Paoay Lake, northern Luzon, the Philippines: a record ofHolocene environmental change
J A N E L L E S T E V E N S O N *, F E R N A N D O S I R I N G A N w , J A N F I N N *, D O M I N G O M A D U L I D zand H E N K H E I J N I S §
*Department of Archaeology and Natural History, ANU College of Asia and the Pacific, Australian National University, Canberra
0200, Australia, wMarine Science Institute, University of the Philippines, Diliman, Quezon City 1101, Philippines, zBotany
Division, National Museum of the Philippines, Ermita, Manila 1000, Philippines, §Institute for Environmental Research, Australian
Nuclear Science and Technology Organisation, Lucas Heights 2234, Australia
Abstract
The last 7000 years of environmental history for Paoay Lake and its surrounding landscape isexamined through the analysis of pollen, diatoms, charcoal, mineral magnetics and AMSdating. Basal sediments contain shells of Cerithiidae and the saline-tolerant diatom Diploneisindicating that this was an estuarine environment before becoming a freshwater lake after6000 BP. Pollen analysis shows that submontane forests, characterized by Pinus pollen,underwent a major disturbance around 5000 years ago, recovering to previous levels by1000 years ago. Charcoal as an indicator of fire is abundant throughout record, although thehighest levels occur in the earlier part of the record, between 6500 and 5000 years ago. Anaspect of the project was to examine whether there is evidence of land clearance andagricultural development in the region during the late Holocene. While a clear signal ofhuman impact in the record remains equivocal, there appears to be a correspondence betweensubmontane forest decline and mid-Holocene ocean data that depict warmer and possiblydrier conditions for the region. The study highlights the vulnerability of these montaneforests to forecasts of a warmer and drier climate in the near future.
Keywords: charcoal, Holocene, Philippines, Pinus, pollen
Received 10 March 2009; revised version received 10 July 2009 and accepted 14 July 2009
Introduction
The Philippine archipelago stretches from the wet tro-
pics in the south to the monsoonal tropics in the north
and although it has an important place within insular
south-east Asia for understanding phenomena such as
the evolution of the Asian Monsoon or the impact of
ENSO, it has few palaeoenvironmental studies of any
description and only one palynological study (Ward &
Bulalacao, 1999). While the central aim of study is to
document environmental change during the Holocene
in the northern Philippines, one of the associated re-
search questions is to assess if any of the changes are
related to human activity. In particular land clearance
and the development of rice agriculture during the
Neolithic, as the timing and development of this is an
unresolved question for the Philippines (Bellwood,
2005; Bellwood & Dizon, 2005). Our study therefore
provides unique information on the landscape changes
that have taken place since the mid-Holocene in north-
western Luzon and makes a significant contribution to
the study of climate change in the western Pacific.
Environmental setting
Paoay Lake is situated in north-western Luzon
(181070N, 1201320E) (Figs 1 and 2) along the western
edge of the Ilocos lowland, a tectonic depression related
to Late Pliocene to Quaternary activity of the Philippine
Fault (Pinet & Stephan, 1990). Coastal progradation and
the subsequent development of a sand dune barrier
during the mid-Holocene are believed to have led to the
formation of the lake (Siringan & Pataray, 1997) and it is
now separated from the sea along its western edge by a
sand dune complex approximately 2.3 km wide and
with an average elevation of 40 m. The Upper Pleisto-
cene Laoag formation bounds the lake in all other
directions (N. P. Punzal et al., unpublished results).
North-western Luzon has a monsoon climate, with a
dry season in the lowlands from November to April and
a wet season from May to October (Argete, 1998).Correspondence: Janelle Stevenson, fax 1 61 2 612 549 17, e-mail:
Global Change Biology (2010) 16, 1672–1688, doi: 10.1111/j.1365-2486.2009.02039.x
1672 r 2009 Blackwell Publishing Ltd
Average annual rainfall is around 2000 mm for the
lowlands and 44000 mm for the upper montane re-
gions of the Central Cordillera, the main mountain
range of northern Luzon. The average annual tempera-
ture is 28 1C for the lowlands and around 15 1C for the
upper montane zone. Rainfall shortages associated with
the ENSO phenomenon occur during the wet season.
During the late 20th century logging throughout the
Ilocos Mountains and Central Cordillera led to massive
landscape transformations; primarily the expansion of
grassland and Pinus forest and the aggradation of river
valleys. In the present day landscape, Pinus kesiya, the
pine species of northern Luzon, is common above an
altitude of 600 m, with the bulk of these forests found on
steep slopes between 1000 and 2000 m altitude (Kowal,
1966; Zamora & Co, 1986). Figure 1b illustrates the
extent of this mountainous terrain in northern Luzon
and its relationship to Paoay Lake.
Pine forests in the Philippines have a grass under-
storey, and like pine forests throughout the world, are
maintained by fire, as low intensity fires prevent the
establishment of hardwood seedlings (Kowal, 1966;
Goldammer & Penafiel, 1990; Richardson & Rundel,
1998). Today the pine forests sit above what is a heavily
modified human landscape, which outside of the irri-
gated valley floors is dry and harsh with skeletal and
easily eroded soils. It is a landscape that is regularly
burnt to promote palatable regrowth for livestock, with
intense fires reducing the number of pine trees found at
their lower altitudinal range (Kowal, 1966). Although
there are few old growth pine forests left in Luzon, pine
savannas are increasing in extent due to the expansion
of human activities into the mountains and the asso-
ciated prevalence of fire. Above the pine forests, be-
tween 2000 and 2600 m, are the cloud or moss forests
and it is this vegetation type in particular that is being
heavily impacted by the expansion of market gardening
throughout the mountains of Luzon.
