratory

sampling, such old samples would be highly susceptible to

even the slightest introduction of younger carbon. Perhaps

this was the case for the two lowermost dates of 41,200 F1400 and 38,100 F 1000 14C yr B.P. at LCH (Fig. 3A),

because they are seemingly the only outliers on the age–

depth curve. We therefore excluded these two ages for

estimating the sedimentation rate between 176 and 251 cm

(31,060–43,400 14C yr B.P.). We extrapolated the rate of

0.06 mm yr�1, giving age estimates of 48,000 14C yr B.P. for

the LCH-1/LCH-2 boundary and 51,000 14C yr B.P. for the

base of LCH-1. All other dates from LCH are in chronolog-

ical order and conform to a consistent age-depth relationship,

strengthening our argument that the entire sedimentary

sequence was deposited continuously, albeit at varying

sedimentation rates, over the past 50,000 yr. Ages for all

pollen zone boundaries at LCH (notwithstanding the base of

LCH-1 and LCH-1/LCH-2) were therefore calculated by

linear interpolation between consecutive radiocarbon dates.

Dates for the LBV Holocene pollen zone boundaries were

calculated similarly, although the absence of any age–depth

relationship for the older dates of this site precludes calcu-

lation of the chronology of Pleistocene pollen changes.

Fig. 4. Laguna Chaplin summary pollen percentage diagram, showing the most common taxa from the full complement of 290 pollen types. Dots on the curves denote <0.5%. Curves showing�10 exaggeration are

depicted for selected taxawhich have low percentages. TLP sums are > 300 grains per sample, except for the four pollen spectra in zone LCH-1which had lower TLP sums of 100–160, due to low pollen concentrations.

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In both records, the Pleistocene sediments are predom-

inantly inorganic clays and sands, whereas the Holocene

sediments consist of organic muds (see Figs. 4 and 7),

indicating fundamentally different depositional environ-

ments in these two periods. Since the lakes currently are

located in a seasonally flooded landscape, connected to

neighboring rivers by small streams (Fig. 1), the Pleistocene

clastic sediments in these basins possibly are fluvial,

originating from floodwaters of the neighboring rivers in

the rainy season. However, we think this is unlikely because

the Rıo Paragua and Rıo Itenez are not white-water rivers,

laden with clastic material eroded from the Andes. Instead,

they are sediment-free, black-water rivers with headwaters

to the southeast on the Brazilian Shield. By contrast, the

lakes in the seasonally flooded forests and savannas of the

Beni, west of the Brazilian Shield, are entirely filled with

clay sediments deposited from flood waters of the white-

water rivers which drain the Andes. We suggest that the

Pleistocene sediments at our study sites (especially the

quartzite sands) were deposited by erosion from the adja-

cent quartzite ridges. The lack of organics in these sedi-

ments perhaps is due to rapid oxidation associated with

lowered water levels caused by a longer dry season.

Furthermore, the square-like morphometry of these basins,

and the substantial distance (15 km) of LCH from the Rıo

Paragua (Fig. 1), clearly shows they are not ox-bow lakes.

Instead, they are most likely pseudo-karst basins formed by

solution of silica from the underlying rocks of the Pre-

Cambrian Shield.

Vegetation and climate history of Noel Kempff Mercado

National Park

The last glaciation (f51,000–10,000 14C yr B.P.)

Our discussion refers predominantly to the LCH record,

since much of the glacial sequence is missing from LBV.

Pollen zone LCH-1 (51,000–48,000 14C yr B.P.) is indic-

ative of a catchment dominated by seasonally flooded

‘‘palm marsh’’ comprising almost exclusively two closely

related species, Mauritia flexuosa and Mauritiella armata

(Fig. 4). Concentration and PAR data (Fig. 5) reveal that the

exceedingly high pollen percentages of these palm taxa

(over 80% TLP) are primarily due to the negligible abun-

dance of other pollen types in this zone. These palms, which

form large, mixed colonies in such habitats today, typically

experience seasonal floods up to 2 m deep (Killeen, 1998).

