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

Paoay , northern , the : a record of Holocene environmental change

JANELLE STEVENSON*, FERNANDO SIRINGANw ,JANFINN*,DOMINGOMADULIDz andHENK HEIJNIS§ *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 Lake and its surrounding landscape is examined through the analysis of pollen, diatoms, charcoal, mineral magnetics and AMS dating. Basal sediments contain shells of Cerithiidae and the saline-tolerant diatom Diploneis indicating that this was an estuarine environment before becoming a freshwater lake after 6000 BP. Pollen analysis shows that submontane forests, characterized by Pinus pollen, underwent a major disturbance around 5000 years ago, recovering to previous levels by 1000 years ago. Charcoal as an indicator of fire is abundant throughout record, although the highest levels occur in the earlier part of the record, between 6500 and 5000 years ago. An aspect of the project was to examine whether there is evidence of land clearance and agricultural development in the region during the late Holocene. While a clear signal of human impact in the record remains equivocal, there appears to be a correspondence between submontane forest decline and mid-Holocene ocean data that depict warmer and possibly drier conditions for the region. The study highlights the vulnerability of these montane forests 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

that have taken place since the mid-Holocene in north- Introduction western Luzon and makes a significant contribution to The Philippine archipelago stretches from the wet tro- the study of climate change in the western Pacific. 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 Environmental setting the evolution of the Asian Monsoon or the impact of is situated in north-western Luzon ENSO, it has few palaeoenvironmental studies of any (181070N, 1201320E) (Figs 1 and 2) along the western description and only one palynological study (Ward & edge of the Ilocos lowland, a tectonic depression related Bulalacao, 1999). While the central aim of study is to to Late Pliocene to Quaternary activity of the Philippine document environmental change during the Holocene Fault (Pinet & Stephan, 1990). Coastal progradation and in the northern Philippines, one of the associated re- the subsequent development of a sand dune barrier search questions is to assess if any of the changes are during the mid-Holocene are believed to have led to the related to human activity. In particular land clearance formation of the lake (Siringan & Pataray, 1997) and it is and the development of rice agriculture during the now separated from the sea along its western edge by a Neolithic, as the timing and development of this is an sand dune complex approximately 2.3 km wide and unresolved question for the Philippines (Bellwood, with an average elevation of 40 m. The Upper Pleisto- 2005; Bellwood & Dizon, 2005). Our study therefore cene formation bounds the lake in all other provides unique information on the landscape changes directions (N. P. Punzal et al., unpublished results). North-western Luzon has a monsoon climate, with a Correspondence: Janelle Stevenson, fax 1 61 2 612 549 17, e-mail: dry season in the lowlands from November to April and [email protected] a wet season from May to October (Argete, 1998).

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Fig. 1 (a) Locality map of Paoay Lake and other locations mentioned in text. (b) Map illustrating the topography of the region.

Average annual rainfall is around 2000 mm for the intense fires reducing the number of pine trees found at lowlands and 44000 mm for the upper montane re- their lower altitudinal range (Kowal, 1966). Although gions of the Central Cordillera, the main mountain there are few old growth pine forests left in Luzon, pine range of northern Luzon. The average annual tempera- savannas are increasing in extent due to the expansion ture is 28 1C for the lowlands and around 15 1C for the of human activities into the mountains and the asso- upper montane zone. Rainfall shortages associated with ciated prevalence of fire. Above the pine forests, be- the ENSO phenomenon occur during the wet season. tween 2000 and 2600 m, are the cloud or moss forests During the late 20th century logging throughout the and it is this vegetation type in particular that is being Ilocos Mountains and Central Cordillera led to massive heavily impacted by the expansion of market gardening landscape transformations; primarily the expansion of throughout the mountains of Luzon. grassland and Pinus forest and the aggradation of river The landscape around the Paoay Lake itself is essen- valleys. In the present day landscape, Pinus kesiya, the tially a cultivated one, constituted primarily by herbac- pine species of northern Luzon, is common above an eous crops, cultivated trees and weeds, with some altitude of 600 m, with the bulk of these forests found on patches of lowland secondary vegetation that are steep slopes between 1000 and 2000 m altitude (Kowal, thought to be natural remnants. The taxa most com- 1966; Zamora & Co, 1986). Figure 1b illustrates the monly occurring in these vegetation remnants are listed extent of this mountainous terrain in northern Luzon in Table 1. During the dry season the exposed shoreline and its relationship to Paoay Lake. is heavily utilized for growing a variety of crops, Pine forests in the Philippines have a grass under- including rice, and fish farming is carried out within storey, and like pine forests throughout the world, are the lake itself. In general, human population pressure maintained by fire, as low intensity fires prevent the and agricultural expansion have heavily transformed establishment of hardwood seedlings (Kowal, 1966; the lowland landscape of . Goldammer & Pen˜afiel, 1990; Richardson & Rundel, 1998). Today the pine forests sit above what is a heavily Site description modified human landscape, which outside of the irri- gated valley floors is dry and harsh with skeletal and Paoay Lake has a surface area of approximately 4.0 km2 easily eroded soils. It is a landscape that is regularly and a relatively small watershed of around 7.5 km2.At burnt to promote palatable regrowth for livestock, with the end of the rainy season the lake has an average r 2009 Blackwell Publishing Ltd, Global Change Biology, 16, 1672–1688 1674 J. STEVENSON et al.

