Landscape change during the Holocene in the Philippine lowlands

Janelle Stevenson¹, Jan Finn¹, Fernando Siringan²
& Domingo Madulid³

Introduction

The palaeoenvironmental history of Paoay Lake in northern Luzon (Figs 1 & 2) is being examined through the analysis of pollen, diatoms, charcoal, mineral magnetics and AMS dating.

The project is part of a larger program of archaeological research examining the theory of whether a Neolithic expansion took place out of Taiwan and into island southeast Asia around 4,000 years ago.

The primary intention of the lake study is to determine if there is evidence of land clearance and agricultural development in the region during the late Holocene.

Setting

Paoay Lake is a large coastal lake in the northwest of Luzon (Figure 1).

It is separated from the sea by sand dunes that are approximately 2.3 km wide and 40 m high (Figure 2). Prior to undertaking this study the age of the lake formation was unknown. The only outlet until recently was a small river on southeast edge. This was closed off during the 1950's.

Two sediment cores (LP-2 and LP-3) were collected in 2002. Additional material from site LP-3 as well as several several other locations across the lake (yellow dots) were collected in 2004.

A grid of lake bed samples (purple dots) were also collected for modern pollen rain and diatoms.

The top 68cm of LP-3 was not recoverable with a livingston corer in 2002 (Figure 3), but has since been recovered with a mud-water interface sampler.

Seven pollen fraction samples were AMS dated from LP-3 revealing that the base of the core collected in 2002 is 5000 yrs old (Figure 3).

An adjacent core collected in 2004 has yielded sediment back into estuarine clay.

Freshwater diatoms and freshwater Pediastrum throughout the overlying sediments suggest that it has been a freshwater lake since the lake was initially cut off around 6000 yrs ago through a combination of dune building and uplift; the interface between the marine clay and overlying freshwater sediments is now 6m above current sea level.

The most important finding to date is that Pinus forest grew around the lake until around 5,000 BP when the pollen signature changes to that more indicative of an open landscape (primarily grass) (Figure 3).

Pine forests today are not found below 600 m in Luzon. This is considered to be their altitudinal limit.

Other forest components include Dacrydium, Dacrycarpus, Phyllocladus, and Podocarpus.

Fire appears to be connected to the destruction / removal of the Pinus forest (Figure 3).

However, Pinus remains a regional component of the pollen rain until the present day. Values of 10% or less (solid red line red line Figure 3) are indicative of the present day Pinus pollen rain from the survey of lake bed samples.

This finding raises the question of whether the forest decline at 5,000 BP is associated with human activity or a climatic shift, possibly an increase in climatic variability.

Untangling the impact of climate change and from human activity

1. Grass seeds found from after the Pinus collapse to just before 2000 BP. The seeds are not Setaria italica (foxtail millet) but they appear to belong to the genus Setaria or possibly one of two closely related genera that contain other less common millet species. A comparative seed collection has just been put together
2. Grass pollen in the size range that rice constitutes rice dominates the pollen spectrum after 2000 BP

Macro-charcoal has been counted in continuous 10 yr intervals for the the last 6000 yrs (Figure 4). The charcoal influx is smoothed by a 100 yr moving average and compared with the ENSO event times series constructed by Moy et al. [2002; Nature, vol 420, 162-165].

No relationship between ENSO curve and charcoal curve 6000-5000 BP and decline of pine forest.

Some similarities in peaks from 5000 - ~2000 BP.

No similarity after 2000 BP. Breakdown in natural system with agriculture? Or inadequate dating in upper sediments?

Ongoing work

• Additional dating: marine freshwater interface as well as younger sediments (AMS and ²¹º Pb)
• ¹(8)0 diatoms to be analysed for palaeo-precipitation record
• Identification of grass seeds
• Palaeoenvironmental work in the montane forests of Luzon to help unravel lowland story