The landscape around the Paoay Lake itself is essen-
tially a cultivated one, constituted primarily by herbac-
eous crops, cultivated trees and weeds, with some
patches of lowland secondary vegetation that are
thought to be natural remnants. The taxa most com-
monly occurring in these vegetation remnants are listed
in Table 1. During the dry season the exposed shoreline
is heavily utilized for growing a variety of crops,
including rice, and fish farming is carried out within
the lake itself. In general, human population pressure
and agricultural expansion have heavily transformed
the lowland landscape of Ilocos Norte.
Site description
Paoay Lake has a surface area of approximately 4.0 km2
and a relatively small watershed of around 7.5 km2. At
the end of the rainy season the lake has an average
Fig. 1 (a) Locality map of Paoay Lake and other locations mentioned in text. (b) Map illustrating the topography of the region.
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water depth of around 4.5 m and the surface of the lake
is around 20 m above mean sea level. Today the lake has
no outflow, but early topographic maps show that it
once flowed into the Quiaoit River to the south when
water levels rose above the 18 m contour, joining the
larger Lawa River before flowing out to the sea. During
the 1960s or 1970s, in conjunction with the building of a
regional irrigation network, this outflow was dammed
raising the water level during the wet season by 2–3 m.
However, during the dry season, the combined effects
of evaporation and extraction of water for irrigation
drops the water level back to the naturally occurring
dry season level, which is below the take-off level for
the irrigation network.
Materials and methods
Preliminary sediment coring was undertaken and ma-
terial collected from two sites, LP2 and LP3 (Fig. 2).
These two cores are from separate embayments on the
landward lake margin. Sediments at both locations
were recovered using a Livingstone corer with the
water depth at each location being just 41 m at the
end of the dry season. Over 6 m of sediment were
collected at each core location; stiff clays prevented
deeper sediment collection. On a return trip a duplicate
core (LP3-1) of the deeper sediment at LP3 was col-
lected with a GEOCORER, a modified Livingstone corer
that allows the sampling barrel to be hammered into the
Fig. 2 Site map of Paoay Lake showing coring locations and general landscape attributes.
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sediments. At the same time the unconsolidated sedi-
ments 470 cm were collected with a mud/water inter-
face sampler. Once back in the lab cores LP2 and LP3
were described and run through a magnetic suscept-
ibility loop to produce a magnetic profile. Core LP3 was
chosen as the core for more detailed analysis as it had
the greatest depth recovered.
Age determinations for LP3 and LP3-1 were carried out
on samples that were given a standard acid–base–acid
pretreatment, which also included HF to remove the
mineral component, and sieving at 10 and 125mm. This
resulted in an organic fraction referred to as the pollen
size fraction that was radiocarbon dated using AMS.
Pollen analyses were undertaken on core LP3 using
standard acetolysis processing techniques and with
approximately 23 000 exotic Lycopodium spores added
to each sample so that the pollen concentration could be
calculated (Bennett & Willis, 2001). Charcoal on the
pollen slides was also counted as an indicator of fire
in the landscape with the concentration of charcoal
calculated using the exotic marker method (Bennett &
Willis, 2001). Only black, opaque angular particles
410 mm were counted as charcoal.
The pollen samples were subsampled at 10 cm inter-
vals from 70 to 695 cm in core LP3 and from 725 to
770 cm in core LP3-1. Pollen analysis of the unconsoli-
dated sediments 470 cm has been halted as the pollen
concentrations are extremely low making a target count
of even 100 grains difficult to obtain. The diatom
assemblage was analysed at 10 cm intervals for core
LP3 and the samples were processed using standard
techniques that included oxidation (30% H2O2) and
removal of soluble salts and carbonates (10% HCl)
(Battarbee et al., 2001).
The pollen diagrams are percentage diagrams plotted
using the program C2 (Juggins, 2003). Northern Luzon
has a large and diverse flora which, in combination with
a very modest pollen reference collection and scarce
published material for the region, limits the level of
identification possible. As a result there are recurring
pollens types that are not yet identified, but are instead
categorized by number. Accounting for unknown pol-
len types in this way ensures that the diversity within
the pollen record is not lost. The unknown types are
quite different to the ‘indeterminate’ category that is
composed of damaged and crumpled grains and grains
that were seen infrequently.
The individual pollen curves are based on a terrestrial
pollen sum that excludes fern spores, aquatic pollen
and Pinus, as this pollen type overwhelms the pollen
rain in the region. Only taxa that acquired a value of 1%
in at least one sample are plotted as they are considered
to carry the bulk of the interpretative information.
Although rice cultivation is of interest to this project,
the identification of rice from the pollen record is
difficult because rice pollen is morphologically similar
to most other grass types. Inferences about cultivated
species in the grass family are easily made for other
parts of the world because crops such as wheat and
maize have particularly large pollen grains. Cultivated
rice, however, has a size range of around 25–49mm, with
a mode of 36mm (Maloney, 1990; Bulalacao, 1997; Wang
et al., 1997). This range includes the median size for
grass pollen of all species. Grass pollen grains in this
study were therefore measured and placed in a size
class to assess whether the diversity of these categories
changed any at point in the record.
To explore rice cultivation further, a phytolith study
was also undertaken. Phytoliths are biogenic silica laid
down in certain plant cells and are most abundant in
grasses; rice phytoliths are diagnostic and can be dis-
tinguished from other grasses (Pearsall et al., 1995).