Fig. 2 Site map of Paoay Lake showing coring locations and general landscape attributes.

water depth of around 4.5 m and the surface of the lake Materials and methods is around 20 m above mean sea level. Today the lake has no outflow, but early topographic maps show that it Preliminary sediment coring was undertaken and ma- once flowed into the Quiaoit River to the south when terial collected from two sites, LP2 and LP3 (Fig. 2). water levels rose above the 18 m contour, joining the These two cores are from separate embayments on the larger Lawa River before flowing out to the sea. During landward lake margin. Sediments at both locations the 1960s or 1970s, in conjunction with the building of a were recovered using a Livingstone corer with the regional irrigation network, this outflow was dammed water depth at each location being just 41 m at the raising the water level during the wet season by 2–3 m. end of the dry season. Over 6 m of sediment were However, during the dry season, the combined effects collected at each core location; stiff clays prevented of evaporation and extraction of water for irrigation deeper sediment collection. On a return trip a duplicate drops the water level back to the naturally occurring core (LP3-1) of the deeper sediment at LP3 was col- dry season level, which is below the take-off level for lected with a GEOCORER, a modified Livingstone corer the irrigation network. that allows the sampling barrel to be hammered into the

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Table 1 Commonly occurring taxa within secondary vegeta- LP3 and the samples were processed using standard tion remnants around Paoay Lake techniques that included oxidation (30% H2O2) and removal of soluble salts and carbonates (10% HCl) Family Genus and species (Battarbee et al., 2001). Apocynaceae Wrightia laniti (Blco.) Merr. The pollen diagrams are percentage diagrams plotted Arecaceae Corypha elata Roxb. using the program C2 (Juggins, 2003). Northern Luzon Capparidaceae Capparis micrantha DC. has a large and diverse flora which, in combination with Casuarinaceae Casuarina equisetifolia L. a very modest pollen reference collection and scarce Cycadaceae Cycas edentata de Laub. (endemic) published material for the region, limits the level of Euphorbiaceae Macaranga tanarius (L.) Muell.-Arg. identification possible. As a result there are recurring Euphorbiaceae Melanolepis multiglandulosa (Reinw.) pollens types that are not yet identified, but are instead Reichb.f.& Zoll. Leguminosae Leucaena leucocephala (Lamk.) de Wit categorized by number. Accounting for unknown pol- (introduced) len types in this way ensures that the diversity within Leguminosae Pterocarpus indicus Willd. Subsp. indicus the pollen record is not lost. The unknown types are Loganiaceae Fagraea obovata Boedj. quite different to the ‘indeterminate’ category that is Moraceae Ficus nota (Blco.) Merr. composed of damaged and crumpled grains and grains Moraceae Ficus ulmifolia Lam. that were seen infrequently. Poaceae Bambusa vulgaris Schrad. ex J.C.Wendl. The individual pollen curves are based on a terrestrial Poaceae Schizostachyum lumampao (Blco.) Merr. pollen sum that excludes fern spores, aquatic pollen (endemic) and Pinus, as this pollen type overwhelms the pollen Rubiaceae Morinda citrifolia L. rain in the region. Only taxa that acquired a value of 1% Rubiaceae Nauclea orientalis (L.) L. in at least one sample are plotted as they are considered Ulmaceae Trema orientalis (L.) Bl. to carry the bulk of the interpretative information. Although rice cultivation is of interest to this project, sediments. At the same time the unconsolidated sedi- the identification of rice from the pollen record is ments 470 cm were collected with a mud/water inter- difficult because rice pollen is morphologically similar face sampler. Once back in the lab cores LP2 and LP3 to most other grass types. Inferences about cultivated