The trace abundances of charcoal (Fig. 6) indicate that this

palm-dominated vegetation was not sufficiently dry to burn

frequently.

In zones LCH-2–LCH-5 (48,000–10,760 14C yr B.P.),

the dominance of Poaceae (>40%) and Cyperaceae pollen

(f15%) and the continuous appearance of Mauritia/Maur-

itiella pollen, especially between 37,750 F 970 and 17,820

F 140 14C yr B.P (zones LCH-3 and LCH-4), indicate the

presence of local low-lying areas covered by seasonally

flooded termite savannas and savanna marsh. This vegeta-

tion most likely occupied those areas that are currently

covered by seasonally inundated evergreen forest. The

consistently low values of Moraceae pollen (<10%) indicate

that humid evergreen forest communities were scarce

regionally, possibly existing only as gallery forests along

levees bordering the Rıo Paragua and in suitable micro-

habitats around the lake shore.

Consistent presence of Paullinia/Roupala, Celtis,

Machaerium/Dalbergia/Byrsonima, and Erythroxylum pol-

len throughout the Pleistocene record suggests that local

upland areas, such as the neighboring quartzite ridge, may

have supported stands of dry forest and/or mixed liana

forest. However, the pollen of Anadenanthera, a key dry

forest indicator (Prado and Gibbs, 1993; Mayle et al., 2004),

is largely absent from glacial sediments of both sites. The

composition of these forest communities probably differed

from those of the contemporary deciduous and semidecid-

uous dry forests.

Clear floristic changes in the community composition of

these savanna and dry forest habitats occurred through the

Pleistocene. The vegetation changes also are accompanied

by variations in charcoal concentrations and inferred fire

dynamics. For example, Machaerium/Dalbergia/Byrsonima

and Erythroxylum pollen are important components of zone

LCH-2 and are associated with peaks in charcoal concen-

trations, suggesting that these savanna and dry forest eco-

systems may have exhibited the highest fire frequencies of

the entire record. Peaks in macroscopic charcoal (e.g., >500

Am) likely represent local fires, whereas microscopic char-

coal noted on pollen slides reflects regional fires. In con-

trast, the peak in Mauritia/Mauritiella pollen and minimal

charcoal concentrations in zones LCH-3 and LCH-4 suggest

a greater coverage of seasonally flooded savanna and

savanna marsh that contained a relatively smaller arboreal

component and hence less fuel for fires. However, interpre-

tation of the charcoal record is complicated by the likeli-

hood that differences in charcoal concentrations could at

least partly be attributable to taphonomic changes associated

with the switch from relatively higher energy clay/sand

facies to lower energy clay facies. The abundance of Isoetes

spores throughout the Pleistocene sediments (40–80% of

the total pollen sum) suggests that water depth was lower

than today, allowing populations of the emergent semiaquat-

ic fern to expand throughout much of the lake. This

vegetation reconstruction is generally corroborated by pol-

len concentrations and PARs. However, PARs of all taxa are

unexpectedly low in zones LCH-4 and LCH-5 (f29,950–

10,760 14C yr B.P.). Perhaps pollen grains oxidized under

low sedimentation rates (f0.02 mm yr�1) and/or within

environments in which sedimentation was at least annually

discontinuous.

These terrestrial and aquatic ecosystem changes indicate

vegetation responses to increased aridity and lowered atmo-

spheric CO2 concentrations (Monnin et al., 2001). Carbon-

Fig. 5. Summary diagrams of (A) pollen concentration and (B) pollen accumulation rate (PAR) for Laguna Chaplin. Pollen assemblage zones are the same as in

Fig. 4. Curves showing �10 exaggeration are depicted for selected taxa which have low PARs.


efficient plants, such as the C4 savanna grasses (Street-

Perrott et al., 1997; Huang et al., 2001) and Isoetes, which

uses the Crassulacean acid metabolism photosynthetic path-

way (Boston and Adams, 1987; Sandquist and Keeley,

1990), would have been preferentially favored under these

conditions. Dynamic climate–vegetation modeling studies

by Sternberg (2001) suggest that dry-season precipitation,

rather than mean annual precipitation, would be critical in

determining forest–savanna ecotonal changes. Although

glacial aridity is the most parsimonious explanation for

the 27,570-yr hiatus at LBV, precipitation cannot have been

drastically below present levels, because the sediment

record for LCH shows no evidence for millennial-scale

sedimentary hiatuses at any time over the past 50,000 yr,

even though the lake is currently only 2 m deep in the dry

season.