Sediment samples from the contemporary lake sedi-
ment surface were also collected to better understand the
modern pollen rain signature. However, because the
pollen content in these samples is very low and many of
the identifications still uncertain, at this stage only the ratio
of pine pollen to all other pollen has been determined.
All statistical analyses were carried out within PSIM-
POLL (Bennett, 2001). Numerical zonation of the pollen
data used optimal splitting by sum of squares analysis.
This was based on the terrestrial pollen sum and
included only those taxa with a value of 1% in at least
one sample.
Table 1 Commonly occurring taxa within secondary vegeta-
tion remnants around Paoay Lake
Family Genus and species
Apocynaceae Wrightia laniti (Blco.) Merr.
Arecaceae Corypha elata Roxb.
Capparidaceae Capparis micrantha DC.
Casuarinaceae Casuarina equisetifolia L.
Cycadaceae Cycas edentata de Laub. (endemic)
Euphorbiaceae Macaranga tanarius (L.) Muell.-Arg.
Euphorbiaceae Melanolepis multiglandulosa (Reinw.)
Reichb.f.& Zoll.
Leguminosae Leucaena leucocephala (Lamk.) de Wit
(introduced)
Leguminosae Pterocarpus indicus Willd. Subsp. indicus
Loganiaceae Fagraea obovata Boedj.
Moraceae Ficus nota (Blco.) Merr.
Moraceae Ficus ulmifolia Lam.
Poaceae Bambusa vulgaris Schrad. ex J.C.Wendl.
Poaceae Schizostachyum lumampao (Blco.) Merr.
(endemic)
Rubiaceae Morinda citrifolia L.
Rubiaceae Nauclea orientalis (L.) L.
Ulmaceae Trema orientalis (L.) Bl.
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Table 2 Stratigraphic description of core LP3 and LP3-1
Depth (cm) Major constituent Description
LP3
0–68 Clay Unconsolidated sediment
68–207 Organics Very dark grey slightly clayey coarse organic sediment
207–215 Clay Dark grey silty clay – diffuse lower boundary, defined upper boundary
215–308 Organics Very dark grey, slightly clayey, coarse organic sediment gradually changing to black, coarse
organic sediment
308–312 Clay Dark grey silty organic clay
312–362 Organics Black, coarse, organic sediment with a fine band of grey silty clay at 342 cm
362–372 Clay Dark grey silty organic clay. Organics coarser from 368 to 372 cm
372–376 Organics Black, coarse, organic sediment
376–407 Clay Dark grey slightly organic silty clay changing gradually to dark grey organic clay
407–409 Organics Black, coarse, organic sediment
409–433 Clay Black slightly organic clay
433–435 Very dark grey silty clay
435–437 Black slightly organic clay
437–443 Organics Black, slightly clayey, organic sediment
443–452 Clay Black, organic silty clay
452–456 Dark grey silty clay
456–468 Black, organic silty clay
468–495 Organics Black, slightly silty, clayey, organic sediment
495–511 Clay Dark reddish grey silty clay
511–615 Organics Black, slightly silty, clayey, organic sediment. Distinct band of black, silty, clay at 577–578.
Less distinct bands of same silty, clay down to 603 cm
615–622 Clay Dark grey organic silty clay
622–627 Dark grey clay
627–632 Dark grey organic silty clay
632–643 Organics Gradual change to black, coarse, organic sediment
643–660 Clay Black, organic clay. Indistinct bands of clay throughout
660–661 Sandy, silty, organic clay
661–680 Black, organic, silty, clay
680–691 Organics Gradual change to black, clayey, organic sediment
691–697 Gradual change to black organic sediment
697–703 Clay Gradual change to black organic silty clay
703–704 Sand Fine sand (pale yellow)
704–705 Clay Black organic silty clay
705–706 Sand Fine sand (pale yellow)
706–733 Clay Black organic silty clay. Increasing clay and sand content with depth.
Charcoal fragments at 708–717 cm
LP3-1
500–572 Organics Black, slightly clayey organic sediment. Diffuse band of dark grey, silty, organic clay from
510 to 515 cm
572–580 Clay Very dark grey, silty, clay. Sharp lower boundary, diffuse upper boundary
580–717 Organics Black, slightly, clayey organic sediment changing gradually to black, silty, organic clay.
Organics getting much finer with depth. Occasional bands of coarser organics at 619–621
and 629–631. Fine grey band of silty clay at 681 cm
717–732 Clay Sharp boundary to dark grey, silty, organic clay with charcoal fragments and occasional shell
732–742 Grades back into black silty organic clay with occasional shell
742–743 Light grey silty clay
743–744 Black silty organic clay
744–745 Light grey silty clay
745–751 Black silty organic clay
751–763 Abrupt boundary to dark grey silty clay with shell fragments
763–792 Grades into very dark grey silty clay. Clay and shell increase with depth
792–885 Grades into dark grey clay. Very stiff
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Results
A detailed stratigraphic description of core LP3/LP3-1 is
reported in Table 2 revealing that the sedimentation
history of the lake has been fairly complex, although
many of the changes are quite subtle and relate to
varying silt and clay contents. In summary, the basal
sediments of core LP3-1 are stiff clays below sandy clay
that contains Cerithiidae shells, a coastal/estuarine
family. The basal marine sediment of the core has high
magnetic susceptibility values (Fig. 3), which along with
the shell and sand provide a strong correlating unit
across the basin. Magnetic susceptibility measurements
can determine the presence of iron-bearing minerals
within the sediments, with the susceptibility controlled
by the concentration and grain size of these ferromag-
netic minerals (Thompson et al., 1975). Samples rich in
magnetizable substances, per unit volume, yield high
readings, while samples that are poor in magnetizable
substances, or contain diamagnetic minerals, yield lower
Fig. 3 Magnetic susceptibility measurements and stratigraphic summaries for cores LP3 and LP3-1.