were described and run through a magnetic suscept- species in the grass family are easily made for other ibility loop to produce a magnetic profile. Core LP3 was parts of the world because crops such as wheat and chosen as the core for more detailed analysis as it had maize have particularly large pollen grains. Cultivated the greatest depth recovered. rice, however, has a size range of around 25–49 mm, with Age determinations for LP3 and LP3-1 were carried out a mode of 36 mm (Maloney, 1990; Bulalacao, 1997; Wang on samples that were given a standard acid–base–acid et al., 1997). This range includes the median size for pretreatment, which also included HF to remove the grass pollen of all species. Grass pollen grains in this mineral component, and sieving at 10 and 125 mm. This study were therefore measured and placed in a size resulted in an organic fraction referred to as the pollen class to assess whether the diversity of these categories size fraction that was radiocarbon dated using AMS. changed any at point in the record. Pollen analyses were undertaken on core LP3 using To explore rice cultivation further, a phytolith study standard acetolysis processing techniques and with was also undertaken. Phytoliths are biogenic silica laid approximately 23 000 exotic Lycopodium spores added down in certain plant cells and are most abundant in to each sample so that the pollen concentration could be grasses; rice phytoliths are diagnostic and can be dis- calculated (Bennett & Willis, 2001). Charcoal on the tinguished from other grasses (Pearsall et al., 1995). pollen slides was also counted as an indicator of fire Sediment samples from the contemporary lake sedi- in the landscape with the concentration of charcoal ment surface were also collected to better understand the calculated using the exotic marker method (Bennett & modern pollen rain signature. However, because the Willis, 2001). Only black, opaque angular particles pollen content in these samples is very low and many of 410 mm were counted as charcoal. the identifications still uncertain, at this stage only the ratio The pollen samples were subsampled at 10 cm inter- of pine pollen to all other pollen has been determined. vals from 70 to 695 cm in core LP3 and from 725 to All statistical analyses were carried out within PSIM- 770 cm in core LP3-1. Pollen analysis of the unconsoli- POLL (Bennett, 2001). Numerical zonation of the pollen dated sediments 470 cm has been halted as the pollen data used optimal splitting by sum of squares analysis. concentrations are extremely low making a target count This was based on the terrestrial pollen sum and of even 100 grains difficult to obtain. The diatom included only those taxa with a value of 1% in at least assemblage was analysed at 10 cm intervals for core one sample. r 2009 Blackwell Publishing Ltd, Global Change Biology, 16, 1672–1688 1676 J. STEVENSON et al.

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 magnetic susceptibility values (Fig. 3), which along with the shell and sand provide a strong correlating unit A detailed stratigraphic description of core LP3/LP3-1 is across the basin. Magnetic susceptibility measurements reported in Table 2 revealing that the sedimentation can determine the presence of iron-bearing minerals history of the lake has been fairly complex, although within the sediments, with the susceptibility controlled many of the changes are quite subtle and relate to by the concentration and grain size of these ferromag- varying silt and clay contents. In summary, the basal netic minerals (Thompson et al., 1975). Samples rich in sediments of core LP3-1 are stiff clays below sandy clay magnetizable substances, per unit volume, yield high that contains Cerithiidae shells, a coastal/estuarine readings, while samples that are poor in magnetizable family. The basal marine sediment of the core has high substances, or contain diamagnetic minerals, yield lower

