Global Change Biology (2010) 16, 1672–1688, doi: 10.1111/j.1365-2486.2009.02039.x Paoay Lake, northern Luzon, the Philippines: 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 Paoay 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 Paoay Lake 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 Laoag 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). 1672 r 2009 Blackwell Publishing Ltd HOLOCENE ENVIRONMENTAL CHANGE 1673 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 Ilocos Norte. 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 r 2009 Blackwell Publishing Ltd, Global Change Biology, 16, 1672–1688 HOLOCENE ENVIRONMENTAL CHANGE 1675 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.