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or negative values. From 700 to 360 cm organic silty clays
alternating with black organic layers dominate the sedi-
ments. The magnetic-susceptibility curve in Fig. 3 shows
that above 360 cm the sedimentation processes changed
significantly, with the sediment being almost pure or-
ganics. It would appear that from 360 cm to the current
surface sedimentation is predominantly from internal
lake productivity. The stratigraphy in combination with
the magnetic measurements also illustrate that the basal
sediments of core LP3 and LP3-1 are offset, but that the
basal sediments of LP3-1 have greater resolution.
The chronology of the sediments has been established
with 15 AMS dates (Table 3), revealing that that the
sediments are Holocene in age with a maximum deter-
mined age of 6570–6950 calibrated (cal) years BP. The
high d13C values in the base of the core are typical of
marine vertebrates, invertebrates and higher plants
(Ariztegui & McKenzie, 1995). An age depth relation-
ship for cores LP3 and LP3-1 is shown in Fig. 4, with the
steepening of the curve after 1600 years possibly asso-
ciated with the higher organic and less consolidated
nature of the sediments. All ages referred to in the text
are calibrated years BP.
The results of the diatom analyses are shown in Fig. 5
and will be reported in full in a forthcoming paper by
Stevenson and colleagues. In summary they reveal that
Diploneis, a marine/saline tolerant genus, is present in
the lower sediments of LP3-1 along with the Cerithiidae
shell. By 6000 BP, however, the system is dominated by
freshwater species. The planktonic taxa dominate the
record and are composed primarily of Aulacoseira,
Cyclotella and Navicula species, while the epiphytic
forms are represented by Cocconeis, Cymbella and
Gomphonema species. Nitzschia species and Diadesmis
confervacia dominate the benthic taxa. The only signifi-
cant changes in species composition occur in the upper
50 cm of unconsolidated lake-bed sediments, when
Cymbella turgida, Eunotia pectinalis, Eunotia praerupta,
Gomphonema clevii, Gomphonema grunowii and Luticola
mutica enter the record for the first time. All are in-
dicative of eutrophy and reflect modern land and lake
use practices at the site. Also of note is the occurrence of
D. confervacia between 670 and 230 cm (5500–1200 BP).
This is an aerophilic or nonpermanent shallow water
diatom that prefers warm alkaline waters (Cocquyt,
1998; Velez et al., 2005). It is able to grow in waters of
high mineral content and is an indicator of intermit-
tently polluted waters when present in large quantities
(Schoeman, 1973; Gasse, 1986). This diatom drops out
with reduced mineral input into the lake system. The
planktonic to epiphytic ratio is used to illustrate chan-
ging water depth through time. Planktonic taxa (includ-
ing the facultative planktonic taxa) can live on a substrate
but more importantly live in the water column with
turbulence, whereas the epiphytic taxa are attached to
plants and by inference suggest shallower water and the
encroachment of littoral habitat on the coring site. The
ratio suggests that water at the coring site was shallow
until around 5500 years ago with water depth fluctuating
since then. A fourier analysis of the ratio data could not
detect any statistically significant cycles within the data.
The pollen concentration for most of the core is low,
particularly in the upper samples. From 670 to 70 cm the
concentration ranges from 15 300 to 1600 grains cm�3
with a mean of 6200 grains cm�3. Below 670 cm the
concentrations range from 8300 to 94 500 grains cm�3,
with a mean of 27 500 grains cm�3. Target pollen counts
of 200 grains were harder to obtain toward the top of the
core as the sediments become increasingly organic,
effectively drowning out the pollen signal on the slides.
Overall the record is dominated by Pinus, Poaceae
and Cyperaceae pollen (Fig. 6). In the modern land-
scape Pinus forest occurs only above 600 m altitude.
Therefore, to disentangle this regional component from
a more local source of pollen, the individual pollen
curves in Fig. 6 are based on a terrestrial pollen sum
that excludes Pinus. The diversity in pollen types from
the site is large, with 135 individual pollen and spore
types counted. The majority of these, however, are
found only occasionally and in very small quantities.
The ratio of pine to all other pollen types has been
determined for 10 lake-bed samples so that the modern
signature of Pinus can be determined for the current
distribution of Pinus in the landscape (Table 4). The
calculation sum used all terrestrial pollen and spore
types. The mean percentage of Pinus pollen across the
seven samples is 24%, with a minimum of 20% and a
high of 36%.
The zonation of the pollen data resulted in four zones.
Each zone is reported with an inferred age range
derived from the age model shown in Fig. 4.
Zone LP3-A: 775–700 cm: inferred age 6500–5500 cal years BP
The pollen of this zone is dominated by Pinus and
Poaceae. Pollen of the aquatic plant Nymphoides only
appears in this zone, along with another aquatic, cf.
Hygrophila, as well as Neonauclea, a tree common along
coastal rivers and the margins of lowland swamps.
Cyperaceae is present but the values are low in compar-
ison with the zones above. Occasional grains of man-
grove pollen were seen, although the Rhizophora values
are too low for the waters to be directly associated with a
mangrove swamp (Grindrod, 1985, 1988; Thanikaimoni,
1987). Other coastal taxa include Lumnitzera and Casuarina.