Fig. 3 Magnetic susceptibility measurements and stratigraphic summaries for cores LP3 and LP3-1. r 2009 Blackwell Publishing Ltd, Global Change Biology, 16, 1672–1688 1678 J. STEVENSON et al. or negative values. From 700 to 360 cm organic silty clays turbulence, whereas the epiphytic taxa are attached to alternating with black organic layers dominate the sedi- plants and by inference suggest shallower water and the ments. The magnetic-susceptibility curve in Fig. 3 shows encroachment of littoral habitat on the coring site. The that above 360 cm the sedimentation processes changed ratio suggests that water at the coring site was shallow significantly, with the sediment being almost pure or- until around 5500 years ago with water depth fluctuating ganics. It would appear that from 360 cm to the current since then. A fourier analysis of the ratio data could not surface sedimentation is predominantly from internal detect any statistically significant cycles within the data. lake productivity. The stratigraphy in combination with The pollen concentration for most of the core is low, the magnetic measurements also illustrate that the basal particularly in the upper samples. From 670 to 70 cm the sediments of core LP3 and LP3-1 are offset, but that the concentration ranges from 15 300 to 1600 grains cm3 basal sediments of LP3-1 have greater resolution. with a mean of 6200 grains cm3. Below 670 cm the The chronology of the sediments has been established concentrations range from 8300 to 94 500 grains cm3, with 15 AMS dates (Table 3), revealing that that the with a mean of 27 500 grains cm3. Target pollen counts sediments are Holocene in age with a maximum deter- of 200 grains were harder to obtain toward the top of the mined age of 6570–6950 calibrated (cal) years BP. The core as the sediments become increasingly organic, high d13C values in the base of the core are typical of effectively drowning out the pollen signal on the slides. marine vertebrates, invertebrates and higher plants Overall the record is dominated by Pinus, Poaceae (Ariztegui & McKenzie, 1995). An age depth relation- and Cyperaceae pollen (Fig. 6). In the modern land- ship for cores LP3 and LP3-1 is shown in Fig. 4, with the scape Pinus forest occurs only above 600 m altitude. steepening of the curve after 1600 years possibly asso- Therefore, to disentangle this regional component from ciated with the higher organic and less consolidated a more local source of pollen, the individual pollen nature of the sediments. All ages referred to in the text curves in Fig. 6 are based on a terrestrial pollen sum are calibrated years BP. that excludes Pinus. The diversity in pollen types from The results of the diatom analyses are shown in Fig. 5 the site is large, with 135 individual pollen and spore and will be reported in full in a forthcoming paper by types counted. The majority of these, however, are Stevenson and colleagues. In summary they reveal that found only occasionally and in very small quantities. Diploneis, a marine/saline tolerant genus, is present in The ratio of pine to all other pollen types has been the lower sediments of LP3-1 along with the Cerithiidae determined for 10 lake-bed samples so that the modern shell. By 6000 BP, however, the system is dominated by signature of Pinus can be determined for the current freshwater species. The planktonic taxa dominate the distribution of Pinus in the landscape (Table 4). The record and are composed primarily of Aulacoseira, calculation sum used all terrestrial pollen and spore Cyclotella and Navicula species, while the epiphytic types. The mean percentage of Pinus pollen across the forms are represented by Cocconeis, Cymbella and seven samples is 24%, with a minimum of 20% and a Gomphonema species. Nitzschia species and Diadesmis high of 36%. confervacia dominate the benthic taxa. The only signifi- The zonation of the pollen data resulted in four zones. cant changes in species composition occur in the upper Each zone is reported with an inferred age range 50 cm of unconsolidated lake-bed sediments, when derived from the age model shown in Fig. 4. Cymbella turgida, Eunotia pectinalis, Eunotia praerupta, Gomphonema clevii, Gomphonema grunowii and Luticola Zone LP3-A: 775–700 cm: inferred age 6500– mutica enter the record for the first time. All are in- 5500 cal years BP dicative of eutrophy and reflect modern land and lake use practices at the site. Also of note is the occurrence of The pollen of this zone is dominated by Pinus and D. confervacia between 670 and 230 cm (5500–1200 BP). Poaceae. Pollen of the Nymphoides only This is an aerophilic or nonpermanent shallow water appears in this zone, along with another aquatic, cf. diatom that prefers warm alkaline waters (Cocquyt, Hygrophila,aswellasNeonauclea, a tree common along 1998; Velez et al., 2005). It is able to grow in waters of coastal rivers and the margins of lowland swamps. high mineral content and is an indicator of intermit- Cyperaceae is present but the values are low in compar- tently polluted waters when present in large quantities ison with the zones above. Occasional grains of man- (Schoeman, 1973; Gasse, 1986). This diatom drops out grove pollen were seen, although the Rhizophora values with reduced mineral input into the lake system. The are too low for the waters to be directly associated with a planktonic to epiphytic ratio is used to illustrate chan- mangrove swamp (Grindrod, 1985, 1988; Thanikaimoni, ging water depth through time. Planktonic taxa (includ- 1987). Other coastal taxa include Lumnitzera and Casuarina. ing the facultative planktonic taxa) can live on a substrate While herb values are low, fern spore percentages are but more importantly live in the water column with relatively high. A range of gymnosperms other than

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Table 3 AMS and calibrated radiocarbon ages from core LP3 Paoay Lake