A peak in pollen of the predominantly Andean genus

Podocarpus between the last glacial maximum (LGM) and

the late glaciation (zone LCH-5) contributes to the

expanding body of paleobotanical (e.g., Behling, 1996,

2001; Behling et al., 1999; Colinvaux et al., 2000; Ledru

et al., 2001) and independent paleoclimate evidence (Guil-

derson et al., 1994; Stute et al., 1995; Thompson et al.,

1998) for cooler temperatures (f5jC below present) and

Fig. 5 (continued).


nonanalog plant communities at this time. Cooling would

reduce evapotranspiration rates and therefore offset, to

some degree, the increased water stress expected by a

reduction in precipitation and/or atmospheric CO2 during

the LGM.

Given that the Pleistocene assemblages are dominated

by pollen types that are poorly dispersed and/or produced

in low quantities relative to wind-pollinated taxa, such as

Moraceae, vegetation interpretations on a regional scale

(i.e., beyond f2 km from the lake shore) are speculative.

In nonflooded areas, however, increased water stress,

caused by a combination of reduced precipitation and

lowered CO2 concentrations, would likely result in in-

creased populations of dry forest species, lianas, and

cerrado (savanna woodland). Certainly, regional differen-

ces existed in the vegetation of NKMNP ca. 40,000–

44,000 14C yr B.P. For example, Alchornea was abundant

around LBV (20–50% pollen, LBV-2) (Fig. 7) but was

largely absent from LCH. Unfortunately, the paleoecolog-

ical significance of this genus for Amazonian Bolivia is

unclear, because it grows in humid evergreen forest,

gallery forest, and seasonally flooded savannas (Killeen

et al., 1993; Killeen, 1998). However, coincident peaks in

macroscopic charcoal (Fig. 9) and Alchornea pollen in

zone LBV-2 suggest a frequently burnt landscape (e.g., a

combination of gallery forests and savanna).

Fig. 6. Microscopic and macroscopic charcoal concentrations for Laguna Chaplin. Pollen assemblage zones are the same as in Fig. 4.


The Holocene (10,000 14C yr B.P.–present)

For the Holocene zones (LCH-6–LCH-8), continued

high percentages of Poaceae pollen (f40%), low percen-

tages of Moraceae pollen (<20%), and similar trends in

pollen concentration and PAR (Figs. 4 and 5) show that

seasonally flooded savannas continued to dominate low-

lying areas around LCH untilf2000 14C yr B.P. Significant

amounts of pollen of Mauritia flexuosa and/or Mauritiella

armata palms and the obligate savanna species, Curatella

americana, which commonly grow together on termite

mounds characteristic of such habitats (Killeen, 1998),

provide further support for this interpretation. Semidecidu-

ous dry forest bordered the lake (most likely in upland,

nonflooded areas such as the quartzite ridge) as indicated by

the presence of Anadenanthera, Astronium fraxinifolium,

Astronium urundeuva, and Gallesia pollen, which are abun-

dant components of the modern Chiquitano Dry Forest,

located 30 km south of LCH (Killeen, 1998; Killeen et al.,

1998). Additional evidence for the establishment of these

dry forest and savanna communities comes from macrofos-

sil charcoal (Fig. 6), which suggests the presence of fire-

prone vegetation. Temporal differences in peak abundances

of these taxa through the Holocene attest to the dynamic

Fig. 7. Laguna Bella Vista summary pollen percentage diagram, showing the most common taxa from the full complement of 263 pollen types. Dots on the curves denote <0.5%. Curves showing �10

exaggeration are depicted for selected taxa which have low percentages. TLP sums are >300 grains per sample, except for sample numbers 92, 247, 257, 289, and 297, which had lower TLP sums of 150–280, due

to low pollen concentrations.