While herb values are low, fern spore percentages are
relatively high. A range of gymnosperms other than
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Pinus, as well as Lycopodium spores are also found in this
zone. The Lycopodium along with the gymnosperms are
good indicators that there is input from the mountains in
this record as the clubmosses, Phyllocladus, Dacrycarpus
and Podocarpus are all confined to wet forests above
2000 m (Kowal, 1966; Zamora & Co, 1986, de Laubenfels,
1988). Charcoal is consistently high throughout the zone.
The samples that constitute this zone are all from the
base of the duplicate core, LP3-1, and so the identifica-
tion of these samples as a separate zone could be an
artefact of a disconformity with the main core. However,
any disconformity is insufficient to affect the slope of the
age model in Fig. 4.
Zone LP3-B: 700–635 cm: inferred age 5500–4800 cal years BP
From this zone upwards all samples are from the
primary core (LP3) and from sediments above the basal
sandy clay layers. Poaceae and Pinus still dominate the
record in this zone, however Pinus declines dramati-
cally in the top of LP3-B. The other gymnosperms,
Dacrydium, Dacrycarpus, Phyllocladus and Podocarpus,
are also the most diverse and abundant in this zone.
Other montane taxa such as Quercus and Theaceae
pollen are also seen for the first time and then disappear
from the record along with the Pinus. The aquatic taxa
from the previous zone are no longer present, and
Cyperaceae, which starts off at around 70% of the total
pollen sum, falls to levels of around 30% by the top of
the zone. There is also the consistent presence of Con-
vovulaceae cf. Merremia pollen in this zone. Charcoal
particles are abundant, dropping to low levels in the top
of the zone with the disappearance of Pinus pollen.
Zone LP3-C: 635–525 cm: inferred age 4800–3600 cal years BP
Once again the dominant pollen type (excluding Pinus)
is Poaceae at 35–70%. Pinus values increase half way
through this zone, but not to the same levels recorded in
the previous two zones, reaching a maximum of 10% of
the total pollen sum. Most of the other gymnosperms
are absent from this zone, with just single grains of
Phyllocladus and Podocarpus seen in sample 570 cm.
Cyperaceae values oscillate between 5% and 30% of
Table 3 AMS and calibrated radiocarbon ages from core LP3 Paoay Lake
Lab number Depth (cm) d13C Conventional radiocarbon age (1s) Calibrated age years BP (2s)
OxA-V-2023-43 111–112 �20.7 802 � 24 670–760
OZI043 161–162 �25.4 990 � 35 790–1000
OxA-V-2023-44 222–223 �23.2 1299 � 25 1180–1290
OZI044 301–302 �25.2 1670 � 30 1520–1690
OxA-V-2023-45 361–362 �26.5 2208 � 26 2130–2330
ANU – 11918 392–393 �24 2650 � 190 2210–3320
ANU – 11917 440–441 �24 2870 � 180 2500–3470
OZI047 443–444* �24.5 3130 � 60 3170–3470
OxA-V-2023-45 511–512 �23.1 3187 � 27 3360–3470
OZI043 611–612 �23.7 4080 � 60 4420–4820
OZI046 649–650 �24.7 4470 � 40 4920–5300
OZI048 650–651* �24.2 4360 � 50 4830–5210
OxA-V-2023-47 696–697 �22.3 4677 � 29 5320–5570
WK-15837 750–751* �14.6 5567 � 39 5950–6260
OZI049 810–811* �16.0 5940 � 70 6570–6950
*Indicates material from a duplicate core LP3-1. Ages have been calibrated using CALIB 4.4 (Stuvier & Reimer, 2002). In all cases the
material dated was the pollen size fraction which equals the organic material between 10 and 125 mm.
Fig. 4 Age depth relationship for cores LP3 and LP3-1. Hor-
izontal bars indicate � 2 SD.
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Fig
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1680 J . S T E V E N S O N et al.
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the total pollen sum and Potamogeton is present, though
at very low values. There are still small amounts of
Rhizophora, Lumnitzera and Neonauclea pollen, and
Casuarina and Macaranga start to reach consistently
higher values. Charcoal values remain low compared
with those in the previous zones and visual inspection
reveals that most of the charcoal in this zone is grass
cuticle.
Zone LP3-D: 525–70 cm: inferred age 3600–310 cal years BP
Grass still dominates the pollen spectra. In general this
zone differs from the previous zones in having greater
input of pollen from disturbance taxa and in particular
herbs other than grass. The main contributors in this
respect are Trema pollen, Urticaceae, Alternanthera and
Amaranthaceae undiff., Asteraceae, and several fern
taxa. Overall there is less input from Cyperaceae in this
zone, with the average value o10%. Charcoal values
remain low relative to the earlier part of the record.
Grass pollen observations
The grass signature in the Paoay Lake record is likely to
come from several sources, such as the montane pine
forests and the grasslands of the lower slopes and
coastal plain. Of note is the change in the grass pollen
signal (Fig. 7). Up until 2000 BP grass pollen o20 mm in
size are common. After 2000 BP, however, this size class
is virtually absent. A similar trend is also seen in the
Table 4 Percentage Pinus pollen in surface lake-bed sediments
Sample no. Location Pinus (%) Total pollen concentration (grains cm�3)
1 Embayment 20 7300
2 Embayment 20 14 000
3 � 75 m from nearest shore 23 12 600
4 � 75 m from nearest shore 36 16 200
5 � 100 m from nearest shore 24 9950
6 � 100 m from nearest shore 28 9920
7 � 100 m from nearest shore 28 7250
Percentages were calculated on the total pollen sum which included aquatic taxa and ferns.