13 Lab number Depth (cm) d C 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.

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 Fig. 4 Age depth relationship for cores LP3 and LP3-1. Hor- pollen sum, falls to levels of around 30% by the top of izontal bars indicate 2 SD. 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. 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 Zone LP3-C: 635–525 cm: inferred age 4800– this record as the clubmosses, Phyllocladus, Dacrycarpus 3600 cal years BP and Podocarpus are all confined to wet forests above 2000 m (Kowal, 1966; Zamora & Co, 1986, de Laubenfels, Once again the dominant pollen type (excluding Pinus) 1988). Charcoal is consistently high throughout the zone. is Poaceae at 35–70%. Pinus values increase half way The samples that constitute this zone are all from the through this zone, but not to the same levels recorded in base of the duplicate core, LP3-1, and so the identifica- the previous two zones, reaching a maximum of 10% of tion of these samples as a separate zone could be an the total pollen sum. Most of the other gymnosperms artefact of a disconformity with the main core. However, are absent from this zone, with just single grains of any disconformity is insufficient to affect the slope of the Phyllocladus and Podocarpus seen in sample 570 cm. age model in Fig. 4. Cyperaceae values oscillate between 5% and 30% of r 2009 Blackwell Publishing Ltd, Global Change Biology, 16, 1672–1688 1680 .STEVENSON J. tal et . r 09BakelPbihn Ltd, Publishing Blackwell 2009 lblCag Biology Change Global

, Fig. 5 Percentage diagram of selected diatom taxa. The planktonic: epiphytic ratio as an indicator of relative water depth is also shown. 16 1672–1688 , r 09BakelPbihn Ltd, Publishing Blackwell 2009 lblCag Biology Change Global , 16 OOEEEVRNETLCHANGE ENVIRONMENTAL HOLOCENE 1672–1688 ,

Fig. 6 Percentage pollen diagrams for Paoay Lake. Hollow curves are based on the total pollen sum that includes ferns and aquatics. The solid pollen curves are based on a terrestrial 1681 pollen sum that excludes Pinus, fern spores and aquatic pollen (including Cyperaceae). Only taxa with a value of 1% in at least one sample are plotted and triangles have been used for taxa that have values consistently o5%. The diagram also includes charcoal accumulation and sedimentation rates. 1682 J. STEVENSON et al.

Table 4 Percentage Pinus pollen in surface lake-bed sediments

Sample no. Location Pinus (%) Total pollen concentration (grains cm3)

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. the total pollen sum and Potamogeton is present, though respect are Trema pollen, Urticaceae, Alternanthera and at very low values. There are still small amounts of Amaranthaceae undiff., Asteraceae, and several fern Rhizophora, Lumnitzera and Neonauclea pollen, and taxa. Overall there is less input from Cyperaceae in this Casuarina and Macaranga start to reach consistently zone, with the average value o10%. Charcoal values higher values. Charcoal values remain low compared remain low relative to the earlier part of the record. with those in the previous zones and visual inspection reveals that most of the charcoal in this zone is grass cuticle. Grass pollen observations

The grass signature in the Paoay Lake record is likely to Zone LP3-D: 525–70 cm: inferred age 3600– come from several sources, such as the montane pine 310 cal years BP forests and the grasslands of the lower slopes and Grass still dominates the pollen spectra. In general this coastal plain. Of note is the change in the grass pollen zone differs from the previous zones in having greater signal (Fig. 7). Up until 2000 BP grass pollen o20 mmin input of pollen from disturbance taxa and in particular size are common. After 2000 BP, however, this size class herbs other than grass. The main contributors in this is virtually absent. A similar trend is also seen in the

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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.