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nature of these ecosystems (e.g., Alchornea in zone LCH-6,

Tapirira guianensis and Erythroxylum in zones LCH-6 and

LCH-7, Mauritia/Mauritiella and Didymopanax in zone

LCH-8).

Between f2000 and 710 F 50 14C yr B.P. (zone LCH-

9), declining Poaceae and increasing Moraceae pollen (Figs.

4 and 5) indicate a reduction in open savannas and an

expansion of forest, the latter likely a mixture of rain forest

and dry forest species. At 710 F 50 14C yr B.P. (zone LCH-

9/LCH-10 boundary), the sharp increase in Moraceae pollen

percentages to present-day levels of 40% and the decline in

Fig. 8. Summary pollen concentration data for Laguna Bella Vista. Pollen assemb

depicted for selected taxa which have low concentrations.

savanna and dry forest taxa to negligible levels mark the

establishment of the modern rain forest-dominated land-

scape around LCH.

A similar pattern of Holocene vegetation changes is

evident at LBV, although significant differences occur in

the times of change. The rain forest/dry forest expansion

began between f6000 (onset of zone LBV-6) and 3000 14C

yr B.P., although the extremely slow sedimentation rate over

this interval (0.02 mm yr�1) precludes a more precise

estimate of the age of this vegetation change. Percentage

data (Fig. 7) suggest that the modern rain forest communi-

lage zones are the same as in Fig. 7. Curves showing �10 exaggeration are


ties appeared f1650 F 40 14C yr B.P. (zone LBV-6/LBV-7

boundary), although pollen concentration data (Fig. 8)

indicate a somewhat earlier arrival at f2000 14C yr B.P.

At both LBVand LCH, reduced concentrations and PARs of

all taxa in the uppermost sediments can be attributed to the

high water content that typifies samples near the sediment/

water interface.

Discerning the relative contribution of atmospheric CO2

concentrations, precipitation, and temperature as controls of

glacial-age vegetation is difficult for NKMNP. Nonetheless,

precipitation changes were most certainly the primary factor

behind the observed Holocene vegetation dynamics, be-

cause atmospheric CO2 concentrations and temperature

Fig. 9. Microscopic and macroscopic charcoal concentrations for Lagun

exhibited only minor fluctuations through the Holocene

(Indermuhle et al., 1999) relative to those of the LGM–

Holocene transition (Monnin et al., 2001). The progressive

vegetation succession through the Holocene in NKMNP

from savanna/semideciduous forest to semideciduous/ever-

green forest and finally to evergreen rain forest can be

attributed to a gradual increase in mean annual precipitation

and reduction in the length/severity of the dry season since

the mid-Holocene.

Evidence of dense populations of indigenous peoples

throughout much of the Bolivian Amazon prior to the

arrival of Europeans (e.g., Denevan, 1966) raises the

possibility that human activity may have contributed to

a Bella Vista. Pollen assemblage zones are the same as in Fig. 7.


these vegetation changes. Although there is no conclusive

pollen evidence (e.g., Zea) of paleo-Indian agriculture

around either of these lakes, ceramics have been recovered

from a soil pit in an interfluve f25 km northwest of LCH

(T.J. Killeen, personal communication, 2002) and abundant

ceramics mixed with charcoal (dated to f400 14C yr B.P.)