Fig. 7 Breakdown of Poaceae (grass) pollen percentages by size class.
1682 J . S T E V E N S O N et al.
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20–30 size class, with both curves closely resembling the
Cyperaceae curve and possibly indicating an aquatic or
wetland origin of the smaller grass pollen grains.
The grass understorey of the Pinus forests is primarily
composed of Imperata cylindrical, Miscanthus sinensis,
Themeda triandra and Eulalia quadrinervis and Eulalia
trispicata (Kowal, 1966). As can be seen by the size
ranges listed in Table 5, most of these are in the 40–
50mm range, a size range that is virtually absent be-
tween 5500 and 4200 years ago overlapping with the
loss of Pinus and other montane taxa from 5100 to
4200 BP (Fig. 7).
Grass pollen in the 30–40mm size class is found
throughout the record, and along with the 40–50mm
class comes to dominate the record after 2000 BP. As
pointed to earlier, rice has a size range of 25–49mm, with
a mode of 36mm (Maloney, 1990; Bulalacao, 1997; Wang
et al., 1997). The 450mm size class, which is also found
throughout the record, increases significantly after 750 BP.
Charcoal observations
Pollen slide charcoal (o125 mm) is thought to be domi-
nated by charcoal from a more regional source (Whit-
lock & Larsen, 2001); however, local fires are responsible
for at least some of the microcharcoal in the base of this
sequence as the charcoal particles larger than 125 mm,
which are removed by sieving during the pollen pro-
cessing procedure, are more abundant in samples older
than 5500 BP. Much of this larger charcoal fraction in the
base of the core has wood structure and no doubt
represents the burning of trees and shrubs. By contrast
the charcoal fraction after 5500 BP is quite different,
being more elongate and made up almost entirely by
grass cuticle.
Phytoliths
An aim of the phytolith analysis was to see if any Oryza
(rice) phytoliths could be found in the sediments, given
the difficulties of using rice pollen. Unfortunately no
such phytoliths were found. While the phytolith record
(Fig. 8) mimics the pollen record in many ways, it also
provided additional information. For instance the pre-
valence of palms (Arecaceae) in the landscape was
greater than could be deduced from the pollen record,
and there were also more trees in the immediate vicinity
of the lake than can be inferred from the pollen diagram.
Discussion
Lake formation
The digitate outline of Paoay Lake defines a drowned
river system. The very linear boundary to the west was
attributed by Siringan & Pataray (1997) to a sand bar
spit which grew across and closed a shallow embay-
ment which eventually became the lake. This is sup-
ported by these results, which show that freshwater
sediments only began accumulating in the basin after
6500 BP, around the time of sea level stabilization. It
Table 5 Pollen size ranges for grass species associated with Pinus forests in northern Luzon
Name Altitudinal range of forest association (m) Pollen size range (mm) Modal size (mm)
Miscanthus sinensis 2200–2300 30–40 33
Imperata cylindrica 1000–2300 35–47 41
Themeda triandra 1000–2000 43–54 43
Eulalia trispicata 1000–2000 41–49* 46*
Eulalia quadrinervis 1000 41–49* 46*
*Pollen reference material of E. trispicata and E. quadrinervis were not available. Measurements of Eulalia are therefore based on
Eulalia sp. held in the ANU pollen reference collection.
Fig. 8 Percentage phytolith diagram based on a count of 200
phytoliths for each sample. The nondiagnostic curve is com-
posed of phytoliths that cannot be attributed to any particular
taxon or life-form category.
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would appear that Paoay Lake was beyond the tidal
limit at this time, as the only evidence of saline indica-
tors are in the sediments below 6500 BP that contain
Cerithiidae shells and marine or saline diatoms. The
Nymphoides pollen in the basal sediments after 6500 BP is
also indicative of freshwater conditions. Coastal pro-
gradation and sand dune development since the mid-
Holocene led to the lake’s complete disconnection to the
sea (Siringan & Pataray, 1997).
The Paoay Lake bed sediments are currently around
15 m a.s.l., with the transition from marine to freshwater
sediments occurring below 7 m in core LP3. Therefore
these marine/saline sediments are now 8 m above pre-
sent sea level. Uplift due to tectonism and eustatic sea
level fall (Maeda et al., 2004) have contributed to the
present elevation of Paoay Lake and may have also
influenced the sedimentation processes within the lake.
In addition the diatom record suggests that lake levels
have fluctuated over time, though apparently not in a
cyclic manner.
Vegetation history
It would appear that there are two palaeovegetation
records within the lake sediments, one that represents
processes taking place in the Central Cordillera and
Ilocos Mountains, some distance from the site, and then
the record of the coastal plain itself. A still widely
accepted model of forest development for the moun-
tains of northern Luzon was first put forward by Kowal
(1966) and suggests that the original vegetation of the
Central Cordillera, before being disturbed by people,
was likely a broadleaf forest. That is, lowland rainforest
at lower altitudes grading into tropical montane forest
and cloud forest at higher elevations (Fig. 9). The model
suggests that pines had a limited distribution within the
montane forests, behaving primarily as pioneers and
expanding into gaps created by natural or human
disturbances (Kowal, 1966; Goldammer & Penafiel,
1990). In other words, the Pinus savannas of today are
considered to be a product of intensified human dis-
turbance. However, if the modern day pine pollen
signature in Paoay Lake is representative of how abun-
dant Pinus is in the montane landscape, then the present
day distribution may have analogues in the past.