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– 50 mm 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 30–40 mm size class is found throughout the record, and along with the 40–50 mm class comes to dominate the record after 2000 BP.As pointed to earlier, rice has a size range of 25–49 mm, with amodeof36mm (Maloney, 1990; Bulalacao, 1997; Wang et al., 1997). The 450 mm size class, which is also found Fig. 8 Percentage phytolith diagram based on a count of 200 throughout the record, increases significantly after 750 BP. phytoliths for each sample. The nondiagnostic curve is com- posed of phytoliths that cannot be attributed to any particular Charcoal observations taxon or life-form category. Pollen slide charcoal (o125 mm) is thought to be domi- nated by charcoal from a more regional source (Whit- (Fig. 8) mimics the pollen record in many ways, it also lock & Larsen, 2001); however, local fires are responsible provided additional information. For instance the pre- for at least some of the microcharcoal in the base of this valence of palms (Arecaceae) in the landscape was sequence as the charcoal particles larger than 125 mm, greater than could be deduced from the pollen record, which are removed by sieving during the pollen pro- and there were also more trees in the immediate vicinity cessing procedure, are more abundant in samples older of the lake than can be inferred from the pollen diagram. than 5500 BP. Much of this larger charcoal fraction in the base of the core has wood structure and no doubt Discussion represents the burning of trees and shrubs. By contrast the charcoal fraction after 5500 BP is quite different, Lake formation being more elongate and made up almost entirely by grass cuticle. 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 Phytoliths spit which grew across and closed a shallow embay- An aim of the phytolith analysis was to see if any Oryza ment which eventually became the lake. This is sup- (rice) phytoliths could be found in the sediments, given ported by these results, which show that freshwater the difficulties of using rice pollen. Unfortunately no sediments only began accumulating in the basin after such phytoliths were found. While the phytolith record 6500 BP, around the time of sea level stabilization. It r 2009 Blackwell Publishing Ltd, Global Change Biology, 16, 1672–1688 1684 J. STEVENSON et al. would appear that Paoay Lake was beyond the tidal expanding into gaps created by natural or human limit at this time, as the only evidence of saline indica- disturbances (Kowal, 1966; Goldammer & Pen˜afiel, tors are in the sediments below 6500 BP that contain 1990). In other words, the Pinus savannas of today are Cerithiidae shells and marine or saline diatoms. The considered to be a product of intensified human dis- Nymphoides pollen in the basal sediments after 6500 BP is turbance. However, if the modern day pine pollen also indicative of freshwater conditions. Coastal pro- signature in Paoay Lake is representative of how abun- gradation and sand dune development since the mid- dant Pinus is in the montane landscape, then the present Holocene led to the lake’s complete disconnection to the day distribution may have analogues in the past. sea (Siringan & Pataray, 1997). Prior to 5000 BP pine pollen accounts for 10–50% of the The Paoay Lake bed sediments are currently around pollen rain to the lake, a range that overlaps with the 15 m a.s.l., with the transition from marine to freshwater present day amounts of 20–36%. Interestingly these sediments occurring below 7 m in core LP3. Therefore early high values of Pinus also coincide with the highest these marine/saline sediments are now 8 m above pre- values of charcoal accumulation in the record. At sent sea level. Uplift due to tectonism and eustatic sea 5000 BP, however, something in the system changes level fall (Maeda et al., 2004) have contributed to the and pine pollen virtually disappears as does the char- present elevation of Paoay Lake and may have also coal. The corresponding high levels of Pinus pollen and influenced the sedimentation processes within the lake. charcoal in the base of the record, in combination with In addition the diatom record suggests that lake levels what we know about the relationship of Pinus with fire, have fluctuated over time, though apparently not in a suggests that these two records may be related. How- cyclic manner. 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 Vegetation history fraction larger than 125 mm, reveal that the biomass It would appear that there are two palaeovegetation being burnt around the lake changes significantly after records within the lake sediments, one that represents 5000 BP, with a shift from woody to grass-dominated processes taking place in the Central Cordillera and charcoal particles. This may be indicative of a shift to a Ilocos Mountains, some distance from the site, and then drier climate after 5000 BP and hence a lower accumula- the record of the coastal plain itself. A still widely tion of woody fuel within the vicinity of the lake. accepted model of forest development for the moun- Although Pinus values increase after 4200 BP the per- tains of northern Luzon was first put forward by Kowal centage input does not return to pre-5000-year levels (1966) and suggests that the original vegetation of the until after 1000 BP. Also of note is that the other montane Central Cordillera, before being disturbed by people, taxa, found earlier in the record in association with the was likely a broadleaf forest. That is, lowland rainforest Pinus (Dacrycarpus, Dacrydium, Phyllocladus, Podocarpus, at lower altitudes grading into tropical montane forest Quercus, Eugenia, Theaceae and Tiliaceae) only appear and cloud forest at higher elevations (Fig. 9). The model sporadically in the record after 4200 BP, suggesting that suggests that pines had a limited distribution within the the distribution of montane taxa differs before and after montane forests, behaving primarily as pioneers and 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).

r 2009 Blackwell Publishing Ltd, Global Change Biology, 16, 1672–1688 HOLOCENE ENVIRONMENTAL CHANGE 1685