were recovered from anthosols (terra preta) throughout La

Chonta (Guarayos province) f150 km west of NKMNP

(C. Paz, personal communication, 2002). Furthermore,

well-documented evidence (e.g., Denevan, 1966; Erickson,

1995, 2000; Langstroth, 1999) indicates that paleo-Indians

caused major landscape modification to the seasonally

flooded savannas of the alluvial plains throughout the

Bolivian Beni, which encompass much of the Bolivian

Amazon northwest of NKMNP. Although many forest

islands in the Moxos plains of the Beni are natural remnants

or fragments of gallery forests on ancient river levees

(Langstroth, 1999), other forest islands are the result of

human activity, whereby forests invaded artificial earth

mounds following abandonment by paleo-Indians (e.g.,

Denevan, 1966, Erickson, 1995). Although an anthropo-

genic explanation for the recent expansion of rain forest in

NKMNP may seem plausible, we discount this possibility

for several reasons. First, in contrast to the forest islands in

the Beni savannas, rain forests in our study area are not

restricted to terra firme environments, but occur in season-

ally flooded habitats. Second, no archaeological earth-

works, such as earth mounds or ridges, have been

discovered in savanna or forested environments. Finally,

if rain forest expansion occurred simply as secondary

succession following human population collapse in the

area, rain forests should have dominated the area prior to

human arrival. Our pollen data clearly discount this hy-

pothesis. Although ceramic evidence shows that paleo-

Indians lived in NKMNP prior to the Spanish Conquest,

possibly around the shores of the two sites, they appear not

to have had any significant impact upon the ecosystems of

our study area. We therefore infer that shifts in precipitation

were the predominant drivers of vegetation change through

the Holocene.

Implications for understanding paleoclimatology of

southwestern Amazonia

Stable carbon isotope data from soil samples taken

along a 200-km transect across savanna ‘‘islands’’ sur-

rounded by rain forests between Porto Velho (Rondonia

State) and Humaita (Amazonas State), Brazil (8j43VS,63j58VW–7j38VS, 63j04VW), show no evidence for savan-

na expansion during the LGM (de Freitas et al., 2001).

These results are in contrast to our findings from NKMNP,

even though the transect lies only 450 km north of

NKMNP. Furthermore, lake-level reconstructions from

Lake Titicaca (Baker et al., 2001a) and the Salar de Uyuni

salt flats (Baker et al., 2001b) and ice-core data from

Sajama Mountain (Thompson et al., 1998), all sites whose

modern source of precipitation is primarily from the

Amazon Basin, demonstrate that LGM precipitation on

the Bolivian Altiplano was perhaps 20–75% higher than

today (Hastenrath and Kutzbach, 1985; Blodgett et al.,

1997). The difference between a wet Altiplano and rela-

tively drier Bolivian Amazon lowlands is supported by

general circulation modeling studies (Hostetler and Mix,

1999), which clearly show that, in contrast to the present

day, these two regions received distinctly different moisture

sources during the LGM.

How can these regional precipitation differences in

southwestern Amazonia and the Bolivian Altiplano be

explained? Predictions based on orbital forcing by the

19,000- to 22,000-yr precession cycle of the Milankovitch

Astronomical Theory (Berger and Loutre, 1991) would

maximize LGM precipitation in southwestern Amazonia

due to maximum austral summer insolation at 10jS and

hence maximum latitudinal, seasonal migration of the

ITCZ. However, expansion of the Antarctic ice sheet

and Patagonian glaciers would likely steepen the latitudi-

nal temperature gradient across South America despite the

5jC equatorial cooling. Wet-season convective activity

over southern Amazonia would consequently lessen,

thereby reducing precipitation in southernmost Amazonia

relative to today. Furthermore, reduced sea-surface temper-

atures in the equatorial Atlantic would curtail oceanic

moisture transfer to the Basin. These factors may explain

why there was an ecotonal response in the Bolivian

Amazonian lowlands but no ecotonal shift farther north

at Porto Velho/Humaita. The precipitation contrast be-

tween the Altiplano and lowland Bolivia could perhaps

be explained by the dominant moisture source for the

Altiplano coming from winter anticyclones off the eastern

Pacific (as simulated by Hostetler and Mix, 1999) rather

than from the Amazon summer monsoons associated with

the ITCZ.