Prior to 5000 BP pine pollen accounts for 10–50% of the
pollen rain to the lake, a range that overlaps with the
present day amounts of 20–36%. Interestingly these
early high values of Pinus also coincide with the highest
values of charcoal accumulation in the record. At
5000 BP, however, something in the system changes
and pine pollen virtually disappears as does the char-
coal. The corresponding high levels of Pinus pollen and
charcoal in the base of the record, in combination with
what we know about the relationship of Pinus with fire,
suggests that these two records may be related. How-
ever, when Pinus values increase after 4200 BP, the
charcoal values remain low. Our observations of the
different size fractions of charcoal, in particular the
fraction larger than 125 mm, reveal that the biomass
being burnt around the lake changes significantly after
5000 BP, with a shift from woody to grass-dominated
charcoal particles. This may be indicative of a shift to a
drier climate after 5000 BP and hence a lower accumula-
tion of woody fuel within the vicinity of the lake.
Although Pinus values increase after 4200 BP the per-
centage input does not return to pre-5000-year levels
until after 1000 BP. Also of note is that the other montane
taxa, found earlier in the record in association with the
Pinus (Dacrycarpus, Dacrydium, Phyllocladus, Podocarpus,
Quercus, Eugenia, Theaceae and Tiliaceae) only appear
sporadically in the record after 4200 BP, suggesting that
the distribution of montane taxa differs before and after
5000 BP.
Fig. 9 Kowal’s model of possible forest distributions in the Central Cordillera of Luzon, (a) scattered occurrence of Pinus kesiya in the
submontane/montane broadleaf dipetrocarp forest, (b) present expansion of the pine savanna, (c) proposed future retreat of pine
savanna if submontane/montane forests are protected from fire (After Kowal 1966).
1684 J . S T E V E N S O N et al.
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Pertinent to our understanding of the vegetation
changes in the lake record are sea surface temperature
and sea surface salinity records being developed from
corals approximately 50 km to the south of Paoay Lake.
From this location fossil Porites that span the mid-
Holocene have been analysed for d18O and produced
records that indicate the sea off north-western Luzon
between 6100 and 4000 BP was warmer than present by
as much as 1.61 as well as more saline, suggesting a
decrease in runoff and hence precipitation (Yokoyama
et al., 2006; Kobayashi et al., 2007). Ocean core 17927-2
off the west coast of northern Luzon (see Fig. 1a)
records a similar drop in d18O during the mid-Holocene
with the UK37 index data also suggesting increased sea
surface salinity (Wang et al., 2005). It would appear from
these two sets of data that north-western Luzon was
warmer during the mid-Holocene and possibly drier.
At a more regional scale the speleothem data from
Dongge Cave in southern China (Wang et al., 2005) also
suggest the region may have been drier after 7000 BP.
This record reveals the gradual weakening of the Asian
Monsoon from 7000 BP to around 1000 BP. After 1000 BP
the strength of the monsoon once again increases. The
overall weakening between 7000 and 1000 BP is thought
to be in response to orbitally induced lowering of the
Northern Hemisphere summer insolation (Wang et al.,
2005), but there are several periods of weakening em-
bedded within this time frame, the most pronounced
being a 500-year period centred around 4400 yrs BP that
overlaps with the Pinus decline in the Paoay Lake
pollen record as well as a period of lower lake levels.
We know from the instrumental record that warmer
sea surface temperatures accompany El Nino years and
that these events lead to reduced precipitation during
the monsoon months and increased stress to forests the
montane forests of Luzon through an increase in tem-
perature during the dry months (Moya & Malayang,
2004). Recent research reports indicate that that rising
global temperatures are leading to increased tree mor-
tality within the coniferous forests of north-west Amer-
ica (van Mantgem et al., 2009). Therefore, the vegetation
changes observed in the Paoay Lake record may well be
related to some of the climatic observations made for the
broader region, as warmer temperatures in combination
with a weakening monsoon would no doubt have
adversely affected both the pine and upper montane
forests during the mid-Holocene. As conditions became
cooler toward the present Pinus forests may have ex-
panded, although not to same extent until conditions
also became wetter after 1000 BP. However, it is also
worth considering that a weaker Asian Monsoon may
also have lead to the pollen transport mechanism being
disrupted. That is, weaker south-east winds may have
resulted in less pine pollen and other montane taxa
being carried to the lake rather than widespread dis-
ruption to these vegetation zones. It seems likely, how-
ever, that the larger accumulation values of charcoal in
the early part of the record were probably the result of a
greater biomass in the lowland landscape when climate
was warmer and wetter, with the following period of
warm dry conditions leading to a decrease in biomass
and as a result a decrease in charcoal accumulation.
When the gymnosperm and aquatic taxa are re-
moved, the remainder of the record has the appearance
of a tropical coastal savanna dominated palynologically
by grass, Casuarina, Macaranga, possibly coastal Ficus
species (recorded as Moraceae/Urticaceae), along with
various coastal herbs such as the Amaranthaceae,
including Alternanthera, and various Convovulaceae
including the strand plant Merremia. All are common
elements outside of the cultivated areas in this lowland
landscape and suggest a significant degree of stability
over this time period.