Pertinent to our understanding of the vegetation being carried to the lake rather than widespread dis- changes in the lake record are sea surface temperature ruption to these vegetation zones. It seems likely, how- and sea surface salinity records being developed from ever, that the larger accumulation values of charcoal in corals approximately 50 km to the south of Paoay Lake. the early part of the record were probably the result of a From this location fossil Porites that span the mid- greater biomass in the lowland landscape when climate Holocene have been analysed for d18O and produced was warmer and wetter, with the following period of records that indicate the sea off north-western Luzon warm dry conditions leading to a decrease in biomass between 6100 and 4000 BP was warmer than present by and as a result a decrease in charcoal accumulation. as much as 1.61 as well as more saline, suggesting a When the gymnosperm and aquatic taxa are re- decrease in runoff and hence precipitation (Yokoyama moved, the remainder of the record has the appearance et al., 2006; Kobayashi et al., 2007). Ocean core 17927-2 of a tropical coastal savanna dominated palynologically off the west coast of northern Luzon (see Fig. 1a) by grass, Casuarina, Macaranga, possibly coastal Ficus records a similar drop in d18O during the mid-Holocene species (recorded as Moraceae/Urticaceae), along with K with the U37 index data also suggesting increased sea various coastal herbs such as the Amaranthaceae, surface salinity (Wang et al., 2005). It would appear from including Alternanthera, and various Convovulaceae these two sets of data that north-western Luzon was including the strand plant Merremia. All are common warmer during the mid-Holocene and possibly drier. elements outside of the cultivated areas in this lowland At a more regional scale the speleothem data from landscape and suggest a significant degree of stability Dongge Cave in southern China (Wang et al., 2005) also over this time period. suggest the region may have been drier after 7000 BP. Today the coastal vegetation of Ilocos Norte has been This record reveals the gradual weakening of the Asian heavily modified by human activities, but how long Monsoon from 7000 BP to around 1000 BP. After 1000 BP people have been practicing agriculture in this region is the strength of the monsoon once again increases. The still an open question. One theory of agricultural devel- overall weakening between 7000 and 1000 BP is thought opment for the Philippines suggests that Neolithic to be in response to orbitally induced lowering of the people expanded out of Taiwan and into northern Northern Hemisphere summer insolation (Wang et al., Luzon during the Holocene, bringing with them rice 2005), but there are several periods of weakening em- agriculture (Bellwood, 1997). While extensive evidence bedded within this time frame, the most pronounced demonstrates a cultural connection between Taiwan being a 500-year period centred around 4400 yrs BP that northern Luzon from around 4000 BP (Bellwood et al., overlaps with the Pinus decline in the Paoay Lake 2003; Bellwood & Dizon, 2005; Hung, 2005) no plant pollen record as well as a period of lower lake levels. remains have been recovered, and direct evidence for We know from the instrumental record that warmer agricultural strategies is slight. In addition, there is no sea surface temperatures accompany El Nin˜o years and archaeological record for this period of prehistory in the that these events lead to reduced precipitation during north-west of Luzon. A recent archaeological survey for the monsoon months and increased stress to forests the the Laoag/Paoay region drew the conclusion that, montane forests of Luzon through an increase in tem- although iron-age pottery and other artefacts are com- perature during the dry months (Moya & Malayang, mon in a number of locales, most of the Neolithic 2004). Recent research reports indicate that that rising archaeology is probably now buried under many global temperatures are leading to increased tree mor- metres of sediment in the massive alluvial plains that tality within the coniferous forests of north-west Amer- constitute most of the flat land in the region (Bellwood ica (van Mantgem et al., 2009). Therefore, the vegetation et al., 2008). At this stage, however, there is no substan- changes observed in the Paoay Lake record may well be tive chronology for the alluvial deposition. related to some of the climatic observations made for the Despite several different approaches to tackling the broader region, as warmer temperatures in combination agriculture question with the Paoay Lake sediments, we with a weakening monsoon would no doubt have have been unable to shed light on this subject for north- adversely affected both the pine and upper montane western Luzon. The most tantalizing result is that forests during the mid-Holocene. As conditions became although pollen of the disturbance taxa Trema and cooler toward the present Pinus forests may have ex- Urticaceae are found throughout the record, there is a panded, although not to same extent until conditions significant increase in these pollen types from 3500 to also became wetter after 1000 BP. However, it is also 1500 BP. The intriguing question, given the overlap with worth considering that a weaker Asian Monsoon may the Neolithic period on the island, is whether or not this also have lead to the pollen transport mechanism being disturbance could be associated with human impact. disrupted. That is, weaker south-east winds may have However, the causes behind this increase in disturbance resulted in less pine pollen and other montane taxa taxa remain unresolved. r 2009 Blackwell Publishing Ltd, Global Change Biology, 16, 1672–1688 1686 J. STEVENSON et al.

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.