In contrast to the marked regional differences during the

LGM, climatic changes through the Holocene are broadly

consistent and synchronous throughout southwestern Ama-

zonia and the Altiplano. The early to mid-Holocene arid

phase at NKMNP correlates with: (1) the expansion of

savanna islands 450 km to the north at the Rondonia/

Amazonas border between 9000 and 3000 14C yr B.P. (de

Freitas et al., 2001); (2) replacement of cloud forest by open,

grass-dominated ecosystems in the eastern Bolivian Andes

(17j50VS, 64j43V08WW) between f11,000 and 4000 14C yr

B.P. (Mourguiart and Ledru, 2003); and (3) a prolonged

lake-level low-stand in Lake Titicaca between 8500 and

3500 cal yr B.P. (7750–3300 14C yr B.P.) (Baker et al.,

2001a). These correlations point to a common climatic

forcing affecting all these sites. The most likely explanation

for this arid episode is that the orbitally driven minimum in

January insolation at 15–10jS (Berger and Loutre, 1991)

would have restricted the southerly penetration of the ITCZ

and deep cell convective activity over the Amazon Basin,


thus reducing the length of the summer rainy season.

Although the close correlation between the Lake Titicaca

low-stand and the sedimentation minimum between 7980

and 3220 14C yr B.P. at LBV (Fig. 3b) suggests that organic

sedimentation in the lake was tightly coupled to climate, the

poor correlation with the sedimentation trends at LCH raises

doubts about the strength of such a causal relationship,

suggesting that local catchment processes were perhaps the

overriding factor.

The trend of increasing precipitation in our study area,

beginning 6000–3000 14C yr B.P., is similarly part of a

widespread phenomenon throughout southwestern Amazo-

nia and the Altiplano, corroborated by: (1) rising water

levels in Lake Titicaca, beginning f4500 cal yr B.P.

(4000 14C yr B.P.) and reaching modern levels byf2000 cal yr B.P. (2000 14C yr B.P.) (Baker et al.,

2001a); (2) increased snow accumulation on Sajama

Mountain at f3000 14C yr B.P. (Thompson et al.,

1998); (3) expansion of cloud forest in the Bolivian Andes

after 4000 14C yr B.P. (Mourguiart and Ledru, 2003); and

(4) contraction of savanna islands in Rondonia/Amazonas

beginning 3000 14C yr B.P. (de Freitas et al., 2001). This

late Holocene increase in humidity reflects progressive

increases in January insolation at 10–15jS since the early

Holocene minimum (Berger and Loutre, 1991) and conse-

quent greater southerly migration of the ITCZ. The 1300-

yr delay in rain forest expansion at LCH compared with

LBV is likely caused by the time-transgressive nature of

precipitation change through the Holocene and/or species’

dispersal lagging behind climatic change. The Holocene

lake-level record from the Bolivian Andes shows that this

rain forest expansion was a function, not only of a

lengthening of the rainy season, but also an absolute

increase in mean annual precipitation.

Acknowledgments

We thank the following: M. Suarez Riglos and M.

Saldias for use of the herbarium at the Natural History

Museum, Santa Cruz, Bolivia; J. Ratter for use of the

herbarium at the Royal Botanic Garden Edinburgh; P.

Colinvaux for use of the Amazon pollen reference collection

at the Smithsonian Tropical Research Institute, Panama; R.

Guillen, L. Sanchez, M. Siles, J. Surubi, and P. Soliz for

assistance with fieldwork; the Fundacion Amigos de la

Naturaleza and the Bolivian Government for permission to

work in NKMNP; and Mark Bush and an anonymous

reviewer, whose comments greatly improved the original

manuscript. This work was funded by a Royal Society

research grant (F.E.M.), two Royal Society Overseas Study

Visits (F.E.M.), a University of Leicester Research Grant

(F.E.M.), and a University of Leicester Ph.D. Studentship

(R.B.). Funding for radiocarbon dates was provided by the

Natural Environment Research Council (26 dates) and the

University of Leicester (1 date).

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