S. G. Haberle
Table 1. Description of pollen zones from the Haeapugua record (figure 3), Tari Basin, Papua New Guinea. (Palynological richness provides a comparative estimate of the expected number of taxa in each sample and is determined by rarefaction analysis (Birks & Line 1992). Key indicators of swamp forest biodiversity are Myrtaceae, Dacrydium and Pandanus.)
low to high
pollen zone description
zone H3 (O30 000–18 000 yr BP) Nothofagus-rich montane forest on valley floor zone H4 (18 000–8500 yr BP)
Grassland dominates with high fire incidence followed by an expansion of mixed montane forest taxa after 14 500 yr BP zone H5 (8500–1700 yr BP) Mixed montane swamp forest dominated by Myrtaceae until around 3000 yr BP when increased disturbance of swamp forest leads to increased dominance of Dacrydium and Pandanus zone H6 (1700 yr BP to the present) Grass and sedge dominated swampland with loss of swamp forest taxa resulting from burning and the initiation of artificial drainage of section of the swamp near the site for agricultural purposes. The increased abundance of Casuarina after 900 yr BP may reflect deliberate planting in nearby gardens
of the pollen zone interpretations are given in table 1. Palynological richness analysis shows that there has been considerable change in pollen-type diversity over at least the past 30 000 years coinciding with major changes in vegetation in this region. Prior to the Last Glacial Maximum, the basin floor is forested with a diverse mosaic of Nothofagus-rich montane forest and mixed swamp forest in the wetter areas and palynolo- gical richness is high. At around the time of the Last Glacial Maximum, grassland is established under the influence of increased burning which is considered to be the consequence of climate change as well as the arrival of humans in the region. Between 18 000 and 9000 yr BP, burning is sufficient to maintain grasslands in the valley floors and while forest is present in the valley the overall diversity is low as are the key indicators of biodiversity. The Early Holocene is marked by the development of swamp forest vegetation and a peak in both the key indicator types and the pollen diversity. This is maintained until around 3000 yr BP when there are indications of swamp forest disturbance, a decline in the pollen diversity and the key indicator taxa, followed by swamp forest clearance commencing around 1700 yr BP (figures 4 and 5).
forest loss under pressure from increased burning and forest clearance activity for agriculture (figure 2). In the Haeapugua record, the combination of both indicators of biodiversity provides a useful insight into the relationship between vegetation community change and biodiversity change. The coincidence in loss of swamp Myrtaceae with lower palynological richness between approximately 18 000 and 14 500 yr BP and after 3000 yr BP to the present provides a strong argument for true biodiversity loss at the site as the former tends to overestimate biodiversity change and the latter underestimates biodiversity change. In contrast, the phases of vegetation change prior to 18 000 yr BP show a lack of consistency between the two indicators in table 1. The low palynological richness values associated with forest cover that included swamp forest indicators during the earliest phase (prior to around 27 500 yr BP; figure 3) in the record may point to a shift in relative dominance of tree species cover (particularly Nothofagus). In this case, swamp forest dynamics driven by disturbance factors or climate change may have led to greater dominance of a single taxonomic group and a perceived loss of diversity at the site.
In the Haeapugua record, lower palynological richness generally coincides with the establishment of open vegetation on the site, which may be explained by a true loss of overall plant diversity from the site or it may reflect the lower pollen taxonomic resolution available from grasslands relative to forests. While there is no evidence to support or deny the latter assertion, this may serve as a cautionary note for interpretation of palynological richness as an indicator of biodiversity. What is certain is that forest biodiversity is reduced through the retreat or total loss of forests from the site. Currently, Haeapugua Swamp does not support any swamp forest, though remnant swamp forest patches survive in other parts of the Tari Basin. The extirpation of swamp forest from the Haeapugua catchment is indicative of a much wider phenomenon evident through an examination of the key pollen indicators of swamp forest from the five major valleys where local swamp forest loss outstrips the overall
5. DISCUSSION (a) Are the present day swamp forests remnants of a once much more extensive forest community? Palaeoecological records from the five major highland valleys examined in New Guinea point to a sustained and gradual intensification of forest clearance and burning from at least 7000 yr BP that led to the loss of swamp forest cover at different times. This process is widespread across the highlands and within the valleys as well. The loss of swamp forest cover reflected in the Haeapugua record is not simply restricted to the local swamp environment. Buried wood in peat sections recorded throughout the basin attest to a much more widespread phenomenon in the Tari Basin (Haberle 1998b), though the variability in the timing of these changes in one single valley has yet to be determined.