There is only one other pollen record from the Phi- differ in that has a much stronger lippines, Laguna de Bay in southern Luzon (Ward & relationship between charcoal and grass, with both Bulalacao, 1999). A summary of this 10 m sediment core, increasing after 2500 BP. At the time, Ward & Bulalacao which covers the last 7000 years, is shown alongside the (1999) found the interpretation of the forest decline after Paoay Lake record in Fig. 10. As Laguna de Baye is 5000 BP problematic, but concluded that it was best situated to the south of the Central Cordillera (Fig. 1) it attributed to the regional climate becoming dryer and has minimal representation of pine pollen due to the that the contribution of human activities to this process prevailing south-westerly winds. However, while the would require study of fire regimes from prehuman vegetation composition of the two records differ, they horizons. Likewise, interpretation of the Paoay Lake have an interesting parallel in forest decline at around record will become more comprehensible as further 5000 BP in the absence of any increase in fire. They also palaeoecological records in our research program are

r 2009 Blackwell Publishing Ltd, Global Change Biology, 16, 1672–1688 HOLOCENE ENVIRONMENTAL CHANGE 1687 developed for the region, in particular from the Caga- to climate change in the past and thereby provide an yan Valley and Mountains Province in the Central assessment of how they might cope in the future. Cordillera. Acknowledgements

Conclusions We thank the office of former Governor Jr. and Governor Michael Marcos Keon for their assistance, as well as The Philippine archipelago is an important location in DENR Laoag City for permission to work at the lake. Thank you island south-east Asia for understanding the evolution to Ludivino Agressor, DENR Paoay Lake, and to Damien Kelle- of the tropical climate system as well as the movement her, Tony Pen˜alosa, Gerald Quina and Alex Pataray for assis- tance with the lake coring. UP Diliman kindly made available the of people through time, yet there is virtually no knowl- use of a vehicle and boat. The National Museum of the Philip- edge of its palaeoenvironmental history. The record pines and Corazon S. Alvina, Director of the National Museum from Paoay Lake therefore makes a fundamental con- are thanked for facilitating our research in the Philippines. Jeff tribution to our understanding of this under-repre- Parr undertook the phytolith analysis and research support was sented region. provided by the Australian Research Council (DP0208831) and AINSE (AINGRA05156). Thanks also to ANU Cartography for Most climate change predictions for this region sug- their excellent work on the figures and to Edward Cushing and gest that temperatures will increase and annual rainfall two anonymous referees who all made suggestions that greatly decrease over the coming decade (Cruz et al., 2007). In improved the manuscript. addition to impacts on the human population we know that an increase in temperature and decrease in rainfall References will also have harmful consequences for the montane biota of Luzon. Human activities in association with the Argete AC (1998) Climate and weather. In: Environmental and Natural widespread use of fire are already reducing cloud forest Resources Atlas of the Philippines (ed. Magdaraog GL), pp. 176–197. Environment Centre of the Philippines Foundation, the Philippines. habitat, and if climate change forecasts are realized, Ariztegui D, McKenzie JA (1995) Temperature-dependent carbon-isotope then even more pressure will be placed on these fragile fractionation of organic matter: a potential paleoclimatic indicator in habitats as clearance activities move higher into the holocene lacustrine sequences. Paleoclimate Research, 15, 17–28. mountains and fires become more intense. The results Battarbee R, Carvalho L, Jones V et al. (2001) Diatoms. In: Tracking from this study suggest that the montane forest systems Environmental Change Using Lake Sediments Volume 3: Terrestrial, Algal and Siliceous Indicators (eds Smol J, Birks H, Last W), pp. 155–202. were altered between 5000 and 4200 years ago, possibly Kluwer Academic Publishers, Dordrecht, the Netherlands. by a period of higher temperatures and lower rainfall. Bellwood P (1997) Prehistory of the Indo-Malaysian Archipelago, 2nd edn. Although the forests appear to have slowly recovered University of Hawaii Press, Honolulu. over the next 3000 years, they did so in the absence of Bellwood P (2005) First Farmers: The Origins of Agricultural Societies. the intense human activities of the modern era. Blackwell, Oxford. Bellwood P, Dizon E (2005) The Batanes Archaological Project and the While we cannot be precise about the causes of ‘Out of Taiwan’ hypothesis for Austronesian dispersal. Journal of environmental change in the mountains of Luzon 5000 Austronesian Studies, 1, 1–33. years ago, these preliminary results are still informative Bellwood P, Stevenson J, Anderson A, Dizon E (2003) Archaeological and and raise a number of questions for further research, palaeoenvironmental research in Batanes and Ilocos Norte Provinces, which include the following: northern Philippines. Bulletin of the Indo-Pacific Prehistory Association, 23, 141–161. 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