Late Quaternary vegetational history of the Enga province of Upland Papua New Guinea

Abstract

Stratigraphies and pollen analyses are reported from three sites within 25 km east and west from Wabag in the highlands of Papua New Guinea, namely: Sirunki, 2500 m above sea level, 32000 to 1500 yr Inferred Ages; Inim, 2500 m above sea level, 10000 to 0 yr Inferred Ages; Birip, 1900 m above sea level, 2300 to 0 yr Inferred Ages. Events evidenced by these data are described against a time scale of Inferred Ages (I.A.) based on radiocarbon dates and stratigraphic considerations. The pollen analytical data from Sirunki are presented in terms of pollen recovery (deposition) rates as grains per square centimetre per year (grains cm -2 a -1 ) and their interpretation controlled by information about total pollen deposition rates and differential pollen production and transport at the present day. Around Sirunki, the composition of the vegetation before 27500 I.A. is enigmatic, although almost certainly it was treeless. From then until 9000 I.A. subalpine and alpine conditions dominated except during two short periods when forest taxa grew in the catchment. Final afforestation began about 9000 I.A. but the composition of the forest did not stabilize until about 3000 years later. This relative stability was shortlived; soon after 5000 I.A. fluctuations in forest composition began. These fluctuations were associated with periodic changes in the proportion of forested to unforested land. The Inim data lead to conclusions generally compatible with those drawn from Sirunki. However, data from the two areas differ in detail, particularly in the later onset of change in the local forests about 2000 I.A. and its intensification, coeval with a diminution in forest area, after 500 I.A. The short record from Birip is dominated by serai changes on the crater wall itself but the main indicators of forest disturbance and unforested areas were already there at its beginning (2300 I.A.). It seems likely that general forest destruction began, or gained greater impetus, around Birip about 450 I.A. In the most general terms, the forest taxa, recorded by pollen analysis, have behaved consistently with their present distributions and ecological relationships throughout the last 30000 years. More detailed resolution, however, exposes many deviations from this generalization. The majority of taxa are usually associated in groups which vary in their composition repeatedly during a few thousand years, yet some of the taxa occasionally behave entirely individualistically. The establishment of forest broadly comparable with that growing around Sirunki today began about 9000 I.A., when the main components entered the catchment, but took about 2500 years to achieve balance and a repeated regeneration process. About 4500 I.A., the relationship between forest canopy trees and forest ephemerals changed from one explicable in terms of the latter’s role in natural forest regeneration to one suggestive of the ephemerals’ wide spread through the forest which could only have been achieved by degradation of the canopy. It is suggested that a rise in the Sirunki basin’s water level and the destruction of the surrounding forest about 13500 I.A. may have been due to seismic activity. The failure of the forest to re-establish there until 9000 i.a. was perhaps due to continued earth movement and partially to climatic conditions. The vegetation record from Sirunki suggests that the mean annual temperature there was similar to that of today between 27000 I.A. and 25500 I.A. but fell irregularly thereafter until between 18500 I.A. and 16000 I.A. it was probably about 10 °C below present. The mean annual temperature rose rapidly after 16000 i.a. and was within 1 °C of its present level by 13500 I.A. The cold episode between 18 500 I.A. and 16000 i.a. corresponds with the last glacial maximum at higher altitudes in New Guinea. Pollen analytical evidence of the altitude of the forest limit and Climap Project Members (1976) estimates of sea surface temperature at that time suggest a temperature lapse rate of about 8.5 °C per 1000 m altitude (compared with 5.8 °C at present), with a firn line kept high, as the geomorphological evidence demands, by low precipitation at high altitudes. In this coldest period the altitudinal forest limit was about 1500 m below its present level of 3800 m. There is some evidence to suggest that the highest altitude forests of that time may have been quite different from those of today, perhaps containing components of the lower mountain forest canopy as well as the plants of the present upper mountain forest. This implies that the upper mountain forest becomes a separate entity only during comparatively short excursions up the mountains during periods of relatively warm climate. The low altitude of the forest limit during the last major cold period and its subsequent rise through 1500 m must have had substantial repercussions on the composition of the forests at lower altitudes. Although there is no archaeological evidence, the pollen analytical data suggest human interference with the forests around Sirunki from about 4300 I.A., which for 1300 years involved clearing of the forest and the enhanced growth of ephemerals of forest and open-land. Subsequently, the forest remained generally degenerate and a new wave of clearing began about 2000 I.A. near both Sirunki and Inim which continued and intensified about 500 I.A. At the lower altitude of Birip, forests were already disturbed by the beginning of the pollen analytical record at about 2300 I.A.