Today the coastal vegetation of Ilocos Norte has been
heavily modified by human activities, but how long
people have been practicing agriculture in this region is
still an open question. One theory of agricultural devel-
opment for the Philippines suggests that Neolithic
people expanded out of Taiwan and into northern
Luzon during the Holocene, bringing with them rice
agriculture (Bellwood, 1997). While extensive evidence
demonstrates a cultural connection between Taiwan
northern Luzon from around 4000 BP (Bellwood et al.,
2003; Bellwood & Dizon, 2005; Hung, 2005) no plant
remains have been recovered, and direct evidence for
agricultural strategies is slight. In addition, there is no
archaeological record for this period of prehistory in the
north-west of Luzon. A recent archaeological survey for
the Laoag/Paoay region drew the conclusion that,
although iron-age pottery and other artefacts are com-
mon in a number of locales, most of the Neolithic
archaeology is probably now buried under many
metres of sediment in the massive alluvial plains that
constitute most of the flat land in the region (Bellwood
et al., 2008). At this stage, however, there is no substan-
tive chronology for the alluvial deposition.
Despite several different approaches to tackling the
agriculture question with the Paoay Lake sediments, we
have been unable to shed light on this subject for north-
western Luzon. The most tantalizing result is that
although pollen of the disturbance taxa Trema and
Urticaceae are found throughout the record, there is a
significant increase in these pollen types from 3500 to
1500 BP. The intriguing question, given the overlap with
the Neolithic period on the island, is whether or not this
disturbance could be associated with human impact.
However, the causes behind this increase in disturbance
taxa remain unresolved.
H O L O C E N E E N V I R O N M E N T A L C H A N G E 1685
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There is only one other pollen record from the Phi-
lippines, Laguna de Bay in southern Luzon (Ward &
Bulalacao, 1999). A summary of this 10 m sediment core,
which covers the last 7000 years, is shown alongside the
Paoay Lake record in Fig. 10. As Laguna de Baye is
situated to the south of the Central Cordillera (Fig. 1) it
has minimal representation of pine pollen due to the
prevailing south-westerly winds. However, while the
vegetation composition of the two records differ, they
have an interesting parallel in forest decline at around
5000 BP in the absence of any increase in fire. They also
differ in that Laguna de Bay has a much stronger
relationship between charcoal and grass, with both
increasing after 2500 BP. At the time, Ward & Bulalacao
(1999) found the interpretation of the forest decline after
5000 BP problematic, but concluded that it was best
attributed to the regional climate becoming dryer and
that the contribution of human activities to this process
would require study of fire regimes from prehuman
horizons. Likewise, interpretation of the Paoay Lake
record will become more comprehensible as further
palaeoecological records in our research program are
Fig. 10 A comparison of the pollen percentage data and charcoal concentrations for Laguna de Baye (Ward and Bulalacao 1999) and
Paoay Lake. The percentage sum for both sites is based on a pollen sum that excludes Cyperaceae and ferns. The percentage calculations
for these categories are outside the pollen sum.
1686 J . S T E V E N S O N et al.
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developed for the region, in particular from the Caga-
yan Valley and Mountains Province in the Central
Cordillera.
Conclusions
The Philippine archipelago is an important location in
island south-east Asia for understanding the evolution
of the tropical climate system as well as the movement
of people through time, yet there is virtually no knowl-
edge of its palaeoenvironmental history. The record
from Paoay Lake therefore makes a fundamental con-
tribution to our understanding of this under-repre-
sented region.
Most climate change predictions for this region sug-
gest that temperatures will increase and annual rainfall
decrease over the coming decade (Cruz et al., 2007). In
addition to impacts on the human population we know
that an increase in temperature and decrease in rainfall
will also have harmful consequences for the montane
biota of Luzon. Human activities in association with the
widespread use of fire are already reducing cloud forest
habitat, and if climate change forecasts are realized,
then even more pressure will be placed on these fragile
habitats as clearance activities move higher into the
mountains and fires become more intense. The results
from this study suggest that the montane forest systems
were altered between 5000 and 4200 years ago, possibly
by a period of higher temperatures and lower rainfall.
Although the forests appear to have slowly recovered
over the next 3000 years, they did so in the absence of
the intense human activities of the modern era.
While we cannot be precise about the causes of
environmental change in the mountains of Luzon 5000
years ago, these preliminary results are still informative
and raise a number of questions for further research,
which include the following:
(1) Was Pinus forest more extensive in the northern
Luzon landscape before 5000 BP?
(2) Is there a regional shift in lowland biomass from
woody to more herbaceous vegetation after 5000 BP?
(3) Are the changes observed between 5000 and 4200 BP
a result of a weakened Asian Monsoon?
(4) Can human activities be related to any of the
observed vegetation changes?
The immediate aim of our ongoing research is to try and
resolve these questions by analysing a lowland site on
the windward side of the Central Cordillera as well as
several lakes within the cloud forest of the Central
Cordillera. With this additional data we hope then to
provide a more comprehensive view of how, in particu-
lar, the mountain landscapes of Luzon have responded
to climate change in the past and thereby provide an
assessment of how they might cope in the future.
Acknowledgements
We thank the office of former Governor Ferdinand Marcos Jr. andGovernor Michael Marcos Keon for their assistance, as well asDENR Laoag City for permission to work at the lake. Thank youto Ludivino Agressor, DENR Paoay Lake, and to Damien Kelle-her, Tony Penalosa, Gerald Quina and Alex Pataray for assis-tance with the lake coring. UP Diliman kindly made available theuse of a vehicle and boat. The National Museum of the Philip-pines and Corazon S. Alvina, Director of the National Museumare thanked for facilitating our research in the Philippines. JeffParr undertook the phytolith analysis and research support wasprovided by the Australian Research Council (DP0208831) andAINSE (AINGRA05156). Thanks also to ANU Cartography fortheir excellent work on the figures and to Edward Cushing andtwo anonymous referees who all made suggestions that greatlyimproved the manuscript.
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