Paoay Lake, northern Luzon, the Philippines: a record of ...scinet.dost.gov.ph/union/Downloads/ACD...

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:

[email protected]

Global Change Biology (2010) 16, 1672–1688, doi: 10.1111/j.1365-2486.2009.02039.x

1672 r 2009 Blackwell Publishing Ltd

mailto:[email protected]

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.

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 1673

r 2009 Blackwell Publishing Ltd, Global Change Biology, 16, 1672–1688

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.

1674 J . S T E V E N S O N et al.


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.



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



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.



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



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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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



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.



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.



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).



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.



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.



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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